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Repository: isocpp/CppCoreGuidelines
Branch: master
Commit: 431d25dc7710
Files: 31
Total size: 1.8 MB
Directory structure:
gitextract_y8p6tgzy/
├── .github/
│ └── workflows/
│ ├── build.yml
│ └── jekyll-gh-pages.yml
├── .gitignore
├── .travis.yml
├── CONTRIBUTING.md
├── CppCoreGuidelines.md
├── LICENSE
├── README.md
├── SECURITY.md
├── _config.yml
├── _includes/
│ ├── head.html
│ └── sidebar.html
├── _layouts/
│ └── default.html
├── docs/
│ ├── dyn_array.md
│ └── gsl-intro.md
├── index.html
├── params.json
├── public/
│ └── css/
│ ├── custom.css
│ ├── hyde.css
│ └── poole.css
├── scripts/
│ ├── Makefile
│ ├── hunspell/
│ │ ├── en_US.aff
│ │ ├── en_US.dic
│ │ └── isocpp.dic
│ ├── nodejs/
│ │ ├── package.json
│ │ └── remark/
│ │ └── .remarkrc
│ └── python/
│ ├── Makefile.in
│ ├── cpplint.py
│ ├── cpplint_wrap.py
│ └── md-split.py
└── talks/
└── README.md
================================================
FILE CONTENTS
================================================
================================================
FILE: .github/workflows/build.yml
================================================
name: build
on: [push, pull_request]
jobs:
build:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@master
- name: Install Hunspell
run: |
sudo apt-get update
sudo apt-get install hunspell
- name: Run cd scripts; make -k
run: cd scripts; make -k
- name: Run cd ..
run: cd ..
================================================
FILE: .github/workflows/jekyll-gh-pages.yml
================================================
# Sample workflow for building and deploying a Jekyll site to GitHub Pages
name: Deploy Jekyll with GitHub Pages dependencies preinstalled
on:
# Runs on pushes targeting the default branch
push:
branches: ["master"]
# Allows you to run this workflow manually from the Actions tab
workflow_dispatch:
# Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
permissions:
contents: read
pages: write
id-token: write
# Allow only one concurrent deployment, skipping runs queued between the run in-progress and latest queued.
# However, do NOT cancel in-progress runs as we want to allow these production deployments to complete.
concurrency:
group: "pages"
cancel-in-progress: false
jobs:
# Build job
build:
runs-on: ubuntu-latest
steps:
- name: Checkout
uses: actions/checkout@v6
- name: Setup Pages
uses: actions/configure-pages@v5
- name: Build with Jekyll
uses: actions/jekyll-build-pages@v1
with:
source: ./
destination: ./_site
- name: Upload artifact
uses: actions/upload-pages-artifact@v4
# Deployment job
deploy:
environment:
name: github-pages
url: ${{ steps.deployment.outputs.page_url }}
runs-on: ubuntu-latest
needs: build
steps:
- name: Deploy to GitHub Pages
id: deployment
uses: actions/deploy-pages@v4
================================================
FILE: .gitignore
================================================
scripts/build
scripts/nodesjs/build
node_modules
_site
scripts/python/__pycache__
scripts/python/*.pyc
# VS Code
.vs/
# MacOS-specific
.DS_Store
================================================
FILE: .travis.yml
================================================
# This is the configuration for Travis CI, a free github-integrated service that runs this script for each pull request (if configured)
# provides gcc, clang, make, scons, cmake
language: c++
# alternatives: gcc, clang, or both (as yaml list)
compiler: clang
addons:
apt:
packages:
- hunspell
install:
-
script:
- cd scripts; make -k
- cd ..
notifications:
email: false
================================================
FILE: CONTRIBUTING.md
================================================
## Contributing to the C++ Core Guidelines
>"Within C++ is a smaller, simpler, safer language struggling to get out."
>-- <cite>Bjarne Stroustrup</cite>
The C++ Core Guidelines are a collaborative effort led by Bjarne Stroustrup, much like the C++ language itself. They are the result of many
person-years of discussion and design across a number of organizations. Their design encourages general applicability and broad adoption but
they can be freely copied and modified to meet your organization's needs.
We encourage contributions to the C++ Core Guidelines in a number of ways:
- **Individual feedback** Are you a developer who is passionate about your code? Join the discussion in
[Issues](https://github.com/isocpp/CppCoreGuidelines/issues). We want to know which rules resonate with you and which don't. Were any rules
inordinately difficult to apply? Does your compiler vendor's Guidelines Support Library (e.g.,
[Microsoft's implementation of the GSL](https://github.com/microsoft/gsl)) suit your needs in adopting these guidelines?
- **Organizational adoption** While the guidelines are designed to be broadly adoptable they are also intended to be modified to fit your
organization's particular needs. We encourage your organization to fork this repo and create your own copy of these guidelines with changes
that reflect your needs. We suggest that you make it clear in the title of your guidelines that these are your organization's fork of the
guidelines and that you provide a link back to the original set of [guidelines](https://github.com/isocpp/CppCoreGuidelines). And if any of
your local changes are appropriate to pull back into the original guidelines, please open an
[Issue](https://github.com/isocpp/CppCoreGuidelines/issues) which can lead to a pull request.
- **Maintain the Guidelines** The C++ Core Guidelines were created from a wealth of knowledge spread across a number of organizations
worldwide. If you or your organization is passionate about helping to create the guidelines, consider becoming an editor or maintainer. If
you're a C++ expert who is serious about participating, please
[email coreguidelines@isocpp.org](mailto:coreguidelines@isocpp.org?subject=Maintain%20the%20C++%20Code%20Guidelines).
## Contributor License Agreement
By contributing content to the C++ Core Guidelines (i.e., submitting a pull request for inclusion in this repository) you agree with the
[Standard C++ Foundation](https://isocpp.org/about) [Terms of Use](https://isocpp.org/home/terms-of-use), especially all of the terms specified
regarding Copyright and Patents.
- You warrant that your material is original, or you have the right to contribute it.
- With respect to the material that you own, you grant a worldwide, non-exclusive, irrevocable, transferable, and royalty-free license to your contributed
material to Standard C++ Foundation to display, reproduce, perform, distribute, and create derivative works of that material for commercial or
non-commercial use. With respect to any other material you contribute, such material must be under a license sufficient to allow Standard C++ Foundation
to display, reproduce, perform, distribute, and create derivative works of that material for commercial or non-commercial use.
- You agree that, if your contributed material is subsequently reflected in the ISO/IEC C++ standard in any form, it will be subject to all ISO/IEC JTC
1 policies including [copyrights](http://www.iso.org/iso/home/policies.htm),
[patents](http://www.iso.org/iso/home/standards_development/governance_of_technical_work/patents.htm), and
[procedures](http://www.itscj.ipsj.or.jp/sc29/29w7proc.htm); please direct any questions about these policies to the
[ISO Central Secretariat](http://www.iso.org/iso/home/about.htm).
## Pull requests
We welcome pull requests for scoped changes to the guidelines--bug fixes in
examples, clarifying ambiguous text, etc. Significant changes should first be
discussed in the [Issues](https://github.com/isocpp/CppCoreGuidelines/issues)
and the Issue number must be included in the pull request. For
guideline-related changes, please specify the rule number in your Issue and/or
Pull Request.
Changes should be made in a child commit of a recent commit in the master
branch. If you are making many small changes, please create separate PRs to
minimize merge issues.
### Document Style Guidelines
Documents in this repository are written in an unspecific flavor of Markdown,
which leaves some ambiguity for formatting text. We ask that pull requests
maintain the following style guidelines, though we are aware that the document
may not already be consistent.
#### Indentation
Code and nested text should use multiples of 4 spaces of indentation, and no
tab characters, like so:
void func(const int x)
{
std::cout << x << '\n';
}
#### Code Blocks
Please use 4-space indentation to trigger code parsing, rather than [fenced code blocks](https://help.github.com/articles/github-flavored-markdown/#fenced-code-blocks) or any other style, like so:
This is some document text, with an example below:
void func()
{
std::cout << "This is code.\n";
}
#### Document style decisions
We've discussed and made decisions on a number of document style. Please do not open PRs that revisit these stylistic points:
- The CppCoreGuidelines.md file is a single GH-flavored Markdown file. It is not split into separate chapters.
- We do not use syntax highlighting in the Core Guidelines. See PRs #33, #96, #328, and #779. If you want syntax highlighting you
can either view the "pretty" version at http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines or do your own post-processing.
- We're sticking with the ASCII character set. We do not use Unicode em-dashes, Unicode spaces, or pretty quotes. Lots of people edit this file with their various text editors. ASCII is simple and universally understood.
### Update dictionary
Code samples in the guidelines are run through a spelling checker. Be sure to add new class and variable names to [scripts/hunspell/isocpp.dic](https://github.com/isocpp/CppCoreGuidelines/blob/master/scripts/hunspell/isocpp.dic).
### Miscellaneous
To avoid line-ending issues, please set `autocrlf = input` and `whitespace = cr-at-eol` in your git configuration.
================================================
FILE: CppCoreGuidelines.md
================================================
# <a name="main"></a>C++ Core Guidelines
Jul 8, 2025
Editors:
* [Bjarne Stroustrup](https://www.stroustrup.com)
* [Herb Sutter](https://herbsutter.com/)
This is a living document under continuous improvement.
Had it been an open-source (code) project, this would have been release 0.8.
Copying, use, modification, and creation of derivative works from this project is licensed under an MIT-style license.
Contributing to this project requires agreeing to a Contributor License. See the accompanying [LICENSE](https://github.com/isocpp/CppCoreGuidelines/blob/master/LICENSE) file for details.
We make this project available to "friendly users" to use, copy, modify, and derive from, hoping for constructive input.
Comments and suggestions for improvements are most welcome.
We plan to modify and extend this document as our understanding improves and the language and the set of available libraries improve.
When commenting, please note [the introduction](#s-introduction) that outlines our aims and general approach.
The list of contributors is [here](#ss-ack).
Problems:
* The sets of rules have not been completely checked for completeness, consistency, or enforceability.
* Triple question marks (???) mark known missing information.
* Update reference sections; many pre-C++11 sources are too old.
* For a more-or-less up-to-date to-do list see: [To-do: Unclassified proto-rules](#s-unclassified).
You can [read an explanation of the scope and structure of this Guide](#s-abstract) or just jump straight in:
* [In: Introduction](#s-introduction)
* [P: Philosophy](#s-philosophy)
* [I: Interfaces](#s-interfaces)
* [F: Functions](#s-functions)
* [C: Classes and class hierarchies](#s-class)
* [Enum: Enumerations](#s-enum)
* [R: Resource management](#s-resource)
* [ES: Expressions and statements](#s-expr)
* [Per: Performance](#s-performance)
* [CP: Concurrency and parallelism](#s-concurrency)
* [E: Error handling](#s-errors)
* [Con: Constants and immutability](#s-const)
* [T: Templates and generic programming](#s-templates)
* [CPL: C-style programming](#s-cpl)
* [SF: Source files](#s-source)
* [SL: The Standard Library](#sl-the-standard-library)
Supporting sections:
* [A: Architectural ideas](#s-a)
* [NR: Non-Rules and myths](#s-not)
* [RF: References](#s-references)
* [Pro: Profiles](#s-profile)
* [GSL: Guidelines support library](#s-gsl)
* [NL: Naming and layout suggestions](#s-naming)
* [FAQ: Answers to frequently asked questions](#s-faq)
* [Appendix A: Libraries](#s-libraries)
* [Appendix B: Modernizing code](#s-modernizing)
* [Appendix C: Discussion](#s-discussion)
* [Appendix D: Supporting tools](#s-tools)
* [Glossary](#s-glossary)
* [To-do: Unclassified proto-rules](#s-unclassified)
You can sample rules for specific language features:
* assignment:
[regular types](#rc-regular) --
[prefer initialization](#rc-initialize) --
[copy](#rc-copy-semantic) --
[move](#rc-move-semantic) --
[other operations](#rc-matched) --
[default](#rc-eqdefault)
* `class`:
[data](#rc-org) --
[invariant](#rc-struct) --
[members](#rc-member) --
[helpers](#rc-helper) --
[concrete types](#ss-concrete) --
[ctors, =, and dtors](#s-ctor) --
[hierarchy](#ss-hier) --
[operators](#ss-overload)
* `concept`:
[rules](#ss-concepts) --
[in generic programming](#rt-raise) --
[template arguments](#rt-concepts) --
[semantics](#rt-low)
* constructor:
[invariant](#rc-struct) --
[establish invariant](#rc-ctor) --
[`throw`](#rc-throw) --
[default](#rc-default0) --
[not needed](#rc-default) --
[`explicit`](#rc-explicit) --
[delegating](#rc-delegating) --
[`virtual`](#rc-ctor-virtual)
* derived `class`:
[when to use](#rh-domain) --
[as interface](#rh-abstract) --
[destructors](#rh-dtor) --
[copy](#rh-copy) --
[getters and setters](#rh-get) --
[multiple inheritance](#rh-mi-interface) --
[overloading](#rh-using) --
[slicing](#rc-copy-virtual) --
[`dynamic_cast`](#rh-dynamic_cast)
* destructor:
[and constructors](#rc-matched) --
[when needed?](#rc-dtor) --
[must not fail](#rc-dtor-fail)
* exception:
[errors](#s-errors) --
[`throw`](#re-throw) --
[for errors only](#re-errors) --
[`noexcept`](#re-noexcept) --
[minimize `try`](#re-catch) --
[what if no exceptions?](#re-no-throw-codes)
* `for`:
[range-for and for](#res-for-range) --
[for and while](#res-for-while) --
[for-initializer](#res-for-init) --
[empty body](#res-empty) --
[loop variable](#res-loop-counter) --
[loop variable type ???](#res-???)
* function:
[naming](#rf-package) --
[single operation](#rf-logical) --
[no throw](#rf-noexcept) --
[arguments](#rf-smart) --
[argument passing](#rf-conventional) --
[multiple return values](#rf-out-multi) --
[pointers](#rf-return-ptr) --
[lambdas](#rf-capture-vs-overload)
* `inline`:
[small functions](#rf-inline) --
[in headers](#rs-inline)
* initialization:
[always](#res-always) --
[prefer `{}`](#res-list) --
[lambdas](#res-lambda-init) --
[default member initializers](#rc-in-class-initializer) --
[class members](#rc-initialize) --
[factory functions](#rc-factory)
* lambda expression:
[when to use](#ss-lambdas)
* operator:
[conventional](#ro-conventional) --
[avoid conversion operators](#ro-conversion) --
[and lambdas](#ro-lambda)
* `public`, `private`, and `protected`:
[information hiding](#rc-private) --
[consistency](#rh-public) --
[`protected`](#rh-protected)
* `static_assert`:
[compile-time checking](#rp-compile-time) --
[and concepts](#rt-check-class)
* `struct`:
[for organizing data](#rc-org) --
[use if no invariant](#rc-struct) --
[no private members](#rc-class)
* `template`:
[abstraction](#rt-raise) --
[containers](#rt-cont) --
[concepts](#rt-concepts)
* `unsigned`:
[and signed](#res-mix) --
[bit manipulation](#res-unsigned)
* `virtual`:
[interfaces](#ri-abstract) --
[not `virtual`](#rc-concrete) --
[destructor](#rc-dtor-virtual) --
[never fail](#rc-dtor-fail)
You can look at design concepts used to express the rules:
* assertion: ???
* error: ???
* exception: exception guarantee (???)
* failure: ???
* invariant: ???
* leak: ???
* library: ???
* precondition: ???
* postcondition: ???
* resource: ???
# <a name="s-abstract"></a>Abstract
This document is a set of guidelines for using C++ well.
The aim of this document is to help people to use modern C++ effectively.
By "modern C++" we mean effective use of the ISO C++ standard (currently C++20, but almost all of our recommendations also apply to C++17, C++14 and C++11).
In other words, what would you like your code to look like in 5 years' time, given that you can start now? In 10 years' time?
The guidelines are focused on relatively high-level issues, such as interfaces, resource management, memory management, and concurrency.
Such rules affect application architecture and library design.
Following the rules will lead to code that is statically type safe, has no resource leaks, and catches many more programming logic errors than is common in code today.
And it will run fast -- you can afford to do things right.
We are less concerned with low-level issues, such as naming conventions and indentation style.
However, no topic that can help a programmer is out of bounds.
Our initial set of rules emphasizes safety (of various forms) and simplicity.
They might very well be too strict.
We expect to have to introduce more exceptions to better accommodate real-world needs.
We also need more rules.
You will find some of the rules contrary to your expectations or even contrary to your experience.
If we haven't suggested you change your coding style in any way, we have failed!
Please try to verify or disprove rules!
In particular, we'd really like to have some of our rules backed up with measurements or better examples.
You will find some of the rules obvious or even trivial.
Please remember that one purpose of a guideline is to help someone who is less experienced or coming from a different background or language to get up to speed.
Many of the rules are designed to be supported by an analysis tool.
Violations of rules will be flagged with references (or links) to the relevant rule.
We do not expect you to memorize all the rules before trying to write code.
One way of thinking about these guidelines is as a specification for tools that happens to be readable by humans.
The rules are meant for gradual introduction into a code base.
We plan to build tools for that and hope others will too.
Comments and suggestions for improvements are most welcome.
We plan to modify and extend this document as our understanding improves and the language and the set of available libraries improve.
# <a name="s-introduction"></a>In: Introduction
This is a set of core guidelines for modern C++ (currently C++20 and C++17) taking likely future enhancements and ISO Technical Specifications (TSs) into account.
The aim is to help C++ programmers to write simpler, more efficient, more maintainable code.
Introduction summary:
* [In.target: Target readership](#ss-readers)
* [In.aims: Aims](#ss-aims)
* [In.not: Non-aims](#ss-non)
* [In.force: Enforcement](#ss-force)
* [In.struct: The structure of this document](#ss-struct)
* [In.sec: Major sections](#ss-sec)
## <a name="ss-readers"></a>In.target: Target readership
All C++ programmers. This includes [programmers who might consider C](#s-cpl).
## <a name="ss-aims"></a>In.aims: Aims
The purpose of this document is to help developers to adopt modern C++ (currently C++20 and C++17) and to achieve a more uniform style across code bases.
We do not suffer the delusion that every one of these rules can be effectively applied to every code base. Upgrading old systems is hard. However, we do believe that a program that uses a rule is less error-prone and more maintainable than one that does not. Often, rules also lead to faster/easier initial development.
As far as we can tell, these rules lead to code that performs as well or better than older, more conventional techniques; they are meant to follow the zero-overhead principle ("what you don't use, you don't pay for" or "when you use an abstraction mechanism appropriately, you get at least as good performance as if you had handcoded using lower-level language constructs").
Consider these rules ideals for new code, opportunities to exploit when working on older code, and try to approximate these ideals as closely as feasible.
Remember:
### <a name="r0"></a>In.0: Don't panic!
Take the time to understand the implications of a guideline rule on your program.
These guidelines are designed according to the "subset of superset" principle ([Stroustrup05](#Stroustrup05)).
They do not simply define a subset of C++ to be used (for reliability, safety, performance, or whatever).
Instead, they strongly recommend the use of a few simple "extensions" ([library components](#gsl-guidelines-support-library))
that make the use of the most error-prone features of C++ redundant, so that they can be banned (in our set of rules).
The rules emphasize static type safety and resource safety.
For that reason, they emphasize possibilities for range checking, for avoiding dereferencing `nullptr`, for avoiding dangling pointers, and the systematic use of exceptions (via RAII).
Partly to achieve that and partly to minimize obscure code as a source of errors, the rules also emphasize simplicity and the hiding of necessary complexity behind well-specified interfaces.
Many of the rules are prescriptive.
We are uncomfortable with rules that simply state "don't do that!" without offering an alternative.
One consequence of that is that some rules can be supported only by heuristics, rather than precise and mechanically verifiable checks.
Other rules articulate general principles. For these more general rules, more detailed and specific rules provide partial checking.
These guidelines address the core of C++ and its use.
We expect that most large organizations, specific application areas, and even large projects will need further rules, possibly further restrictions, and further library support.
For example, hard-real-time programmers typically can't use free store (dynamic memory) freely and will be restricted in their choice of libraries.
We encourage the development of such more specific rules as addenda to these core guidelines.
Build your ideal small foundation library and use that, rather than lowering your level of programming to glorified assembly code.
The rules are designed to allow [gradual adoption](#s-modernizing).
Some rules aim to increase various forms of safety while others aim to reduce the likelihood of accidents, many do both.
The guidelines aimed at preventing accidents often ban perfectly legal C++.
However, when there are two ways of expressing an idea and one has shown itself a common source of errors and the other has not, we try to guide programmers towards the latter.
## <a name="ss-non"></a>In.not: Non-aims
The rules are not intended to be minimal or orthogonal.
In particular, general rules can be simple, but unenforceable.
Also, it is often hard to understand the implications of a general rule.
More specialized rules are often easier to understand and to enforce, but without general rules, they would just be a long list of special cases.
We provide rules aimed at helping novices as well as rules supporting expert use.
Some rules can be completely enforced, but others are based on heuristics.
These rules are not meant to be read serially, like a book.
You can browse through them using the links.
However, their main intended use is to be targets for tools.
That is, a tool looks for violations and the tool returns links to violated rules.
The rules then provide reasons, examples of potential consequences of the violation, and suggested remedies.
These guidelines are not intended to be a substitute for a tutorial treatment of C++.
If you need a tutorial for some given level of experience, see [the references](#s-references).
This is not a guide on how to convert old C++ code to more modern code.
It is meant to articulate ideas for new code in a concrete fashion.
However, see [the modernization section](#s-modernizing) for some possible approaches to modernizing/rejuvenating/upgrading.
Importantly, the rules support gradual adoption: It is typically infeasible to completely convert a large code base all at once.
These guidelines are not meant to be complete or exact in every language-technical detail.
For the final word on language definition issues, including every exception to general rules and every feature, see the ISO C++ standard.
The rules are not intended to force you to write in an impoverished subset of C++.
They are *emphatically* not meant to define a, say, Java-like subset of C++.
They are not meant to define a single "one true C++" language.
We value expressiveness and uncompromised performance.
The rules are not value-neutral.
They are meant to make code simpler and more correct/safer than most existing C++ code, without loss of performance.
They are meant to inhibit perfectly valid C++ code that correlates with errors, spurious complexity, and poor performance.
The rules are not precise to the point where a person (or machine) can follow them without thinking.
The enforcement parts try to be that, but we would rather leave a rule or a definition a bit vague
and open to interpretation than specify something precisely and wrong.
Sometimes, precision comes only with time and experience.
Design is not (yet) a form of Math.
The rules are not perfect.
A rule can do harm by prohibiting something that is useful in a given situation.
A rule can do harm by failing to prohibit something that enables a serious error in a given situation.
A rule can do a lot of harm by being vague, ambiguous, unenforceable, or by enabling every solution to a problem.
It is impossible to completely meet the "do no harm" criteria.
Instead, our aim is the less ambitious: "Do the most good for most programmers";
if you cannot live with a rule, object to it, ignore it, but don't water it down until it becomes meaningless.
Also, suggest an improvement.
## <a name="ss-force"></a>In.force: Enforcement
Rules with no enforcement are unmanageable for large code bases.
Enforcement of all rules is possible only for a small weak set of rules or for a specific user community.
* But we want lots of rules, and we want rules that everybody can use.
* But different people have different needs.
* But people don't like to read lots of rules.
* But people can't remember many rules.
So, we need subsetting to meet a variety of needs.
* But arbitrary subsetting leads to chaos.
We want guidelines that help a lot of people, make code more uniform, and strongly encourage people to modernize their code.
We want to encourage best practices, rather than leave all to individual choices and management pressures.
The ideal is to use all rules; that gives the greatest benefits.
This adds up to quite a few dilemmas.
We try to resolve those using tools.
Each rule has an **Enforcement** section listing ideas for enforcement.
Enforcement might be done by code review, by static analysis, by compiler, or by run-time checks.
Wherever possible, we prefer "mechanical" checking (humans are slow, inaccurate, and bore easily) and static checking.
Run-time checks are suggested only rarely where no alternative exists; we do not want to introduce "distributed bloat".
Where appropriate, we label a rule (in the **Enforcement** sections) with the name of groups of related rules (called "profiles").
A rule can be part of several profiles, or none.
For a start, we have a few profiles corresponding to common needs (desires, ideals):
* **type**: No type violations (reinterpreting a `T` as a `U` through casts, unions, or varargs)
* **bounds**: No bounds violations (accessing beyond the range of an array)
* **lifetime**: No leaks (failing to `delete` or multiple `delete`) and no access to invalid objects (dereferencing `nullptr`, using a dangling reference).
The profiles are intended to be used by tools, but also serve as an aid to the human reader.
We do not limit our comment in the **Enforcement** sections to things we know how to enforce; some comments are mere wishes that might inspire some tool builder.
Tools that implement these rules shall respect the following syntax to explicitly suppress a rule:
[[gsl::suppress("tag")]]
and optionally with a message (following usual C++11 standard attribute syntax):
[[gsl::suppress("tag", justification: "message")]]
where
* `"tag"` is a string literal with the anchor name of the item where the Enforcement rule appears (e.g., for [C.134](#rh-public) it is "rh-public"), the
name of a profile group-of-rules ("type", "bounds", or "lifetime"),
or a specific rule in a profile ([type.4](#pro-type-cstylecast), or [bounds.2](#pro-bounds-arrayindex)). Any text that is not one of those should be rejected.
* `"message"` is a string literal
## <a name="ss-struct"></a>In.struct: The structure of this document
Each rule (guideline, suggestion) can have several parts:
* The rule itself -- e.g., **no naked `new`**
* A rule reference number -- e.g., **C.7** (the 7th rule related to classes).
Since the major sections are not inherently ordered, we use letters as the first part of a rule reference "number".
We leave gaps in the numbering to minimize "disruption" when we add or remove rules.
* **Reason**s (rationales) -- because programmers find it hard to follow rules they don't understand
* **Example**s -- because rules are hard to understand in the abstract; can be positive or negative
* **Alternative**s -- for "don't do this" rules
* **Exception**s -- we prefer simple general rules. However, many rules apply widely, but not universally, so exceptions must be listed
* **Enforcement** -- ideas about how the rule might be checked "mechanically"
* **See also**s -- references to related rules and/or further discussion (in this document or elsewhere)
* **Note**s (comments) -- something that needs saying that doesn't fit the other classifications
* **Discussion** -- references to more extensive rationale and/or examples placed outside the main lists of rules
Some rules are hard to check mechanically, but they all meet the minimal criteria that an expert programmer can spot many violations without too much trouble.
We hope that "mechanical" tools will improve with time to approximate what such an expert programmer notices.
Also, we assume that the rules will be refined over time to make them more precise and checkable.
A rule is aimed at being simple, rather than carefully phrased to mention every alternative and special case.
Such information is found in the **Alternative** paragraphs and the [Discussion](#s-discussion) sections.
If you don't understand a rule or disagree with it, please visit its **Discussion**.
If you feel that a discussion is missing or incomplete, enter an [Issue](https://github.com/isocpp/CppCoreGuidelines/issues)
explaining your concerns and possibly a corresponding PR.
Examples are written to illustrate rules.
* Examples are not intended to be production quality or to cover all tutorial dimensions.
For example, many examples are language-technical and use names like `f`, `base`, and `x`.
* We try to ensure that "good" examples follow the Core Guidelines.
* Comments are often illustrating rules where they would be unnecessary and/or distracting in "real code."
* We assume knowledge of the standard library. For example, we use plain `vector` rather than `std::vector`.
This is not a language manual.
It is meant to be helpful, rather than complete, fully accurate on technical details, or a guide to existing code.
Recommended information sources can be found in [the references](#s-references).
## <a name="ss-sec"></a>In.sec: Major sections
* [In: Introduction](#s-introduction)
* [P: Philosophy](#s-philosophy)
* [I: Interfaces](#s-interfaces)
* [F: Functions](#s-functions)
* [C: Classes and class hierarchies](#s-class)
* [Enum: Enumerations](#s-enum)
* [R: Resource management](#s-resource)
* [ES: Expressions and statements](#s-expr)
* [Per: Performance](#s-performance)
* [CP: Concurrency and parallelism](#s-concurrency)
* [E: Error handling](#s-errors)
* [Con: Constants and immutability](#s-const)
* [T: Templates and generic programming](#s-templates)
* [CPL: C-style programming](#s-cpl)
* [SF: Source files](#s-source)
* [SL: The Standard Library](#sl-the-standard-library)
Supporting sections:
* [A: Architectural ideas](#s-a)
* [NR: Non-Rules and myths](#s-not)
* [RF: References](#s-references)
* [Pro: Profiles](#s-profile)
* [GSL: Guidelines support library](#gsl-guidelines-support-library)
* [NL: Naming and layout suggestions](#s-naming)
* [FAQ: Answers to frequently asked questions](#s-faq)
* [Appendix A: Libraries](#s-libraries)
* [Appendix B: Modernizing code](#s-modernizing)
* [Appendix C: Discussion](#s-discussion)
* [Appendix D: Supporting tools](#s-tools)
* [Glossary](#s-glossary)
* [To-do: Unclassified proto-rules](#s-unclassified)
These sections are not orthogonal.
Each section (e.g., "P" for "Philosophy") and each subsection (e.g., "C.hier" for "Class Hierarchies (OOP)") have an abbreviation for ease of searching and reference.
The main section abbreviations are also used in rule numbers (e.g., "C.11" for "Make concrete types regular").
# <a name="s-philosophy"></a>P: Philosophy
The rules in this section are very general.
Philosophy rules summary:
* [P.1: Express ideas directly in code](#rp-direct)
* [P.2: Write in ISO Standard C++](#rp-cplusplus)
* [P.3: Express intent](#rp-what)
* [P.4: Ideally, a program should be statically type safe](#rp-typesafe)
* [P.5: Prefer compile-time checking to run-time checking](#rp-compile-time)
* [P.6: What cannot be checked at compile time should be checkable at run time](#rp-run-time)
* [P.7: Catch run-time errors early](#rp-early)
* [P.8: Don't leak any resources](#rp-leak)
* [P.9: Don't waste time or space](#rp-waste)
* [P.10: Prefer immutable data to mutable data](#rp-mutable)
* [P.11: Encapsulate messy constructs, rather than spreading through the code](#rp-library)
* [P.12: Use supporting tools as appropriate](#rp-tools)
* [P.13: Use support libraries as appropriate](#rp-lib)
Philosophical rules are generally not mechanically checkable.
However, individual rules reflecting these philosophical themes are.
Without a philosophical basis, the more concrete/specific/checkable rules lack rationale.
### <a name="rp-direct"></a>P.1: Express ideas directly in code
##### Reason
Compilers don't read comments (or design documents) and neither do many programmers (consistently).
What is expressed in code has defined semantics and can (in principle) be checked by compilers and other tools.
##### Example
class Date {
public:
Month month() const; // do
int month(); // don't
// ...
};
The first declaration of `month` is explicit about returning a `Month` and about not modifying the state of the `Date` object.
The second version leaves the reader guessing and opens more possibilities for uncaught bugs.
##### Example, bad
This loop is a restricted form of `std::find`:
void f(vector<string>& v)
{
string val;
cin >> val;
// ...
int index = -1; // bad, plus should use gsl::index
for (int i = 0; i < v.size(); ++i) {
if (v[i] == val) {
index = i;
break;
}
}
// ...
}
##### Example, good
A much clearer expression of intent would be:
void f(vector<string>& v)
{
string val;
cin >> val;
// ...
auto p = find(begin(v), end(v), val); // better
// ...
}
A well-designed library expresses intent (what is to be done, rather than just how something is being done) far better than direct use of language features.
A C++ programmer should know the basics of the standard library, and use it where appropriate.
Any programmer should know the basics of the foundation libraries of the project being worked on, and use them appropriately.
Any programmer using these guidelines should know the [guidelines support library](#gsl-guidelines-support-library), and use it appropriately.
##### Example
change_speed(double s); // bad: what does s signify?
// ...
change_speed(2.3);
A better approach is to be explicit about the meaning of the double (new speed or delta on old speed?) and the unit used:
change_speed(Speed s); // better: the meaning of s is specified
// ...
change_speed(2.3); // error: no unit
change_speed(23_m / 10s); // meters per second
We could have accepted a plain (unit-less) `double` as a delta, but that would have been error-prone.
If we wanted both absolute speed and deltas, we would have defined a `Delta` type.
##### Enforcement
Very hard in general.
* use `const` consistently (check if member functions modify their object; check if functions modify arguments passed by pointer or reference)
* flag uses of casts (casts neuter the type system)
* detect code that mimics the standard library (hard)
### <a name="rp-cplusplus"></a>P.2: Write in ISO Standard C++
##### Reason
This is a set of guidelines for writing ISO Standard C++.
##### Note
There are environments where extensions are necessary, e.g., to access system resources.
In such cases, localize the use of necessary extensions and control their use with non-core Coding Guidelines. If possible, build interfaces that encapsulate the extensions so they can be turned off or compiled away on systems that do not support those extensions.
Extensions often do not have rigorously defined semantics. Even extensions that
are common and implemented by multiple compilers might have slightly different
behaviors and edge case behavior as a direct result of *not* having a rigorous
standard definition. With sufficient use of any such extension, expected
portability will be impacted.
##### Note
Using valid ISO C++ does not guarantee portability (let alone correctness).
Avoid dependence on undefined behavior (e.g., [undefined order of evaluation](#res-order))
and be aware of constructs with implementation defined meaning (e.g., `sizeof(int)`).
##### Note
There are environments where restrictions on use of standard C++ language or library features are necessary, e.g., to avoid dynamic memory allocation as required by aircraft control software standards.
In such cases, control their (dis)use with an extension of these Coding Guidelines customized to the specific environment.
##### Enforcement
Use an up-to-date C++ compiler (currently C++20 or C++17) with a set of options that do not accept extensions.
### <a name="rp-what"></a>P.3: Express intent
##### Reason
Unless the intent of some code is stated (e.g., in names or comments), it is impossible to tell whether the code does what it is supposed to do.
##### Example
gsl::index i = 0;
while (i < v.size()) {
// ... do something with v[i] ...
}
The intent of "just" looping over the elements of `v` is not expressed here. The implementation detail of an index is exposed (so that it might be misused), and `i` outlives the scope of the loop, which might or might not be intended. The reader cannot know from just this section of code.
Better:
for (const auto& x : v) { /* do something with the value of x */ }
Now, there is no explicit mention of the iteration mechanism, and the loop operates on a reference to `const` elements so that accidental modification cannot happen. If modification is desired, say so:
for (auto& x : v) { /* modify x */ }
For more details about for-statements, see [ES.71](#res-for-range).
Sometimes better still, use a named algorithm. This example uses the `for_each` from the Ranges TS because it directly expresses the intent:
for_each(v, [](int x) { /* do something with the value of x */ });
for_each(par, v, [](int x) { /* do something with the value of x */ });
The last variant makes it clear that we are not interested in the order in which the elements of `v` are handled.
A programmer should be familiar with
* [The guidelines support library](#gsl-guidelines-support-library)
* [The ISO C++ Standard Library](#sl-the-standard-library)
* Whatever foundation libraries are used for the current project(s)
##### Note
Alternative formulation: Say what should be done, rather than just how it should be done.
##### Note
Some language constructs express intent better than others.
##### Example
If two `int`s are meant to be the coordinates of a 2D point, say so:
draw_line(int, int, int, int); // obscure: (x1,y1,x2,y2)? (x,y,h,w)? ...?
// need to look up documentation to know
draw_line(Point, Point); // clearer
##### Enforcement
Look for common patterns for which there are better alternatives
* simple `for` loops vs. range-`for` loops
* `f(T*, int)` interfaces vs. `f(span<T>)` interfaces
* loop variables in too large a scope
* naked `new` and `delete`
* functions with many parameters of built-in types
There is a huge scope for cleverness and semi-automated program transformation.
### <a name="rp-typesafe"></a>P.4: Ideally, a program should be statically type safe
##### Reason
Ideally, a program would be completely statically (compile-time) type safe.
Unfortunately, that is not possible. Problem areas:
* unions
* casts
* array decay
* range errors
* narrowing conversions
##### Note
These areas are sources of serious problems (e.g., crashes and security violations).
We try to provide alternative techniques.
##### Enforcement
We can ban, restrain, or detect the individual problem categories separately, as required and feasible for individual programs.
Always suggest an alternative.
For example:
* unions -- use `variant` (in C++17)
* casts -- minimize their use; templates can help
* array decay -- use `span` (from the GSL)
* range errors -- use `span`
* narrowing conversions -- minimize their use and use `narrow` or `narrow_cast` (from the GSL) where they are necessary
### <a name="rp-compile-time"></a>P.5: Prefer compile-time checking to run-time checking
##### Reason
Code clarity and performance.
You don't need to write error handlers for errors caught at compile time.
##### Example
// Int is an alias used for integers
int bits = 0; // don't: avoidable code
for (Int i = 1; i; i <<= 1)
++bits;
if (bits < 32)
cerr << "Int too small\n";
This example fails to achieve what it is trying to achieve (because overflow is undefined) and should be replaced with a simple `static_assert`:
// Int is an alias used for integers
static_assert(sizeof(Int) >= 4); // do: compile-time check
Or better still just use the type system and replace `Int` with `int32_t`.
##### Example
void read(int* p, int n); // read max n integers into *p
int a[100];
read(a, 1000); // bad, off the end
better
void read(span<int> r); // read into the range of integers r
int a[100];
read(a); // better: let the compiler figure out the number of elements
**Alternative formulation**: Don't postpone to run time what can be done well at compile time.
##### Enforcement
* Look for pointer arguments.
* Look for run-time checks for range violations.
### <a name="rp-run-time"></a>P.6: What cannot be checked at compile time should be checkable at run time
##### Reason
Leaving hard-to-detect errors in a program is asking for crashes and bad results.
##### Note
Ideally, we catch all errors (that are not errors in the programmer's logic) at either compile time or run time. It is impossible to catch all errors at compile time and often not affordable to catch all remaining errors at run time. However, we should endeavor to write programs that in principle can be checked, given sufficient resources (analysis programs, run-time checks, machine resources, time).
##### Example, bad
// separately compiled, possibly dynamically loaded
extern void f(int* p);
void g(int n)
{
// bad: the number of elements is not passed to f()
f(new int[n]);
}
Here, a crucial bit of information (the number of elements) has been so thoroughly "obscured" that static analysis is probably rendered infeasible and dynamic checking can be very difficult when `f()` is part of an ABI so that we cannot "instrument" that pointer. We could embed helpful information into the free store, but that requires global changes to a system and maybe to the compiler. What we have here is a design that makes error detection very hard.
##### Example, bad
We can of course pass the number of elements along with the pointer:
// separately compiled, possibly dynamically loaded
extern void f2(int* p, int n);
void g2(int n)
{
// bad: the wrong number of elements can be passed to f2()
f2(new int[n], n);
}
Passing the number of elements as an argument is better (and far more common) than just passing the pointer and relying on some (unstated) convention for knowing or discovering the number of elements. However (as shown), a simple typo can introduce a serious error. The connection between the two arguments of `f2()` is conventional, rather than explicit.
Also, it is implicit that `f2()` is supposed to `delete` its argument (or did the caller make a second mistake?).
##### Example, bad
The standard library resource management pointers fail to pass the size when they point to an object:
// separately compiled, possibly dynamically loaded
// NB: this assumes the calling code is ABI-compatible, using a
// compatible C++ compiler and the same stdlib implementation
extern void f3(unique_ptr<int[]>, int n);
void g3(int n)
{
f3(make_unique<int[]>(n), m); // bad: pass ownership and size separately
}
##### Example
We need to pass the pointer and the number of elements as an integral object:
extern void f4(vector<int>&); // separately compiled, possibly dynamically loaded
extern void f4(span<int>); // separately compiled, possibly dynamically loaded
// NB: this assumes the calling code is ABI-compatible, using a
// compatible C++ compiler and the same stdlib implementation
void g3(int n)
{
vector<int> v(n);
f4(v); // pass a reference, retain ownership
f4(span<int>{v}); // pass a view, retain ownership
}
This design carries the number of elements along as an integral part of an object, so that errors are unlikely and dynamic (run-time) checking is always feasible, if not always affordable.
##### Example
How do we transfer both ownership and all information needed for validating use?
vector<int> f5(int n) // OK: move
{
vector<int> v(n);
// ... initialize v ...
return v;
}
unique_ptr<int[]> f6(int n) // bad: loses n
{
auto p = make_unique<int[]>(n);
// ... initialize *p ...
return p;
}
owner<int*> f7(int n) // bad: loses n and we might forget to delete
{
owner<int*> p = new int[n];
// ... initialize *p ...
return p;
}
##### Example
* ???
* show how possible checks are avoided by interfaces that pass polymorphic base classes around, when they actually know what they need?
Or strings as "free-style" options
##### Enforcement
* Flag (pointer, count)-style interfaces (this will flag a lot of examples that can't be fixed for compatibility reasons)
* ???
### <a name="rp-early"></a>P.7: Catch run-time errors early
##### Reason
Avoid "mysterious" crashes.
Avoid errors leading to (possibly unrecognized) wrong results.
##### Example
void increment1(int* p, int n) // bad: error-prone
{
for (int i = 0; i < n; ++i) ++p[i];
}
void use1(int m)
{
const int n = 10;
int a[n] = {};
// ...
increment1(a, m); // maybe typo, maybe m <= n is supposed
// but assume that m == 20
// ...
}
Here we made a small error in `use1` that will lead to corrupted data or a crash.
The (pointer, count)-style interface leaves `increment1()` with no realistic way of defending itself against out-of-range errors.
If we could check subscripts for out of range access, then the error would not be discovered until `p[10]` was accessed.
We could check earlier and improve the code:
void increment2(span<int> p)
{
for (int& x : p) ++x;
}
void use2(int m)
{
const int n = 10;
int a[n] = {};
// ...
increment2({a, m}); // maybe typo, maybe m <= n is supposed
// ...
}
Now, `m <= n` can be checked at the point of call (early) rather than later.
If all we had was a typo so that we meant to use `n` as the bound, the code could be further simplified (eliminating the possibility of an error):
void use3(int m)
{
const int n = 10;
int a[n] = {};
// ...
increment2(a); // the number of elements of a need not be repeated
// ...
}
##### Example, bad
Don't repeatedly check the same value. Don't pass structured data as strings:
Date read_date(istream& is); // read date from istream
Date extract_date(const string& s); // extract date from string
void user1(const string& date) // manipulate date
{
auto d = extract_date(date);
// ...
}
void user2()
{
Date d = read_date(cin);
// ...
user1(d.to_string());
// ...
}
The date is validated twice (by the `Date` constructor) and passed as a character string (unstructured data).
##### Example
Excess checking can be costly.
There are cases where checking early is inefficient because you might never need the value, or might only need part of the value that is more easily checked than the whole. Similarly, don't add validity checks that change the asymptotic behavior of your interface (e.g., don't add a `O(n)` check to an interface with an average complexity of `O(1)`).
class Jet { // Physics says: e * e < x * x + y * y + z * z
float x;
float y;
float z;
float e;
public:
Jet(float x, float y, float z, float e)
:x(x), y(y), z(z), e(e)
{
// Should I check here that the values are physically meaningful?
}
float m() const
{
// Should I handle the degenerate case here?
return sqrt(x * x + y * y + z * z - e * e);
}
???
};
The physical law for a jet (`e * e < x * x + y * y + z * z`) is not an invariant because of the possibility for measurement errors.
???
##### Enforcement
* Look at pointers and arrays: Do range-checking early and not repeatedly
* Look at conversions: Eliminate or mark narrowing conversions
* Look for unchecked values coming from input
* Look for structured data (objects of classes with invariants) being converted into strings
* ???
### <a name="rp-leak"></a>P.8: Don't leak any resources
##### Reason
Even a slow growth in resources will, over time, exhaust the availability of those resources.
This is particularly important for long-running programs, but is an essential piece of responsible programming behavior.
##### Example, bad
void f(const char* name)
{
FILE* input = fopen(name, "r");
// ...
if (something) return; // bad: if something == true, a file handle is leaked
// ...
fclose(input);
}
Prefer [RAII](#rr-raii):
void f(const char* name)
{
ifstream input {name};
// ...
if (something) return; // OK: no leak
// ...
}
**See also**: [The resource management section](#s-resource)
##### Note
A leak is colloquially "anything that isn't cleaned up."
The more important classification is "anything that can no longer be cleaned up."
For example, allocating an object on the heap and then losing the last pointer that points to that allocation.
This rule should not be taken as requiring that allocations within long-lived objects must be returned during program shutdown.
For example, relying on system guaranteed cleanup such as file closing and memory deallocation upon process shutdown can simplify code.
However, relying on abstractions that implicitly clean up can be as simple, and often safer.
##### Note
Enforcing [the lifetime safety profile](#ss-lifetime) eliminates leaks.
When combined with resource safety provided by [RAII](#rr-raii), it eliminates the need for "garbage collection" (by generating no garbage).
Combine this with enforcement of [the type and bounds profiles](#ss-force) and you get complete type- and resource-safety, guaranteed by tools.
##### Enforcement
* Look at pointers: Classify them into non-owners (the default) and owners.
Where feasible, replace owners with standard-library resource handles (as in the example above).
Alternatively, mark an owner as such using `owner` from [the GSL](#gsl-guidelines-support-library).
* Look for naked `new` and `delete`
* Look for known resource allocating functions returning raw pointers (such as `fopen`, `malloc`, and `strdup`)
### <a name="rp-waste"></a>P.9: Don't waste time or space
##### Reason
This is C++.
##### Note
Time and space that you spend well to achieve a goal (e.g., speed of development, resource safety, or simplification of testing) is not wasted.
"Another benefit of striving for efficiency is that the process forces you to understand the problem in more depth." - Alex Stepanov
##### Example, bad
struct X {
char ch;
int i;
string s;
char ch2;
X& operator=(const X& a);
X(const X&);
};
X waste(const char* p)
{
if (!p) throw Nullptr_error{};
int n = strlen(p);
auto buf = new char[n];
if (!buf) throw Allocation_error{};
for (int i = 0; i < n; ++i) buf[i] = p[i];
// ... manipulate buffer ...
X x;
x.ch = 'a';
x.s = string(n); // give x.s space for *p
for (gsl::index i = 0; i < x.s.size(); ++i) x.s[i] = buf[i]; // copy buf into x.s
delete[] buf;
return x;
}
void driver()
{
X x = waste("Typical argument");
// ...
}
Yes, this is a caricature, but we have seen every individual mistake in production code, and worse.
Note that the layout of `X` guarantees that at least 6 bytes (and most likely more) are wasted.
The spurious definition of copy operations disables move semantics so that the return operation is slow
(please note that the Return Value Optimization, RVO, is not guaranteed here).
The use of `new` and `delete` for `buf` is redundant; if we really needed a local string, we should use a local `string`.
There are several more performance bugs and gratuitous complication.
##### Example, bad
void lower(zstring s)
{
for (int i = 0; i < strlen(s); ++i) s[i] = tolower(s[i]);
}
This is actually an example from production code.
We can see that in our condition we have `i < strlen(s)`. This expression will be evaluated on every iteration of the loop, which means that `strlen` must walk through string every loop to discover its length. While the string contents are changing, it's assumed that `tolower` will not affect the length of the string, so it's better to cache the length outside the loop and not incur that cost each iteration.
##### Note
An individual example of waste is rarely significant, and where it is significant, it is typically easily eliminated by an expert.
However, waste spread liberally across a code base can easily be significant and experts are not always as available as we would like.
The aim of this rule (and the more specific rules that support it) is to eliminate most waste related to the use of C++ before it happens.
After that, we can look at waste related to algorithms and requirements, but that is beyond the scope of these guidelines.
##### Enforcement
Many more specific rules aim at the overall goals of simplicity and elimination of gratuitous waste.
* Flag an unused return value from a user-defined non-defaulted postfix `operator++` or `operator--` function. Prefer using the prefix form instead. (Note: "User-defined non-defaulted" is intended to reduce noise. Review this enforcement if it's still too noisy in practice.)
### <a name="rp-mutable"></a>P.10: Prefer immutable data to mutable data
##### Reason
It is easier to reason about constants than about variables.
Something immutable cannot change unexpectedly.
Sometimes immutability enables better optimization.
You can't have a data race on a constant.
See [Con: Constants and immutability](#s-const)
### <a name="rp-library"></a>P.11: Encapsulate messy constructs, rather than spreading through the code
##### Reason
Messy code is more likely to hide bugs and harder to write.
A good interface is easier and safer to use.
Messy, low-level code breeds more such code.
##### Example
int sz = 100;
int* p = (int*) malloc(sizeof(int) * sz);
int count = 0;
// ...
for (;;) {
// ... read an int into x, exit loop if end of file is reached ...
// ... check that x is valid ...
if (count == sz)
p = (int*) realloc(p, sizeof(int) * sz * 2);
p[count++] = x;
// ...
}
This is low-level, verbose, and error-prone.
For example, we "forgot" to test for memory exhaustion and assign new value to `sz`.
Instead, we could use `vector`:
vector<int> v;
v.reserve(100);
// ...
for (int x; cin >> x; ) {
// ... check that x is valid ...
v.push_back(x);
}
##### Note
The standards library and the GSL are examples of this philosophy.
For example, instead of messing with the arrays, unions, cast, tricky lifetime issues, `gsl::owner`, etc.,
that are needed to implement key abstractions, such as `vector`, `span`, `lock_guard`, and `future`, we use the libraries
designed and implemented by people with more time and expertise than we usually have.
Similarly, we can and should design and implement more specialized libraries, rather than leaving the users (often ourselves)
with the challenge of repeatedly getting low-level code well.
This is a variant of the [subset of superset principle](#r0) that underlies these guidelines.
##### Enforcement
* Look for "messy code" such as complex pointer manipulation and casting outside the implementation of abstractions.
### <a name="rp-tools"></a>P.12: Use supporting tools as appropriate
##### Reason
There are many things that are done better "by machine".
Computers don't tire or get bored by repetitive tasks.
We typically have better things to do than repeatedly do routine tasks.
##### Example
Run a static analyzer to verify that your code follows the guidelines you want it to follow.
##### Note
See
* [Static analysis tools](https://en.wikipedia.org/wiki/List_of_tools_for_static_code_analysis)
* [Concurrency tools](#rconc-tools)
* [Testing tools](https://github.com/isocpp/CppCoreGuidelines/tree/master)
There are many other kinds of tools, such as source code repositories, build tools, etc.,
but those are beyond the scope of these guidelines.
##### Note
Be careful not to become dependent on over-elaborate or over-specialized tool chains.
Those can make your otherwise portable code non-portable.
### <a name="rp-lib"></a>P.13: Use support libraries as appropriate
##### Reason
Using a well-designed, well-documented, and well-supported library saves time and effort;
its quality and documentation are likely to be greater than what you could do
if the majority of your time must be spent on an implementation.
The cost (time, effort, money, etc.) of a library can be shared over many users.
A widely used library is more likely to be kept up-to-date and ported to new systems than an individual application.
Knowledge of a widely-used library can save time on other/future projects.
So, if a suitable library exists for your application domain, use it.
##### Example
std::sort(begin(v), end(v), std::greater<>());
Unless you are an expert in sorting algorithms and have plenty of time,
this is more likely to be correct and to run faster than anything you write for a specific application.
You need a reason not to use the standard library (or whatever foundational libraries your application uses) rather than a reason to use it.
##### Note
By default use
* The [ISO C++ Standard Library](#sl-the-standard-library)
* The [Guidelines Support Library](#gsl-guidelines-support-library)
##### Note
If no well-designed, well-documented, and well-supported library exists for an important domain,
maybe you should design and implement it, and then use it.
# <a name="s-interfaces"></a>I: Interfaces
An interface is a contract between two parts of a program. Precisely stating what is expected of a supplier of a service and a user of that service is essential.
Having good (easy-to-understand, encouraging efficient use, not error-prone, supporting testing, etc.) interfaces is probably the most important single aspect of code organization.
Interface rule summary:
* [I.1: Make interfaces explicit](#ri-explicit)
* [I.2: Avoid non-`const` global variables](#ri-global)
* [I.3: Avoid singletons](#ri-singleton)
* [I.4: Make interfaces precisely and strongly typed](#ri-typed)
* [I.5: State preconditions (if any)](#ri-pre)
* [I.6: Prefer `Expects()` for expressing preconditions](#ri-expects)
* [I.7: State postconditions](#ri-post)
* [I.8: Prefer `Ensures()` for expressing postconditions](#ri-ensures)
* [I.9: If an interface is a template, document its parameters using concepts](#ri-concepts)
* [I.10: Use exceptions to signal a failure to perform a required task](#ri-except)
* [I.11: Never transfer ownership by a raw pointer (`T*`) or reference (`T&`)](#ri-raw)
* [I.12: Declare a pointer that must not be null as `not_null`](#ri-nullptr)
* [I.13: Do not pass an array as a single pointer](#ri-array)
* [I.22: Avoid complex initialization of global objects](#ri-global-init)
* [I.23: Keep the number of function arguments low](#ri-nargs)
* [I.24: Avoid adjacent parameters that can be invoked by the same arguments in either order with different meaning](#ri-unrelated)
* [I.25: Prefer empty abstract classes as interfaces to class hierarchies](#ri-abstract)
* [I.26: If you want a cross-compiler ABI, use a C-style subset](#ri-abi)
* [I.27: For stable library ABI, consider the Pimpl idiom](#ri-pimpl)
* [I.30: Encapsulate rule violations](#ri-encapsulate)
**See also**:
* [F: Functions](#s-functions)
* [C.concrete: Concrete types](#ss-concrete)
* [C.hier: Class hierarchies](#ss-hier)
* [C.over: Overloading and overloaded operators](#ss-overload)
* [C.con: Containers and other resource handles](#ss-containers)
* [E: Error handling](#s-errors)
* [T: Templates and generic programming](#s-templates)
### <a name="ri-explicit"></a>I.1: Make interfaces explicit
##### Reason
Correctness. Assumptions not stated in an interface are easily overlooked and hard to test.
##### Example, bad
Controlling the behavior of a function through a global (namespace scope) variable (a call mode) is implicit and potentially confusing. For example:
int round(double d)
{
return (round_up) ? ceil(d) : d; // don't: "invisible" dependency
}
It will not be obvious to a caller that the meaning of two calls of `round(7.2)` might give different results.
##### Exception
Sometimes we control the details of a set of operations by an environment variable, e.g., normal vs. verbose output or debug vs. optimized.
The use of a non-local control is potentially confusing, but controls only implementation details of otherwise fixed semantics.
##### Example, bad
Reporting through non-local variables (e.g., `errno`) is easily ignored. For example:
// don't: no test of fprintf's return value
fprintf(connection, "logging: %d %d %d\n", x, y, s);
What if the connection goes down so that no logging output is produced? See I.???.
**Alternative**: Throw an exception. An exception cannot be ignored.
**Alternative formulation**: Avoid passing information across an interface through non-local or implicit state.
Note that non-`const` member functions pass information to other member functions through their object's state.
**Alternative formulation**: An interface should be a function or a set of functions.
Functions can be function templates and sets of functions can be classes or class templates.
##### Enforcement
* (Simple) A function should not make control-flow decisions based on the values of variables declared at namespace scope.
* (Simple) A function should not write to variables declared at namespace scope.
### <a name="ri-global"></a>I.2: Avoid non-`const` global variables
##### Reason
Non-`const` global variables hide dependencies and make the dependencies subject to unpredictable changes.
##### Example
struct Data {
// ... lots of stuff ...
} data; // non-const data
void compute() // don't
{
// ... use data ...
}
void output() // don't
{
// ... use data ...
}
Who else might modify `data`?
**Warning**: The initialization of global objects is not totally ordered.
If you use a global object initialize it with a constant.
Note that it is possible to get undefined initialization order even for `const` objects.
##### Exception
A global object is often better than a singleton.
##### Note
Global constants are useful.
##### Note
The rule against global variables applies to namespace scope variables as well.
**Alternative**: If you use global (more generally namespace scope) data to avoid copying, consider passing the data as an object by reference to `const`.
Another solution is to define the data as the state of some object and the operations as member functions.
**Warning**: Beware of data races: If one thread can access non-local data (or data passed by reference) while another thread executes the callee, we can have a data race.
Every pointer or reference to mutable data is a potential data race.
Using global pointers or references to access and change non-const, and otherwise non-global,
data isn't a better alternative to non-const global variables since that doesn't solve the issues of hidden dependencies or potential race conditions.
##### Note
You cannot have a race condition on immutable data.
**References**: See the [rules for calling functions](#ss-call).
##### Note
The rule is "avoid", not "don't use." Of course there will be (rare) exceptions, such as `cin`, `cout`, and `cerr`.
##### Enforcement
(Simple) Report all non-`const` variables declared at namespace scope and global pointers/references to non-const data.
### <a name="ri-singleton"></a>I.3: Avoid singletons
##### Reason
Singletons are basically complicated global objects in disguise.
##### Example
class Singleton {
// ... lots of stuff to ensure that only one Singleton object is created,
// that it is initialized properly, etc.
};
There are many variants of the singleton idea.
That's part of the problem.
##### Note
If you don't want a global object to change, declare it `const` or `constexpr`.
##### Exception
You can use the simplest "singleton" (so simple that it is often not considered a singleton) to get initialization on first use, if any:
X& myX()
{
static X my_x {3};
return my_x;
}
This is one of the most effective solutions to problems related to initialization order.
In a multi-threaded environment, the initialization of the static object does not introduce a race condition
(unless you carelessly access a shared object from within its constructor).
Note that the initialization of a local `static` does not imply a race condition.
However, if the destruction of `X` involves an operation that needs to be synchronized we must use a less simple solution.
For example:
X& myX()
{
static auto p = new X {3};
return *p; // potential leak
}
Now someone must `delete` that object in some suitably thread-safe way.
That's error-prone, so we don't use that technique unless
* `myX` is in multi-threaded code,
* that `X` object needs to be destroyed (e.g., because it releases a resource), and
* `X`'s destructor's code needs to be synchronized.
If you, as many do, define a singleton as a class for which only one object is created, functions like `myX` are not singletons, and this useful technique is not an exception to the no-singleton rule.
##### Enforcement
Very hard in general.
* Look for classes with names that include `singleton`.
* Look for classes for which only a single object is created (by counting objects or by examining constructors).
* If a class X has a public static function that contains a function-local static of the class' type X and returns a pointer or reference to it, ban that.
### <a name="ri-typed"></a>I.4: Make interfaces precisely and strongly typed
##### Reason
Types are the simplest and best documentation, improve legibility due to their well-defined meaning, and are checked at compile time.
Also, precisely typed code is often optimized better.
##### Example, don't
Consider:
void pass(void* data); // weak and under-qualified type void* is suspicious
Callers are unsure what types are allowed and if the data may
be mutated as `const` is not specified. Note all pointer types
implicitly convert to `void*`, so it is easy for callers to provide this value.
The callee must `static_cast` data to an unverified type to use it.
That is error-prone and verbose.
Only use `const void*` for passing in data in designs that are indescribable in C++. Consider using a `variant` or a pointer to base instead.
**Alternative**: Often, a template parameter can eliminate the `void*` turning it into a `T*` or `T&`.
For generic code these `T`s can be general or concept constrained template parameters.
##### Example, bad
Consider:
draw_rect(100, 200, 100, 500); // what do the numbers specify?
draw_rect(p.x, p.y, 10, 20); // what units are 10 and 20 in?
It is clear that the caller is describing a rectangle, but it is unclear what parts they relate to. Also, an `int` can carry arbitrary forms of information, including values of many units, so we must guess about the meaning of the four `int`s. Most likely, the first two are an `x`,`y` coordinate pair, but what are the last two?
Comments and parameter names can help, but we could be explicit:
void draw_rectangle(Point top_left, Point bottom_right);
void draw_rectangle(Point top_left, Size height_width);
draw_rectangle(p, Point{10, 20}); // two corners
draw_rectangle(p, Size{10, 20}); // one corner and a (height, width) pair
Obviously, we cannot catch all errors through the static type system
(e.g., the fact that a first argument is supposed to be a top-left point is left to convention (naming and comments)).
##### Example, bad
Consider:
set_settings(true, false, 42); // what do the numbers specify?
The parameter types and their values do not communicate what settings are being specified or what those values mean.
This design is more explicit, safe and legible:
alarm_settings s{};
s.enabled = true;
s.displayMode = alarm_settings::mode::spinning_light;
s.frequency = alarm_settings::every_10_seconds;
set_settings(s);
For the case of a set of boolean values consider using a flags `enum`; a pattern that expresses a set of boolean values.
enable_lamp_options(lamp_option::on | lamp_option::animate_state_transitions);
##### Example, bad
In the following example, it is not clear from the interface what `time_to_blink` means: Seconds? Milliseconds?
void blink_led(int time_to_blink) // bad -- the unit is ambiguous
{
// ...
// do something with time_to_blink
// ...
}
void use()
{
blink_led(2);
}
##### Example, good
`std::chrono::duration` types help making the unit of time duration explicit.
void blink_led(milliseconds time_to_blink) // good -- the unit is explicit
{
// ...
// do something with time_to_blink
// ...
}
void use()
{
blink_led(1500ms);
}
The function can also be written in such a way that it will accept any time duration unit.
template<class rep, class period>
void blink_led(duration<rep, period> time_to_blink) // good -- accepts any unit
{
// assuming that millisecond is the smallest relevant unit
auto milliseconds_to_blink = duration_cast<milliseconds>(time_to_blink);
// ...
// do something with milliseconds_to_blink
// ...
}
void use()
{
blink_led(2s);
blink_led(1500ms);
}
##### Enforcement
* (Simple) Report the use of `void*` as a parameter or return type.
* (Simple) Report the use of more than one `bool` parameter.
* (Hard to do well) Look for functions that use too many primitive type arguments.
### <a name="ri-pre"></a>I.5: State preconditions (if any)
##### Reason
Arguments have meaning that might constrain their proper use in the callee.
##### Example
Consider:
double sqrt(double x);
Here `x` must be non-negative. The type system cannot (easily and naturally) express that, so we must use other means. For example:
double sqrt(double x); // x must be non-negative
Some preconditions can be expressed as assertions. For example:
double sqrt(double x) { Expects(x >= 0); /* ... */ }
Ideally, that `Expects(x >= 0)` should be part of the interface of `sqrt()` but that's not easily done. For now, we place it in the definition (function body).
**References**: `Expects()` is described in [GSL](#gsl-guidelines-support-library).
##### Note
Prefer a formal specification of requirements, such as `Expects(p);`.
If that is infeasible, use English text in comments, such as `// the sequence [p:q) is ordered using <`.
##### Note
Most member functions have as a precondition that some class invariant holds.
That invariant is established by a constructor and must be reestablished upon exit by every member function called from outside the class.
We don't need to mention it for each member function.
##### Enforcement
(Not enforceable)
**See also**: The rules for passing pointers. ???
### <a name="ri-expects"></a>I.6: Prefer `Expects()` for expressing preconditions
##### Reason
To make it clear that the condition is a precondition and to enable tool use.
##### Example
int area(int height, int width)
{
Expects(height > 0 && width > 0); // good
if (height <= 0 || width <= 0) my_error(); // obscure
// ...
}
##### Note
Preconditions can be stated in many ways, including comments, `if`-statements, and `assert()`.
This can make them hard to distinguish from ordinary code, hard to update, hard to manipulate by tools, and might have the wrong semantics (do you always want to abort in debug mode and check nothing in productions runs?).
##### Note
Preconditions should be part of the interface rather than part of the implementation,
but we don't yet have the language facilities to do that.
Once language support becomes available (e.g., see the [contract proposal](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0380r1.pdf)) we will adopt the standard version of preconditions, postconditions, and assertions.
##### Note
`Expects()` can also be used to check a condition in the middle of an algorithm.
##### Note
No, using `unsigned` is not a good way to sidestep the problem of [ensuring that a value is non-negative](#res-nonnegative).
##### Enforcement
(Not enforceable) Finding the variety of ways preconditions can be asserted is not feasible. Warning about those that can be easily identified (`assert()`) has questionable value in the absence of a language facility.
### <a name="ri-post"></a>I.7: State postconditions
##### Reason
To detect misunderstandings about the result and possibly catch erroneous implementations.
##### Example, bad
Consider:
int area(int height, int width) { return height * width; } // bad
Here, we (incautiously) left out the precondition specification, so it is not explicit that height and width must be positive.
We also left out the postcondition specification, so it is not obvious that the algorithm (`height * width`) is wrong for areas larger than the largest integer.
Overflow can happen.
Consider using:
int area(int height, int width)
{
auto res = height * width;
Ensures(res > 0);
return res;
}
##### Example, bad
Consider a famous security bug:
void f() // problematic
{
char buffer[MAX];
// ...
memset(buffer, 0, sizeof(buffer));
}
There was no postcondition stating that the buffer should be cleared and the optimizer eliminated the apparently redundant `memset()` call:
void f() // better
{
char buffer[MAX];
// ...
memset(buffer, 0, sizeof(buffer));
Ensures(buffer[0] == 0);
}
##### Note
Postconditions are often informally stated in a comment that states the purpose of a function; `Ensures()` can be used to make this more systematic, visible, and checkable.
##### Note
Postconditions are especially important when they relate to something that is not directly reflected in a returned result, such as a state of a data structure used.
##### Example
Consider a function that manipulates a `Record`, using a `mutex` to avoid race conditions:
mutex m;
void manipulate(Record& r) // don't
{
m.lock();
// ... no m.unlock() ...
}
Here, we "forgot" to state that the `mutex` should be released, so we don't know if the failure to ensure release of the `mutex` was a bug or a feature.
Stating the postcondition would have made it clear:
void manipulate(Record& r) // postcondition: m is unlocked upon exit
{
m.lock();
// ... no m.unlock() ...
}
The bug is now obvious (but only to a human reading comments).
Better still, use [RAII](#rr-raii) to ensure that the postcondition ("the lock must be released") is enforced in code:
void manipulate(Record& r) // best
{
lock_guard<mutex> _ {m};
// ...
}
##### Note
Ideally, postconditions are stated in the interface/declaration so that users can easily see them.
Only postconditions related to the users can be stated in the interface.
Postconditions related only to internal state belong in the definition/implementation.
##### Enforcement
(Not enforceable) This is a philosophical guideline that is infeasible to check
directly in the general case. Domain specific checkers (like lock-holding
checkers) exist for many toolchains.
### <a name="ri-ensures"></a>I.8: Prefer `Ensures()` for expressing postconditions
##### Reason
To make it clear that the condition is a postcondition and to enable tool use.
##### Example
void f()
{
char buffer[MAX];
// ...
memset(buffer, 0, MAX);
Ensures(buffer[0] == 0);
}
##### Note
Postconditions can be stated in many ways, including comments, `if`-statements, and `assert()`.
This can make them hard to distinguish from ordinary code, hard to update, hard to manipulate by tools, and might have the wrong semantics.
**Alternative**: Postconditions of the form "this resource must be released" are best expressed by [RAII](#rr-raii).
##### Note
Ideally, that `Ensures` should be part of the interface, but that's not easily done.
For now, we place it in the definition (function body).
Once language support becomes available (e.g., see the [contract proposal](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0380r1.pdf)) we will adopt the standard version of preconditions, postconditions, and assertions.
##### Enforcement
(Not enforceable) Finding the variety of ways postconditions can be asserted is not feasible. Warning about those that can be easily identified (`assert()`) has questionable value in the absence of a language facility.
### <a name="ri-concepts"></a>I.9: If an interface is a template, document its parameters using concepts
##### Reason
Make the interface precisely specified and compile-time checkable in the (not so distant) future.
##### Example
Use the C++20 style of requirements specification. For example:
template<typename Iter, typename Val>
requires input_iterator<Iter> && equality_comparable_with<iter_value_t<Iter>, Val>
Iter find(Iter first, Iter last, Val v)
{
// ...
}
**See also**: [Generic programming](#ss-gp) and [concepts](#ss-concepts).
##### Enforcement
Warn if any non-variadic template parameter is not constrained by a concept (in its declaration or mentioned in a `requires` clause).
### <a name="ri-except"></a>I.10: Use exceptions to signal a failure to perform a required task
##### Reason
It should not be possible to ignore an error because that could leave the system or a computation in an undefined (or unexpected) state.
This is a major source of errors.
##### Example
int printf(const char* ...); // bad: return negative number if output fails
template<class F, class ...Args>
// good: throw system_error if unable to start the new thread
explicit thread(F&& f, Args&&... args);
##### Note
What is an error?
An error means that the function cannot achieve its advertised purpose (including establishing postconditions).
Calling code that ignores an error could lead to wrong results or undefined systems state.
For example, not being able to connect to a remote server is not by itself an error:
the server can refuse a connection for all kinds of reasons, so the natural thing is to return a result that the caller should always check.
However, if failing to make a connection is considered an error, then a failure should throw an exception.
##### Exception
Many traditional interface functions (e.g., UNIX signal handlers) use error codes (e.g., `errno`) to report what are really status codes, rather than errors. You don't have a good alternative to using such, so calling these does not violate the rule.
##### Alternative
If you can't use exceptions (e.g., because your code is full of old-style raw-pointer use or because there are hard-real-time constraints), consider using a style that returns a pair of values:
int val;
int error_code;
tie(val, error_code) = do_something();
if (error_code) {
// ... handle the error or exit ...
}
// ... use val ...
This style unfortunately leads to uninitialized variables.
Since C++17 the "structured bindings" feature can be used to initialize variables directly from the return value:
auto [val, error_code] = do_something();
if (error_code) {
// ... handle the error or exit ...
}
// ... use val ...
##### Note
We don't consider "performance" a valid reason not to use exceptions.
* Often, explicit error checking and handling consume as much time and space as exception handling.
* Often, cleaner code yields better performance with exceptions (simplifying the tracing of paths through the program and their optimization).
* A good rule for performance critical code is to move checking outside the [critical](#rper-critical) part of the code.
* In the longer term, more regular code gets better optimized.
* Always carefully [measure](#rper-measure) before making performance claims.
**See also**: [I.5](#ri-pre) and [I.7](#ri-post) for reporting precondition and postcondition violations.
##### Enforcement
* (Not enforceable) This is a philosophical guideline that is infeasible to check directly.
* Look for `errno`.
### <a name="ri-raw"></a>I.11: Never transfer ownership by a raw pointer (`T*`) or reference (`T&`)
##### Reason
If there is any doubt whether the caller or the callee owns an object, leaks or premature destruction will occur.
##### Example
Consider:
X* compute(args) // don't
{
X* res = new X{};
// ...
return res;
}
Who deletes the returned `X`? The problem would be harder to spot if `compute` returned a reference.
Consider returning the result by value (use move semantics if the result is large):
vector<double> compute(args) // good
{
vector<double> res(10000);
// ...
return res;
}
**Alternative**: [Pass ownership](#rr-smartptrparam) using a "smart pointer", such as `unique_ptr` (for exclusive ownership) and `shared_ptr` (for shared ownership).
However, that is less elegant and often less efficient than returning the object itself,
so use smart pointers only if reference semantics are needed.
**Alternative**: Sometimes older code can't be modified because of ABI compatibility requirements or lack of resources.
In that case, mark owning pointers using `owner` from the [guidelines support library](#gsl-guidelines-support-library):
owner<X*> compute(args) // It is now clear that ownership is transferred
{
owner<X*> res = new X{};
// ...
return res;
}
This tells analysis tools that `res` is an owner.
That is, its value must be `delete`d or transferred to another owner, as is done here by the `return`.
`owner` is used similarly in the implementation of resource handles.
##### Note
Every object passed as a raw pointer (or iterator) is assumed to be owned by the
caller, so that its lifetime is handled by the caller. Viewed another way:
ownership transferring APIs are relatively rare compared to pointer-passing APIs,
so the default is "no ownership transfer."
**See also**: [Argument passing](#rf-conventional), [use of smart pointer arguments](#rr-smartptrparam), and [value return](#rf-value-return).
##### Enforcement
* (Simple) Warn on `delete` of a raw pointer that is not an `owner<T>`. Suggest use of standard-library resource handle or use of `owner<T>`.
* (Simple) Warn on failure to either `reset` or explicitly `delete` an `owner` pointer on every code path.
* (Simple) Warn if the return value of `new` or a function call with an `owner` return value is assigned to a raw pointer or non-`owner` reference.
### <a name="ri-nullptr"></a>I.12: Declare a pointer that must not be null as `not_null`
##### Reason
To help avoid dereferencing `nullptr` errors.
To improve performance by avoiding redundant checks for `nullptr`.
##### Example
int length(const char* p); // it is not clear whether length(nullptr) is valid
length(nullptr); // OK?
int length(not_null<const char*> p); // better: we can assume that p cannot be nullptr
int length(const char* p); // we must assume that p can be nullptr
By stating the intent in source, implementers and tools can provide better diagnostics, such as finding some classes of errors through static analysis, and perform optimizations, such as removing branches and null tests.
##### Note
`not_null` is defined in the [guidelines support library](#gsl-guidelines-support-library).
##### Note
The assumption that the pointer to `char` pointed to a C-style string (a zero-terminated string of characters) was still implicit, and a potential source of confusion and errors. Use `czstring` in preference to `const char*`.
// we can assume that p cannot be nullptr
// we can assume that p points to a zero-terminated array of characters
int length(not_null<czstring> p);
Note: `length()` is, of course, `std::strlen()` in disguise.
##### Enforcement
* (Simple) ((Foundation)) If a function checks a pointer parameter against `nullptr` before access, on all control-flow paths, then warn it should be declared `not_null`.
* (Complex) If a function with pointer return value ensures it is not `nullptr` on all return paths, then warn the return type should be declared `not_null`.
### <a name="ri-array"></a>I.13: Do not pass an array as a single pointer
##### Reason
(pointer, size)-style interfaces are error-prone. Also, a plain pointer (to array) must rely on some convention to allow the callee to determine the size.
##### Example
Consider:
void copy_n(const T* p, T* q, int n); // copy from [p:p+n) to [q:q+n)
What if there are fewer than `n` elements in the array pointed to by `q`? Then, we overwrite some probably unrelated memory.
What if there are fewer than `n` elements in the array pointed to by `p`? Then, we read some probably unrelated memory.
Either is undefined behavior and a potentially very nasty bug.
##### Alternative
Consider using explicit spans:
void copy(span<const T> r, span<T> r2); // copy r to r2
##### Example, bad
Consider:
void draw(Shape* p, int n); // poor interface; poor code
Circle arr[10];
// ...
draw(arr, 10);
Passing `10` as the `n` argument might be a mistake: the most common convention is to assume `[0:n)` but that is nowhere stated. Worse is that the call of `draw()` compiled at all: there was an implicit conversion from array to pointer (array decay) and then another implicit conversion from `Circle` to `Shape`. There is no way that `draw()` can safely iterate through that array: it has no way of knowing the size of the elements.
**Alternative**: Use a support class that ensures that the number of elements is correct and prevents dangerous implicit conversions. For example:
void draw2(span<Circle>);
Circle arr[10];
// ...
draw2(span<Circle>(arr)); // deduce the number of elements
draw2(arr); // deduce the element type and array size
void draw3(span<Shape>);
draw3(arr); // error: cannot convert Circle[10] to span<Shape>
This `draw2()` passes the same amount of information to `draw()`, but makes the fact that it is supposed to be a range of `Circle`s explicit. See ???.
##### Exception
Use `zstring` and `czstring` to represent C-style, zero-terminated strings.
But when doing so, use `std::string_view` or `span<char>` from the [GSL](#gsl-guidelines-support-library) to prevent range errors.
##### Enforcement
* (Simple) ((Bounds)) Warn for any expression that would rely on implicit conversion of an array type to a pointer type. Allow exception for zstring/czstring pointer types.
* (Simple) ((Bounds)) Warn for any arithmetic operation on an expression of pointer type that results in a value of pointer type. Allow exception for zstring/czstring pointer types.
### <a name="ri-global-init"></a>I.22: Avoid complex initialization of global objects
##### Reason
Complex initialization can lead to undefined order of execution.
##### Example
// file1.c
extern const X x;
const Y y = f(x); // read x; write y
// file2.c
extern const Y y;
const X x = g(y); // read y; write x
Since `x` and `y` are in different translation units the order of calls to `f()` and `g()` is undefined;
one will access an uninitialized `const`.
This shows that the order-of-initialization problem for global (namespace scope) objects is not limited to global *variables*.
##### Note
Order of initialization problems become particularly difficult to handle in concurrent code.
It is usually best to avoid global (namespace scope) objects altogether.
##### Enforcement
* Flag initializers of globals that call non-`constexpr` functions
* Flag initializers of globals that access `extern` objects
### <a name="ri-nargs"></a>I.23: Keep the number of function arguments low
##### Reason
Having many arguments opens opportunities for confusion. Passing lots of arguments is often costly compared to alternatives.
##### Discussion
The two most common reasons why functions have too many parameters are:
1. *Missing an abstraction.*
There is an abstraction missing, so that a compound value is being
passed as individual elements instead of as a single object that enforces an invariant.
This not only expands the parameter list, but it leads to errors because the component values
are no longer protected by an enforced invariant.
2. *Violating "one function, one responsibility."*
The function is trying to do more than one job and should probably be refactored.
##### Example
The standard-library `merge()` is at the limit of what we can comfortably handle:
template<class InputIterator1, class InputIterator2, class OutputIterator, class Compare>
OutputIterator merge(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result, Compare comp);
Note that this is because of problem 1 above -- missing abstraction. Instead of passing a range (abstraction), STL passed iterator pairs (unencapsulated component values).
Here, we have four template arguments and six function arguments.
To simplify the most frequent and simplest uses, the comparison argument can be defaulted to `<`:
template<class InputIterator1, class InputIterator2, class OutputIterator>
OutputIterator merge(InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2,
OutputIterator result);
This doesn't reduce the total complexity, but it reduces the surface complexity presented to many users.
To really reduce the number of arguments, we need to bundle the arguments into higher-level abstractions:
template<class InputRange1, class InputRange2, class OutputIterator>
OutputIterator merge(InputRange1 r1, InputRange2 r2, OutputIterator result);
Grouping arguments into "bundles" is a general technique to reduce the number of arguments and to increase the opportunities for checking.
Alternatively, we could use a standard library concept to define the notion of three types that must be usable for merging:
template<class In1, class In2, class Out>
requires mergeable<In1, In2, Out>
Out merge(In1 r1, In2 r2, Out result);
##### Example
The safety Profiles recommend replacing
void f(int* some_ints, int some_ints_length); // BAD: C style, unsafe
with
void f(gsl::span<int> some_ints); // GOOD: safe, bounds-checked
Here, using an abstraction has safety and robustness benefits, and naturally also reduces the number of parameters.
##### Note
How many parameters are too many? Try to use fewer than four (4) parameters.
There are functions that are best expressed with four individual parameters, but not many.
**Alternative**: Use better abstraction: Group arguments into meaningful objects and pass the objects (by value or by reference).
**Alternative**: Use default arguments or overloads to allow the most common forms of calls to be done with fewer arguments.
##### Enforcement
* Warn when a function declares two iterators (including pointers) of the same type instead of a range or a view.
* (Not enforceable) This is a philosophical guideline that is infeasible to check directly.
### <a name="ri-unrelated"></a>I.24: Avoid adjacent parameters that can be invoked by the same arguments in either order with different meaning
##### Reason
Adjacent arguments of the same type are easily swapped by mistake.
##### Example, bad
Consider:
void copy_n(T* p, T* q, int n); // copy from [p:p + n) to [q:q + n)
This is a nasty variant of a K&R C-style interface. It is easy to reverse the "to" and "from" arguments.
Use `const` for the "from" argument:
void copy_n(const T* p, T* q, int n); // copy from [p:p + n) to [q:q + n)
##### Exception
If the order of the parameters is not important, there is no problem:
int max(int a, int b);
##### Alternative
Don't pass arrays as pointers, pass an object representing a range (e.g., a `span`):
void copy_n(span<const T> p, span<T> q); // copy from p to q
##### Alternative
Define a `struct` as the parameter type and name the fields for those parameters accordingly:
struct SystemParams {
string config_file;
string output_path;
seconds timeout;
};
void initialize(SystemParams p);
This tends to make invocations of this clear to future readers, as the parameters
are often filled in by name at the call site.
##### Note
Only the interface's designer can adequately address the source of violations of this guideline.
##### Enforcement strategy
(Simple) Warn if two consecutive parameters share the same type.
We are still looking for a less-simple enforcement.
### <a name="ri-abstract"></a>I.25: Prefer empty abstract classes as interfaces to class hierarchies
##### Reason
Abstract classes that are empty (have no non-static member data) are more likely to be stable than base classes with state.
##### Example, bad
You just knew that `Shape` would turn up somewhere :-)
class Shape { // bad: interface class loaded with data
public:
Point center() const { return c; }
virtual void draw() const;
virtual void rotate(int);
// ...
private:
Point c;
vector<Point> outline;
Color col;
};
This will force every derived class to compute a center -- even if that's non-trivial and the center is never used. Similarly, not every `Shape` has a `Color`, and many `Shape`s are best represented without an outline defined as a sequence of `Point`s. Using an abstract class is better:
class Shape { // better: Shape is a pure interface
public:
virtual Point center() const = 0; // pure virtual functions
virtual void draw() const = 0;
virtual void rotate(int) = 0;
// ...
// ... no data members ...
// ...
virtual ~Shape() = default;
};
##### Enforcement
(Simple) Warn if a pointer/reference to a class `C` is assigned to a pointer/reference to a base of `C` and the base class contains data members.
### <a name="ri-abi"></a>I.26: If you want a cross-compiler ABI, use a C-style subset
##### Reason
Different compilers implement different binary layouts for classes, exception handling, function names, and other implementation details.
##### Exception
Common ABIs are emerging on some platforms freeing you from the more draconian restrictions.
##### Note
If you use a single compiler, you can use full C++ in interfaces. That might require recompilation after an upgrade to a new compiler version.
##### Enforcement
(Not enforceable) It is difficult to reliably identify where an interface forms part of an ABI.
### <a name="ri-pimpl"></a>I.27: For stable library ABI, consider the Pimpl idiom
##### Reason
Because private data members participate in class layout and private member functions participate in overload resolution, changes to those
implementation details require recompilation of all users of a class that uses them. A non-polymorphic interface class holding a pointer to
implementation (Pimpl) can isolate the users of a class from changes in its implementation at the cost of an indirection.
##### Example
interface (widget.h)
class widget {
class impl;
std::unique_ptr<impl> pimpl;
public:
void draw(); // public API that will be forwarded to the implementation
widget(int); // defined in the implementation file
~widget(); // defined in the implementation file, where impl is a complete type
widget(widget&&) noexcept; // defined in the implementation file
widget(const widget&) = delete;
widget& operator=(widget&&) noexcept; // defined in the implementation file
widget& operator=(const widget&) = delete;
};
implementation (widget.cpp)
class widget::impl {
int n; // private data
public:
void draw(const widget& w) { /* ... */ }
impl(int n) : n(n) {}
};
void widget::draw() { pimpl->draw(*this); }
widget::widget(int n) : pimpl{std::make_unique<impl>(n)} {}
widget::widget(widget&&) noexcept = default;
widget::~widget() = default;
widget& widget::operator=(widget&&) noexcept = default;
##### Notes
See [GOTW #100](https://herbsutter.com/gotw/_100/) and [cppreference](https://en.cppreference.com/w/cpp/language/pimpl) for the trade-offs and additional implementation details associated with this idiom.
##### Enforcement
(Not enforceable) It is difficult to reliably identify where an interface forms part of an ABI.
### <a name="ri-encapsulate"></a>I.30: Encapsulate rule violations
##### Reason
To keep code simple and safe.
Sometimes, ugly, unsafe, or error-prone techniques are necessary for logical or performance reasons.
If so, keep them local, rather than "infecting" interfaces so that larger groups of programmers have to be aware of the
subtleties.
Implementation complexity should, if at all possible, not leak through interfaces into user code.
##### Example
Consider a program that, depending on some form of input (e.g., arguments to `main`), should consume input
from a file, from the command line, or from standard input.
We might write
bool owned;
owner<istream*> inp;
switch (source) {
case std_in: owned = false; inp = &cin; break;
case command_line: owned = true; inp = new istringstream{argv[2]}; break;
case file: owned = true; inp = new ifstream{argv[2]}; break;
}
istream& in = *inp;
This violated the rule [against uninitialized variables](#res-always),
the rule against [ignoring ownership](#ri-raw),
and the rule [against magic constants](#res-magic).
In particular, someone has to remember to somewhere write
if (owned) delete inp;
We could handle this particular example by using `unique_ptr` with a special deleter that does nothing for `cin`,
but that's complicated for novices (who can easily encounter this problem) and the example is an example of a more general
problem where a property that we would like to consider static (here, ownership) needs infrequently be addressed
at run time.
The common, most frequent, and safest examples can be handled statically, so we don't want to add cost and complexity to those.
But we must also cope with the uncommon, less-safe, and necessarily more expensive cases.
Such examples are discussed in [[Str15]](https://www.stroustrup.com/resource-model.pdf).
So, we write a class
class Istream { [[gsl::suppress("lifetime")]]
public:
enum Opt { from_line = 1 };
Istream() { }
Istream(czstring p) : owned{true}, inp{new ifstream{p}} {} // read from file
Istream(czstring p, Opt) : owned{true}, inp{new istringstream{p}} {} // read from command line
~Istream() { if (owned) delete inp; }
operator istream&() { return *inp; }
private:
bool owned = false;
istream* inp = &cin;
};
Now, the dynamic nature of `istream` ownership has been encapsulated.
Presumably, a bit of checking for potential errors would be added in real code.
##### Enforcement
* Hard, it is hard to decide what rule-breaking code is essential
* Flag rule suppression that enable rule-violations to cross interfaces
# <a name="s-functions"></a>F: Functions
A function specifies an action or a computation that takes the system from one consistent state to the next. It is the fundamental building block of programs.
It should be possible to name a function meaningfully, to specify the requirements of its argument, and clearly state the relationship between the arguments and the result. An implementation is not a specification. Try to think about what a function does as well as about how it does it.
Functions are the most critical part in most interfaces, so see the interface rules.
Function rule summary:
Function definition rules:
* [F.1: "Package" meaningful operations as carefully named functions](#rf-package)
* [F.2: A function should perform a single logical operation](#rf-logical)
* [F.3: Keep functions short and simple](#rf-single)
* [F.4: If a function might have to be evaluated at compile time, declare it `constexpr`](#rf-constexpr)
* [F.5: If a function is very small and time-critical, declare it inline](#rf-inline)
* [F.6: If your function must not throw, declare it `noexcept`](#rf-noexcept)
* [F.7: For general use, take `T*` or `T&` arguments rather than smart pointers](#rf-smart)
* [F.8: Prefer pure functions](#rf-pure)
* [F.9: Unused parameters should be unnamed](#rf-unused)
* [F.10: If an operation can be reused, give it a name](#rf-name)
* [F.11: Use an unnamed lambda if you need a simple function object in one place only](#rf-lambda)
Parameter passing expression rules:
* [F.15: Prefer simple and conventional ways of passing information](#rf-conventional)
* [F.16: For "in" parameters, pass cheaply-copied types by value and others by reference to `const`](#rf-in)
* [F.17: For "in-out" parameters, pass by reference to non-`const`](#rf-inout)
* [F.18: For "will-move-from" parameters, pass by `X&&` and `std::move` the parameter](#rf-consume)
* [F.19: For "forward" parameters, pass by `TP&&` and only `std::forward` the parameter](#rf-forward)
* [F.20: For "out" output values, prefer return values to output parameters](#rf-out)
* [F.21: To return multiple "out" values, prefer returning a struct](#rf-out-multi)
* [F.60: Prefer `T*` over `T&` when "no argument" is a valid option](#rf-ptr-ref)
Parameter passing semantic rules:
* [F.22: Use `T*` or `owner<T*>` to designate a single object](#rf-ptr)
* [F.23: Use a `not_null<T>` to indicate that "null" is not a valid value](#rf-nullptr)
* [F.24: Use a `span<T>` or a `span_p<T>` to designate a half-open sequence](#rf-range)
* [F.25: Use a `zstring` or a `not_null<zstring>` to designate a C-style string](#rf-zstring)
* [F.26: Use a `unique_ptr<T>` to transfer ownership where a pointer is needed](#rf-unique_ptr)
* [F.27: Use a `shared_ptr<T>` to share ownership](#rf-shared_ptr)
<a name="rf-value-return"></a>Value return semantic rules:
* [F.42: Return a `T*` to indicate a position (only)](#rf-return-ptr)
* [F.43: Never (directly or indirectly) return a pointer or a reference to a local object](#rf-dangle)
* [F.44: Return a `T&` when copy is undesirable and "returning no object" isn't needed](#rf-return-ref)
* [F.45: Don't return a `T&&`](#rf-return-ref-ref)
* [F.46: `int` is the return type for `main()`](#rf-main)
* [F.47: Return `T&` from assignment operators](#rf-assignment-op)
* [F.48: Don't return `std::move(local)`](#rf-return-move-local)
* [F.49: Don't return `const T`](#rf-return-const)
Other function rules:
* [F.50: Use a lambda when a function won't do (to capture local variables, or to write a local function)](#rf-capture-vs-overload)
* [F.51: Where there is a choice, prefer default arguments over overloading](#rf-default-args)
* [F.52: Prefer capturing by reference in lambdas that will be used locally, including passed to algorithms](#rf-reference-capture)
* [F.53: Avoid capturing by reference in lambdas that will be used non-locally, including returned, stored on the heap, or passed to another thread](#rf-value-capture)
* [F.54: When writing a lambda that captures `this` or any class data member, don't use `[=]` default capture](#rf-this-capture)
* [F.55: Don't use `va_arg` arguments](#f-varargs)
* [F.56: Avoid unnecessary condition nesting](#f-nesting)
Functions have strong similarities to lambdas and function objects.
**See also**: [C.lambdas: Function objects and lambdas](#ss-lambdas)
## <a name="ss-fct-def"></a>F.def: Function definitions
A function definition is a function declaration that also specifies the function's implementation, the function body.
### <a name="rf-package"></a>F.1: "Package" meaningful operations as carefully named functions
##### Reason
Factoring out common code makes code more readable, more likely to be reused, and limit errors from complex code.
If something is a well-specified action, separate it out from its surrounding code and give it a name.
##### Example, don't
void read_and_print(istream& is) // read and print an int
{
int x;
if (is >> x)
cout << "the int is " << x << '\n';
else
cerr << "no int on input\n";
}
Almost everything is wrong with `read_and_print`.
It reads, it writes (to a fixed `ostream`), it writes error messages (to a fixed `ostream`), it handles only `int`s.
There is nothing to reuse, logically separate operations are intermingled and local variables are in scope after the end of their logical use.
For a tiny example, this looks OK, but if the input operation, the output operation, and the error handling had been more complicated the tangled
mess could become hard to understand.
##### Note
If you write a non-trivial lambda that potentially can be used in more than one place, give it a name by assigning it to a (usually non-local) variable.
##### Example
sort(a, b, [](T x, T y) { return x.rank() < y.rank() && x.value() < y.value(); });
Naming that lambda breaks up the expression into its logical parts and provides a strong hint to the meaning of the lambda.
auto lessT = [](T x, T y) { return x.rank() < y.rank() && x.value() < y.value(); };
sort(a, b, lessT);
The shortest code is not always the best for performance or maintainability.
##### Exception
Loop bodies, including lambdas used as loop bodies, rarely need to be named.
However, large loop bodies (e.g., dozens of lines or dozens of pages) can be a problem.
The rule [Keep functions short and simple](#rf-single) implies "Keep loop bodies short."
Similarly, lambdas used as callback arguments are sometimes non-trivial, yet unlikely to be reusable.
##### Enforcement
* See [Keep functions short and simple](#rf-single)
* Flag identical and very similar lambdas used in different places.
### <a name="rf-logical"></a>F.2: A function should perform a single logical operation
##### Reason
A function that performs a single operation is simpler to understand, test, and reuse.
##### Example
Consider:
void read_and_print() // bad
{
int x;
cin >> x;
// check for errors
cout << x << "\n";
}
This is a monolith that is tied to a specific input and will never find another (different) use. Instead, break functions up into suitable logical parts and parameterize:
int read(istream& is) // better
{
int x;
is >> x;
// check for errors
return x;
}
void print(ostream& os, int x)
{
os << x << "\n";
}
These can now be combined where needed:
void read_and_print()
{
auto x = read(cin);
print(cout, x);
}
If there was a need, we could further templatize `read()` and `print()` on the data type, the I/O mechanism, the response to errors, etc. Example:
auto read = [](auto& input, auto& value) // better
{
input >> value;
// check for errors
};
void print(auto& output, const auto& value)
{
output << value << "\n";
}
##### Enforcement
* Consider functions with more than one "out" parameter suspicious. Use return values instead, including `tuple` for multiple return values.
* Consider "large" functions that don't fit on one editor screen suspicious. Consider factoring such a function into smaller well-named suboperations.
* Consider functions with 7 or more parameters suspicious.
### <a name="rf-single"></a>F.3: Keep functions short and simple
##### Reason
Large functions are hard to read, more likely to contain complex code, and more likely to have variables in larger than minimal scopes.
Functions with complex control structures are more likely to be long and more likely to hide logical errors
##### Example
Consider:
double simple_func(double val, int flag1, int flag2)
// simple_func: takes a value and calculates the expected ASIC output,
// given the two mode flags.
{
double intermediate;
if (flag1 > 0) {
intermediate = func1(val);
if (flag2 % 2)
intermediate = sqrt(intermediate);
}
else if (flag1 == -1) {
intermediate = func1(-val);
if (flag2 % 2)
intermediate = sqrt(-intermediate);
flag1 = -flag1;
}
if (abs(flag2) > 10) {
intermediate = func2(intermediate);
}
switch (flag2 / 10) {
case 1: if (flag1 == -1) return finalize(intermediate, 1.171);
break;
case 2: return finalize(intermediate, 13.1);
default: break;
}
return finalize(intermediate, 0.);
}
This is too complex.
How would you know if all possible alternatives have been correctly handled?
Yes, it breaks other rules also.
We can refactor:
double func1_muon(double val, int flag)
{
// ???
}
double func1_tau(double val, int flag1, int flag2)
{
// ???
}
double simple_func(double val, int flag1, int flag2)
// simple_func: takes a value and calculates the expected ASIC output,
// given the two mode flags.
{
if (flag1 > 0)
return func1_muon(val, flag2);
if (flag1 == -1)
// handled by func1_tau: flag1 = -flag1;
return func1_tau(-val, flag1, flag2);
return 0.;
}
##### Note
"It doesn't fit on a screen" is often a good practical definition of "far too large."
One-to-five-line functions should be considered normal.
##### Note
Break large functions up into smaller cohesive and named functions.
Small simple functions are easily inlined where the cost of a function call is significant.
##### Enforcement
* Flag functions that do not "fit on a screen."
How big is a screen? Try 60 lines by 140 characters; that's roughly the maximum that's comfortable for a book page.
* Flag functions that are too complex. How complex is too complex?
You could use cyclomatic complexity. Try "more than 10 logical paths through." Count a simple switch as one path.
### <a name="rf-constexpr"></a>F.4: If a function might have to be evaluated at compile time, declare it `constexpr`
##### Reason
`constexpr` is needed to tell the compiler to allow compile-time evaluation.
##### Example
The (in)famous factorial:
constexpr int fac(int n)
{
constexpr int max_exp = 17; // constexpr enables max_exp to be used in Expects
Expects(0 <= n && n < max_exp); // prevent silliness and overflow
int x = 1;
for (int i = 2; i <= n; ++i) x *= i;
return x;
}
This is C++14.
For C++11, use a recursive formulation of `fac()`.
##### Note
`constexpr` does not guarantee compile-time evaluation;
it just guarantees that the function can be evaluated at compile time for constant expression arguments if the programmer requires it or the compiler decides to do so to optimize.
constexpr int min(int x, int y) { return x < y ? x : y; }
void test(int v)
{
int m1 = min(-1, 2); // probably compile-time evaluation
constexpr int m2 = min(-1, 2); // compile-time evaluation
int m3 = min(-1, v); // run-time evaluation
constexpr int m4 = min(-1, v); // error: cannot evaluate at compile time
}
##### Note
Don't try to make all functions `constexpr`.
Most computation is best done at run time.
##### Note
Any API that might eventually depend on high-level run-time configuration or
business logic should not be made `constexpr`. Such customization can not be
evaluated by the compiler, and any `constexpr` functions that depended upon
that API would have to be refactored or drop `constexpr`.
##### Enforcement
Impossible and unnecessary.
The compiler gives an error if a non-`constexpr` function is called where a constant is required.
### <a name="rf-inline"></a>F.5: If a function is very small and time-critical, declare it `inline`
##### Reason
Some optimizers are good at inlining without hints from the programmer, but don't rely on it.
Measure! Over the last 40 years or so, we have been promised compilers that can inline better than humans without hints from humans.
We are still waiting.
Specifying inline (explicitly, or implicitly when writing member functions inside a class definition) encourages the compiler to do a better job.
##### Example
inline string cat(const string& s, const string& s2) { return s + s2; }
##### Exception
Do not put an `inline` function in what is meant to be a stable interface unless you are certain that it will not change.
An inline function is part of the ABI.
##### Note
`constexpr` implies `inline`.
##### Note
Member functions defined in-class are `inline` by default.
##### Exception
Function templates (including member functions of class templates `A<T>::function()` and member function templates `A::function<T>()`) are normally defined in headers and therefore inline.
##### Note
Consider making functions out of line if they are more than three statements and can be declared out of line (such as class member functions).
### <a name="rf-noexcept"></a>F.6: If your function must not throw, declare it `noexcept`
##### Reason
If an exception is not supposed to be thrown, the program cannot be assumed to cope with the error and should be terminated as soon as possible. Declaring a function `noexcept` helps optimizers by reducing the number of alternative execution paths. It also speeds up the exit after failure.
##### Example
Put `noexcept` on every function written completely in C or in any other language without exceptions.
The C++ Standard Library does that implicitly for all functions in the C Standard Library.
##### Note
`constexpr` functions can throw when evaluated at run time, so you might need conditional `noexcept` for some of those.
##### Example
You can use `noexcept` even on functions that can throw:
vector<string> collect(istream& is) noexcept
{
vector<string> res;
for (string s; is >> s;)
res.push_back(s);
return res;
}
If `collect()` runs out of memory, the program crashes.
Unless the program is crafted to survive memory exhaustion, that might be just the right thing to do;
`terminate()` might generate suitable error log information (but after memory runs out it is hard to do anything clever).
##### Note
You must be aware of the execution environment that your code is running when
deciding whether to tag a function `noexcept`, especially because of the issue
of throwing and allocation. Code that is intended to be perfectly general (like
the standard library and other utility code of that sort) needs to support
environments where a `bad_alloc` exception could be handled meaningfully.
However, most programs and execution environments cannot meaningfully
handle a failure to allocate, and aborting the program is the cleanest and
simplest response to an allocation failure in those cases. If you know that
your application code cannot respond to an allocation failure, it could be
appropriate to add `noexcept` even on functions that allocate.
Put another way: In most programs, most functions can throw (e.g., because they
use `new`, call functions that do, or use library functions that report failure
by throwing), so don't just sprinkle `noexcept` all over the place without
considering whether the possible exceptions can be handled.
`noexcept` is most useful (and most clearly correct) for frequently used,
low-level functions.
##### Note
Destructors, `swap` functions, move operations, and default constructors should never throw.
See also [C.44](#rc-default00).
##### Note
Care must be taken on base virtual functions and functions part of a public interface because declaring a function `noexcept` is establishing a guarantee that all current and future implementations must abide by. For virtual function, all overriders must also be `noexcept` and removing `noexcept` from a function could break calling functions.
##### Enforcement
* (hard) Flag low-level functions that are not `noexcept`, yet cannot throw.
* Flag throwing `swap`, `move`, destructors, and default constructors.
### <a name="rf-smart"></a>F.7: For general use, take `T*` or `T&` arguments rather than smart pointers
##### Reason
Passing a smart pointer transfers or shares ownership and should only be used when ownership semantics are intended.
A function that does not manipulate lifetime should take raw pointers or references instead.
Passing by smart pointer restricts the use of a function to callers that use smart pointers.
A function that needs a `widget` should be able to accept any `widget` object, not just ones whose lifetimes are managed by a particular kind of smart pointer.
Passing a shared smart pointer (e.g., `std::shared_ptr`) implies a run-time cost.
##### Example
// accepts any int*
void f(int*);
// can only accept ints for which you want to transfer ownership
void g(unique_ptr<int>);
// can only accept ints for which you are willing to share ownership
void g(shared_ptr<int>);
// doesn't change ownership, but requires a particular ownership of the caller
void h(const unique_ptr<int>&);
// accepts any int
void h(int&);
##### Example, bad
// callee
void f(shared_ptr<widget>& w)
{
// ...
use(*w); // only use of w -- the lifetime is not used at all
// ...
};
// caller
shared_ptr<widget> my_widget = /* ... */;
f(my_widget);
widget stack_widget;
f(stack_widget); // error
##### Example, good
// callee
void f(widget& w)
{
// ...
use(w);
// ...
};
// caller
shared_ptr<widget> my_widget = /* ... */;
f(*my_widget);
widget stack_widget;
f(stack_widget); // ok -- now this works
##### Note
We can catch many common cases of dangling pointers statically (see [lifetime safety profile](#ss-lifetime)). Function arguments naturally live for the lifetime of the function call, and so have fewer lifetime problems.
##### Enforcement
* (Simple) Warn if a function takes a parameter of a smart pointer type (that overloads `operator->` or `operator*`) that is copyable but the function only calls any of: `operator*`, `operator->` or `get()`.
Suggest using a `T*` or `T&` instead.
* Flag a parameter of a smart pointer type (a type that overloads `operator->` or `operator*`) that is copyable/movable but never copied/moved from in the function body, and that is never modified, and that is not passed along to another function that could do so. That means the ownership semantics are not used.
Suggest using a `T*` or `T&` instead.
**See also**:
* [Prefer `T*` over `T&` when "no argument" is a valid option](#rf-ptr-ref)
* [Smart pointer rule summary](#rr-summary-smartptrs)
### <a name="rf-pure"></a>F.8: Prefer pure functions
##### Reason
Pure functions are easier to reason about, sometimes easier to optimize (and even parallelize), and sometimes can be memoized.
##### Example
template<class T>
auto square(T t) { return t * t; }
##### Enforcement
Not possible.
### <a name="rf-unused"></a>F.9: Unused parameters should be unnamed
##### Reason
Readability.
Suppression of unused parameter warnings.
##### Example
widget* find(const set<widget>& s, const widget& w, Hint); // once upon a time, a hint was used
##### Note
Allowing parameters to be unnamed was introduced in the early 1980s to address this problem.
If parameters are conditionally unused, declare them with the `[[maybe_unused]]` attribute.
For example:
template <typename Value>
Value* find(const set<Value>& s, const Value& v, [[maybe_unused]] Hint h)
{
if constexpr (sizeof(Value) > CacheSize)
{
// a hint is used only if Value is of a certain size
}
}
##### Enforcement
Flag named unused parameters.
### <a name="rf-name"></a>F.10: If an operation can be reused, give it a name
##### Reason
Documentation, readability, opportunity for reuse.
##### Example
struct Rec {
string name;
string addr;
int id; // unique identifier
};
bool same(const Rec& a, const Rec& b)
{
return a.id == b.id;
}
vector<Rec*> find_id(const string& name); // find all records for "name"
auto x = find_if(vr.begin(), vr.end(),
[&](Rec& r) {
if (r.name.size() != n.size()) return false; // name to compare to is in n
for (int i = 0; i < r.name.size(); ++i)
if (tolower(r.name[i]) != tolower(n[i])) return false;
return true;
}
);
There is a useful function lurking here (case insensitive string comparison), as there often is when lambda arguments get large.
bool compare_insensitive(const string& a, const string& b)
{
if (a.size() != b.size()) return false;
for (int i = 0; i < a.size(); ++i) if (tolower(a[i]) != tolower(b[i])) return false;
return true;
}
auto x = find_if(vr.begin(), vr.end(),
[&](Rec& r) { return compare_insensitive(r.name, n); }
);
Or maybe (if you prefer to avoid the implicit name binding to n):
auto cmp_to_n = [&n](const string& a) { return compare_insensitive(a, n); };
auto x = find_if(vr.begin(), vr.end(),
[](const Rec& r) { return cmp_to_n(r.name); }
);
##### Note
whether functions, lambdas, or operators.
##### Exception
* Lambdas logically used only locally, such as an argument to `for_each` and similar control flow algorithms.
* Lambdas as [initializers](#???)
##### Enforcement
* (hard) flag similar lambdas
* ???
### <a name="rf-lambda"></a>F.11: Use an unnamed lambda if you need a simple function object in one place only
##### Reason
That makes the code concise and gives better locality than alternatives.
##### Example
auto earlyUsersEnd = std::remove_if(users.begin(), users.end(),
[](const User &a) { return a.id > 100; });
##### Exception
Naming a lambda can be useful for clarity even if it is used only once.
##### Enforcement
* Look for identical and near identical lambdas (to be replaced with named functions or named lambdas).
## <a name="ss-call"></a>F.call: Parameter passing
There are a variety of ways to pass parameters to a function and to return values.
### <a name="rf-conventional"></a>F.15: Prefer simple and conventional ways of passing information
##### Reason
Using "unusual and clever" techniques causes surprises, slows understanding by other programmers, and encourages bugs.
If you really feel the need for an optimization beyond the common techniques, measure to ensure that it really is an improvement, and document/comment because the improvement might not be portable.
The following tables summarize the advice in the following Guidelines, F.16-21.
Normal parameter passing:
Advanced parameter passing:
Use the advanced techniques only after demonstrating need, and document that need in a comment.
For passing sequences of characters see [String](#ss-string).
##### Exception
To express shared ownership using `shared_ptr` types, rather than following guidelines F.16-21,
follow [R.34](#rr-sharedptrparam-owner), [R.35](#rr-sharedptrparam), and [R.36](#rr-sharedptrparam-const).
### <a name="rf-in"></a>F.16: For "in" parameters, pass cheaply-copied types by value and others by reference to `const`
##### Reason
Both let the caller know that a function will not modify the argument, and both allow initialization by rvalues.
What is "cheap to copy" depends on the machine architecture, but two or three words (doubles, pointers, references) are usually best passed by value.
When copying is cheap, nothing beats the simplicity and safety of copying, and for small objects (up to two or three words) it is also faster than passing by reference because it does not require an extra indirection to access from the function.
##### Example
void f1(const string& s); // OK: pass by reference to const; always cheap
void f2(string s); // bad: potentially expensive
void f3(int x); // OK: Unbeatable
void f4(const int& x); // bad: overhead on access in f4()
For advanced uses (only), where you really need to optimize for rvalues passed to "input-only" parameters:
* If the function is going to unconditionally move from the argument, take it by `&&`. See [F.18](#rf-consume).
* If the function is going to keep a locally modifiable copy of the argument only for its own local use, taking it by value is fine.
* If the function is going to keep a copy of the argument to pass to another destination (to another function, or store in a non-local location), in addition to passing by `const&` (for lvalues),
add an overload that passes the parameter by `&&` (for rvalues) and in the body `std::move`s it to its destination. Essentially this overloads a "will-move-from"; see [F.18](#rf-consume).
* In special cases, such as multiple "input + copy" parameters, consider using perfect forwarding. See [F.19](#rf-forward).
##### Example
int multiply(int, int); // just input ints, pass by value
// suffix is input-only but not as cheap as an int, pass by const&
string& concatenate(string&, const string& suffix);
void sink(unique_ptr<widget>); // input only, and moves ownership of the widget
Avoid "esoteric techniques" such as passing arguments as `T&&` "for efficiency".
Most rumors about performance advantages from passing by `&&` are false or brittle (but see [F.18](#rf-consume) and [F.19](#rf-forward)).
##### Notes
A reference can be assumed to refer to a valid object (language rule).
There is no (legitimate) "null reference."
If you need the notion of an optional value, use a pointer, `std::optional`, or a special value used to denote "no value."
##### Enforcement
* (Simple) ((Foundation)) Warn when a parameter being passed by value has a size greater than `2 * sizeof(void*)`.
Suggest using a reference to `const` instead.
* (Simple) ((Foundation)) Warn when a parameter passed by reference to `const` has a size less or equal than `2 * sizeof(void*)`. Suggest passing by value instead.
* (Simple) ((Foundation)) Warn when a parameter passed by reference to `const` is `move`d.
##### Exception
To express shared ownership using `shared_ptr` types, follow [R.34](#rr-sharedptrparam-owner) or [R.36](#rr-sharedptrparam-const),
depending on whether or not the function unconditionally takes a reference to the argument.
### <a name="rf-inout"></a>F.17: For "in-out" parameters, pass by reference to non-`const`
##### Reason
This makes it clear to callers that the object is assumed to be modified.
##### Example
void update(Record& r); // assume that update writes to r
##### Note
Some user-defined and standard library types, such as `span<T>` or the iterators
are [cheap to copy](#rf-in) and may be passed by value, while doing so has
mutable (in-out) reference semantics:
void increment_all(span<int> a)
{
for (auto&& e : a)
++e;
}
##### Note
A `T&` argument can pass information into a function as well as out of it.
Thus `T&` could be an in-out-parameter. That can in itself be a problem and a source of errors:
void f(string& s)
{
s = "New York"; // non-obvious error
}
void g()
{
string buffer = ".................................";
f(buffer);
// ...
}
Here, the writer of `g()` is supplying a buffer for `f()` to fill, but `f()` simply replaces it (at a somewhat higher cost than a simple copy of the characters).
A bad logic error can happen if the writer of `g()` incorrectly assumes the size of the `buffer`.
##### Enforcement
* (Moderate) ((Foundation)) Warn about functions regarding reference to non-`const` parameters that do *not* write to them.
* (Simple) ((Foundation)) Warn when a non-`const` parameter being passed by reference is `move`d.
### <a name="rf-consume"></a>F.18: For "will-move-from" parameters, pass by `X&&` and `std::move` the parameter
##### Reason
It's efficient and eliminates bugs at the call site: `X&&` binds to rvalues, which requires an explicit `std::move` at the call site if passing an lvalue.
##### Example
void sink(vector<int>&& v) // sink takes ownership of whatever the argument owned
{
// usually there might be const accesses of v here
store_somewhere(std::move(v));
// usually no more use of v here; it is moved-from
}
Note that the `std::move(v)` makes it possible for `store_somewhere()` to leave `v` in a moved-from state.
[That could be dangerous](#rc-move-semantic).
##### Exception
Unique owner types that are move-only and cheap-to-move, such as `unique_ptr`, can also be passed by value which is simpler to write and achieves the same effect. Passing by value does generate one extra (cheap) move operation, but prefer simplicity and clarity first.
For example:
template<class T>
void sink(std::unique_ptr<T> p)
{
// use p ... possibly std::move(p) onward somewhere else
} // p gets destroyed
##### Exception
If the "will-move-from" parameter is a `shared_ptr` follow [R.34](#rr-sharedptrparam-owner) and pass the `shared_ptr` by value.
##### Enforcement
* Flag all `X&&` parameters (where `X` is not a template type parameter name) where the function body uses them without `std::move`.
* Flag access to moved-from objects.
* Don't conditionally move from objects
### <a name="rf-forward"></a>F.19: For "forward" parameters, pass by `TP&&` and only `std::forward` the parameter
##### Reason
If the object is to be passed onward to other code and not directly used by this function, we want to make this function agnostic to the argument `const`-ness and rvalue-ness.
In that case, and only that case, make the parameter `TP&&` where `TP` is a template type parameter -- it both *ignores* and *preserves* `const`-ness and rvalue-ness. Therefore any code that uses a `TP&&` is implicitly declaring that it itself doesn't care about the variable's `const`-ness and rvalue-ness (because it is ignored), but that intends to pass the value onward to other code that does care about `const`-ness and rvalue-ness (because it is preserved). When used as a parameter `TP&&` is safe because any temporary objects passed from the caller will live for the duration of the function call. A parameter of type `TP&&` should essentially always be passed onward via `std::forward` in the body of the function.
##### Example
Usually you forward the entire parameter (or parameter pack, using `...`) exactly once on every static control flow path:
template<class F, class... Args>
inline decltype(auto) invoke(F&& f, Args&&... args)
{
return forward<F>(f)(forward<Args>(args)...);
}
##### Example
Sometimes you may forward a composite parameter piecewise, each subobject once on every static control flow path:
template<class PairLike>
inline auto test(PairLike&& pairlike)
{
// ...
f1(some, args, and, forward<PairLike>(pairlike).first); // forward .first
f2(and, forward<PairLike>(pairlike).second, in, another, call); // forward .second
}
##### Enforcement
* Flag a function that takes a `TP&&` parameter (where `TP` is a template type parameter name) and does anything with it other than `std::forward`ing it exactly once on every static path, or `std::forward`ing it more than once but qualified with a different data member exactly once on every static path.
### <a name="rf-out"></a>F.20: For "out" output values, prefer return values to output parameters
##### Reason
A return value is self-documenting, whereas an `&` could be either in-out or out-only and is liable to be misused.
This includes large objects like standard containers that use implicit move operations for performance and to avoid explicit memory management.
If you have multiple values to return, [use a tuple](#rf-out-multi) or similar multi-member type.
##### Example
// OK: return pointers to elements with the value x
vector<const int*> find_all(const vector<int>&, int x);
// Bad: place pointers to elements with value x in-out
void find_all(const vector<int>&, vector<const int*>& out, int x);
##### Note
A `struct` of many (individually cheap-to-move) elements might be in aggregate expensive to move.
##### Exceptions
* For non-concrete types, such as types in an inheritance hierarchy, return the object by `unique_ptr` or `shared_ptr`.
* If a type is expensive to move (e.g., `array<BigTrivial>`), consider allocating it on the free store and return a handle (e.g., `unique_ptr`), or passing it in a reference to non-`const` target object to fill (to be used as an out-parameter).
* To reuse an object that carries capacity (e.g., `std::string`, `std::vector`) across multiple calls to the function in an inner loop: [treat it as an in/out parameter and pass by reference](#rf-out-multi).
##### Example
Assuming that `Matrix` has move operations (possibly by keeping its elements in a `std::vector`):
Matrix operator+(const Matrix& a, const Matrix& b)
{
Matrix res;
// ... fill res with the sum ...
return res;
}
Matrix x = m1 + m2; // move constructor
y = m3 + m3; // move assignment
##### Note
The return value optimization doesn't handle the assignment case, but the move assignment does.
##### Example
struct Package { // exceptional case: expensive-to-move object
char header[16];
char load[2024 - 16];
};
Package fill(); // Bad: large return value
void fill(Package&); // OK
int val(); // OK
void val(int&); // Bad: Is val reading its argument
##### Enforcement
* Flag reference to non-`const` parameters that are not read before being written to and are a type that could be cheaply returned; they should be "out" return values.
### <a name="rf-out-multi"></a>F.21: To return multiple "out" values, prefer returning a struct
##### Reason
A return value is self-documenting as an "output-only" value.
Note that C++ does have multiple return values, by convention of using tuple-like types (`struct`, `array`, `tuple`, etc.),
possibly with the extra convenience of structured bindings (C++17) at the call site.
Prefer using a named `struct` if possible.
Otherwise, a `tuple` is useful in variadic templates.
##### Example
// BAD: output-only parameter documented in a comment
int f(const string& input, /*output only*/ string& output_data)
{
// ...
output_data = something();
return status;
}
// GOOD: self-documenting
struct f_result { int status; string data; };
f_result f(const string& input)
{
// ...
return {status, something()};
}
C++98's standard library used this style in places, by returning `pair` in some functions.
For example, given a `set<string> my_set`, consider:
// C++98
pair<set::iterator, bool> result = my_set.insert("Hello");
if (result.second)
do_something_with(result.first); // workaround
With C++17 we are able to use "structured bindings" to give each member a name:
if (auto [ iter, success ] = my_set.insert("Hello"); success)
do_something_with(iter);
A `struct` with meaningful names is more common in modern C++.
See for example `ranges::min_max_result`, `from_chars_result`, and others.
##### Exception
Sometimes, we need to pass an object to a function to manipulate its state.
In such cases, passing the object by reference [`T&`](#rf-inout) is usually the right technique.
Explicitly passing an in-out parameter back out again as a return value is often not necessary.
For example:
istream& operator>>(istream& in, string& s); // much like std::operator>>()
for (string s; in >> s; ) {
// do something with line
}
Here, both `s` and `in` are used as in-out parameters.
We pass `in` by (non-`const`) reference to be able to manipulate its state.
We pass `s` to avoid repeated allocations.
By reusing `s` (passed by reference), we allocate new memory only when we need to expand `s`'s capacity.
This technique is sometimes called the "caller-allocated out" pattern and is particularly useful for types,
such as `string` and `vector`, that need to do free store allocations.
To compare, if we passed out all values as return values, we would write something like this:
struct get_string_result { istream& in; string s; };
get_string_result get_string(istream& in) // not recommended
{
string s;
in >> s;
return { in, move(s) };
}
for (auto [in, s] = get_string(cin); in; s = get_string(in).s) {
// do something with string
}
We consider that significantly less elegant with significantly less performance.
For a truly strict reading of this rule (F.21), the exception isn't really an exception because it relies on in-out parameters,
rather than the plain out parameters mentioned in the rule.
However, we prefer to be explicit, rather than subtle.
##### Note
In most cases, it is useful to return a specific, user-defined type.
For example:
struct Distance {
int value;
int unit = 1; // 1 means meters
};
Distance d1 = measure(obj1); // access d1.value and d1.unit
auto d2 = measure(obj2); // access d2.value and d2.unit
auto [value, unit] = measure(obj3); // access value and unit; somewhat redundant
// to people who know measure()
auto [x, y] = measure(obj4); // don't; it's likely to be confusing
The overly generic `pair` and `tuple` should be used only when the value returned represents independent entities rather than an abstraction.
Another option is to use `optional<T>` or `expected<T, error_code>`, rather than `pair` or `tuple`.
When used appropriately these types convey more information about what the members mean than `pair<T, bool>` or `pair<T, error_code>` do.
##### Note
When the object to be returned is initialized from local variables that are expensive to copy,
explicit `move` may be helpful to avoid copying:
pair<LargeObject, LargeObject> f(const string& input)
{
LargeObject large1 = g(input);
LargeObject large2 = h(input);
// ...
return { move(large1), move(large2) }; // no copies
}
Alternatively,
pair<LargeObject, LargeObject> f(const string& input)
{
// ...
return { g(input), h(input) }; // no copies, no moves
}
Note this is different from the `return move(...)` anti-pattern from [ES.56](#res-move).
##### Enforcement
* Output parameters should be replaced by return values.
An output parameter is one that the function writes to, invokes a non-`const` member function, or passes on as a non-`const`.
* `pair` or `tuple` return types should be replaced by `struct`, if possible.
In variadic templates, `tuple` is often unavoidable.
### <a name="rf-ptr-ref"></a>F.60: Prefer `T*` over `T&` when "no argument" is a valid option
##### Reason
A pointer (`T*`) can be a `nullptr` and a reference (`T&`) cannot, there is no valid "null reference".
Sometimes having `nullptr` as an alternative to indicated "no object" is useful, but if it is not, a reference is notationally simpler and might yield better code.
##### Example
string zstring_to_string(zstring p) // zstring is a char*; that is a C-style string
{
if (!p) return string{}; // p might be nullptr; remember to check
return string{p};
}
void print(const vector<int>& r)
{
// r refers to a vector<int>; no check needed
}
##### Note
It is possible, but not valid C++ to construct a reference that is essentially a `nullptr` (e.g., `T* p = nullptr; T& r = *p;`).
That error is very uncommon.
##### Note
If you prefer the pointer notation (`->` and/or `*` vs. `.`), `not_null<T*>` provides the same guarantee as `T&`.
##### Enforcement
* Flag ???
### <a name="rf-ptr"></a>F.22: Use `T*` or `owner<T*>` to designate a single object
##### Reason
Readability: it makes the meaning of a plain pointer clear.
Enables significant tool support.
##### Note
In traditional C and C++ code, plain `T*` is used for many weakly-related purposes, such as:
* Identify a (single) object (not to be deleted by this function)
* Point to an object allocated on the free store (and delete it later)
* Hold the `nullptr`
* Identify a C-style string (zero-terminated array of characters)
* Identify an array with a length specified separately
* Identify a location in an array
This makes it hard to understand what the code does and is supposed to do.
It complicates checking and tool support.
##### Example
void use(int* p, int n, char* s, int* q)
{
p[n - 1] = 666; // Bad: we don't know if p points to n elements;
// assume it does not or use span<int>
cout << s; // Bad: we don't know if that s points to a zero-terminated array of char;
// assume it does not or use zstring
delete q; // Bad: we don't know if *q is allocated on the free store;
// assume it does not or use owner
}
better
void use2(span<int> p, zstring s, owner<int*> q)
{
p[p.size() - 1] = 666; // OK, a range error can be caught
cout << s; // OK
delete q; // OK
}
##### Note
`owner<T*>` represents ownership, `zstring` represents a C-style string.
**Also**: Assume that a `T*` obtained from a smart pointer to `T` (e.g., `unique_ptr<T>`) points to a single element.
**See also**: [Support library](#gsl-guidelines-support-library)
**See also**: [Do not pass an array as a single pointer](#ri-array)
##### Enforcement
* (Simple) ((Bounds)) Warn for any arithmetic operation on an expression of pointer type that results in a value of pointer type.
### <a name="rf-nullptr"></a>F.23: Use a `not_null<T>` to indicate that "null" is not a valid value
##### Reason
Clarity. A function with a `not_null<T>` parameter makes it clear that the caller of the function is responsible for any `nullptr` checks that might be necessary.
Similarly, a function with a return value of `not_null<T>` makes it clear that the caller of the function does not need to check for `nullptr`.
##### Example
`not_null<T*>` makes it obvious to a reader (human or machine) that a test for `nullptr` is not necessary before dereference.
Additionally, when debugging, `owner<T*>` and `not_null<T>` can be instrumented to check for correctness.
Consider:
int length(Record* p);
When I call `length(p)` should I check if `p` is `nullptr` first? Should the implementation of `length()` check if `p` is `nullptr`?
// it is the caller's job to make sure p != nullptr
int length(not_null<Record*> p);
// the implementor of length() must assume that p == nullptr is possible
int length(Record* p);
##### Note
A `not_null<T*>` is assumed not to be the `nullptr`; a `T*` might be the `nullptr`; both can be represented in memory as a `T*` (so no run-time overhead is implied).
##### Note
`not_null` is not just for built-in pointers. It works for `unique_ptr`, `shared_ptr`, and other pointer-like types.
##### Enforcement
* (Simple) Warn if a raw pointer is dereferenced without being tested against `nullptr` (or equivalent) within a function, suggest it is declared `not_null` instead.
* (Simple) Error if a raw pointer is sometimes dereferenced after first being tested against `nullptr` (or equivalent) within the function and sometimes is not.
* (Simple) Warn if a `not_null` pointer is tested against `nullptr` within a function.
### <a name="rf-range"></a>F.24: Use a `span<T>` or a `span_p<T>` to designate a half-open sequence
##### Reason
Informal/non-explicit ranges are a source of errors.
##### Example
X* find(span<X> r, const X& v); // find v in r
vector<X> vec;
// ...
auto p = find({vec.begin(), vec.end()}, X{}); // find X{} in vec
##### Note
Ranges are extremely common in C++ code. Typically, they are implicit and their correct use is very hard to ensure.
In particular, given a pair of arguments `(p, n)` designating an array `[p:p+n)`,
it is in general impossible to know if there really are `n` elements to access following `*p`.
`span<T>` and `span_p<T>` are simple helper classes designating a `[p:q)` range and a range starting with `p` and ending with the first element for which a predicate is true, respectively.
##### Example
A `span` represents a range of elements, but how do we manipulate elements of that range?
void f(span<int> s)
{
// range traversal (guaranteed correct)
for (int x : s) cout << x << '\n';
// C-style traversal (potentially checked)
for (gsl::index i = 0; i < s.size(); ++i) cout << s[i] << '\n';
// random access (potentially checked)
s[7] = 9;
// extract pointers (potentially checked)
std::sort(&s[0], &s[s.size() / 2]);
}
##### Note
A `span<T>` object does not own its elements and is so small that it can be passed by value.
Passing a `span` object as an argument is exactly as efficient as passing a pair of pointer arguments or passing a pointer and an integer count.
**See also**: [Support library](#gsl-guidelines-support-library)
##### Enforcement
(Complex) Warn where accesses to pointer parameters are bounded by other parameters that are integral types and suggest they could use `span` instead.
### <a name="rf-zstring"></a>F.25: Use a `zstring` or a `not_null<zstring>` to designate a C-style string
##### Reason
C-style strings are ubiquitous. They are defined by convention: zero-terminated arrays of characters.
We must distinguish C-style strings from a pointer to a single character or an old-fashioned pointer to an array of characters.
If you don't need null termination, use `string_view`.
##### Example
Consider:
int length(const char* p);
When I call `length(s)` should I check if `s` is `nullptr` first? Should the implementation of `length()` check if `p` is `nullptr`?
// the implementor of length() must assume that p == nullptr is possible
int length(zstring p);
// it is the caller's job to make sure p != nullptr
int length(not_null<zstring> p);
##### Note
`zstring` does not represent ownership.
**See also**: [Support library](#gsl-guidelines-support-library)
### <a name="rf-unique_ptr"></a>F.26: Use a `unique_ptr<T>` to transfer ownership where a pointer is needed
##### Reason
Using `unique_ptr` is the cheapest way to pass a pointer safely.
**See also**: [C.50](#rc-factory) regarding when to return a `shared_ptr` from a factory.
##### Example
unique_ptr<Shape> get_shape(istream& is) // assemble shape from input stream
{
auto kind = read_header(is); // read header and identify the next shape on input
switch (kind) {
case kCircle:
return make_unique<Circle>(is);
case kTriangle:
return make_unique<Triangle>(is);
// ...
}
}
##### Note
You need to pass a pointer rather than an object if what you are transferring is an object from a class hierarchy that is to be used through an interface (base class).
##### Enforcement
(Simple) Warn if a function returns a locally allocated raw pointer. Suggest using either `unique_ptr` or `shared_ptr` instead.
### <a name="rf-shared_ptr"></a>F.27: Use a `shared_ptr<T>` to share ownership
##### Reason
Using `std::shared_ptr` is the standard way to represent shared ownership. That is, the last owner deletes the object.
##### Example
{
shared_ptr<const Image> im { read_image(somewhere) };
std::thread t0 {shade, args0, top_left, im};
std::thread t1 {shade, args1, top_right, im};
std::thread t2 {shade, args2, bottom_left, im};
std::thread t3 {shade, args3, bottom_right, im};
// detaching threads requires extra care (e.g., to join before
// main ends), but even if we do detach the four threads here ...
}
// ... shared_ptr ensures that eventually the last thread to
// finish safely deletes the image
##### Note
Prefer a `unique_ptr` over a `shared_ptr` if there is never more than one owner at a time.
`shared_ptr` is for shared ownership.
Note that pervasive use of `shared_ptr` has a cost (atomic operations on the `shared_ptr`'s reference count have a measurable aggregate cost).
##### Alternative
Have a single object own the shared object (e.g. a scoped object) and destroy that (preferably implicitly) when all users have completed.
##### Enforcement
(Not enforceable) This is a too complex pattern to reliably detect.
### <a name="rf-return-ptr"></a>F.42: Return a `T*` to indicate a position (only)
##### Reason
That's what pointers are good for.
Returning a `T*` to transfer ownership is a misuse.
##### Example
Node* find(Node* t, const string& s) // find s in a binary tree of Nodes
{
if (!t || t->name == s) return t;
if ((auto p = find(t->left, s))) return p;
if ((auto p = find(t->right, s))) return p;
return nullptr;
}
If it isn't the `nullptr`, the pointer returned by `find` indicates a `Node` holding `s`.
Importantly, that does not imply a transfer of ownership of the pointed-to object to the caller.
##### Note
Positions can also be transferred by iterators, indices, and references.
A reference is often a superior alternative to a pointer [if there is no need to use `nullptr`](#rf-ptr-ref) or [if the object referred to should not change](#s-const).
##### Note
Do not return a pointer to something that is not in the caller's scope; see [F.43](#rf-dangle).
**See also**: [discussion of dangling pointer prevention](#???)
##### Enforcement
* Flag `delete`, `std::free()`, etc. applied to a plain `T*`.
Only owners should be deleted.
* Flag `new`, `malloc()`, etc. assigned to a plain `T*`.
Only owners should be responsible for deletion.
### <a name="rf-dangle"></a>F.43: Never (directly or indirectly) return a pointer or a reference to a local object
##### Reason
To avoid the crashes and data corruption that can result from the use of such a dangling pointer.
##### Example, bad
After the return from a function its local objects no longer exist:
int* f()
{
int fx = 9;
return &fx; // BAD
}
void g(int* p) // looks innocent enough
{
int gx;
cout << "*p == " << *p << '\n';
*p = 999;
cout << "gx == " << gx << '\n';
}
void h()
{
int* p = f();
int z = *p; // read from abandoned stack frame (bad)
g(p); // pass pointer to abandoned stack frame to function (bad)
}
Here on one popular implementation I got the output:
*p == 999
gx == 999
I expected that because the call of `g()` reuses the stack space abandoned by the call of `f()` so `*p` refers to the space now occupied by `gx`.
* Imagine what would happen if `fx` and `gx` were of different types.
* Imagine what would happen if `fx` or `gx` was a type with an invariant.
* Imagine what would happen if that dangling pointer was passed around among a larger set of functions.
* Imagine what a cracker could do with that dangling pointer.
Fortunately, most (all?) modern compilers catch and warn against this simple case.
##### Note
This applies to references as well:
int& f()
{
int x = 7;
// ...
return x; // Bad: returns reference to object that is about to be destroyed
}
##### Note
This applies only to non-`static` local variables.
All `static` variables are (as their name indicates) statically allocated, so that pointers to them cannot dangle.
##### Example, bad
Not all examples of leaking a pointer to a local variable are that obvious:
int* glob; // global variables are bad in so many ways
template<class T>
void steal(T x)
{
glob = x(); // BAD
}
void f()
{
int i = 99;
steal([&] { return &i; });
}
int main()
{
f();
cout << *glob << '\n';
}
Here I managed to read the location abandoned by the call of `f`.
The pointer stored in `glob` could be used much later and cause trouble in unpredictable ways.
##### Note
The address of a local variable can be "returned"/leaked by a return statement, by a `T&` out-parameter, as a member of a returned object, as an element of a returned array, and more.
##### Note
Similar examples can be constructed "leaking" a pointer from an inner scope to an outer one;
such examples are handled equivalently to leaks of pointers out of a function.
A slightly different variant of the problem is placing pointers in a container that outlives the objects pointed to.
**See also**: Another way of getting dangling pointers is [pointer invalidation](#???).
It can be detected/prevented with similar techniques.
##### Enforcement
* Compilers tend to catch return of reference to locals and could in many cases catch return of pointers to locals.
* Static analysis can catch many common patterns of the use of pointers indicating positions (thus eliminating dangling pointers)
### <a name="rf-return-ref"></a>F.44: Return a `T&` when copy is undesirable and "returning no object" isn't needed
##### Reason
The language guarantees that a `T&` refers to an object, so that testing for `nullptr` isn't necessary.
**See also**: The return of a reference must not imply transfer of ownership:
[discussion of dangling pointer prevention](#???) and [discussion of ownership](#???).
##### Example
class Car
{
array<wheel, 4> w;
// ...
public:
wheel& get_wheel(int i) { Expects(i < w.size()); return w[i]; }
// ...
};
void use()
{
Car c;
wheel& w0 = c.get_wheel(0); // w0 has the same lifetime as c
}
##### Enforcement
Flag functions where no `return` expression could yield `nullptr`.
### <a name="rf-return-ref-ref"></a>F.45: Don't return a `T&&`
##### Reason
It's asking to return a reference to a destroyed temporary object.
An `&&` is a magnet for temporary objects.
##### Example
A returned rvalue reference goes out of scope at the end of the full expression to which it is returned:
auto&& x = max(0, 1); // OK, so far
foo(x); // Undefined behavior
This kind of use is a frequent source of bugs, often incorrectly reported as a compiler bug.
An implementer of a function should avoid setting such traps for users.
The [lifetime safety profile](#ss-lifetime) will (when completely implemented) catch such problems.
##### Example
Returning an rvalue reference is fine when the reference to the temporary is being passed "downward" to a callee;
then, the temporary is guaranteed to outlive the function call (see [F.18](#rf-consume) and [F.19](#rf-forward)).
However, it's not fine when passing such a reference "upward" to a larger caller scope.
For passthrough functions that pass in parameters (by ordinary reference or by perfect forwarding) and want to return values, use simple `auto` return type deduction (not `auto&&`).
Assume that `F` returns by value:
template<class F>
auto&& wrapper(F f)
{
log_call(typeid(f)); // or whatever instrumentation
return f(); // BAD: returns a reference to a temporary
}
Better:
template<class F>
auto wrapper(F f)
{
log_call(typeid(f)); // or whatever instrumentation
return f(); // OK
}
##### Exception
`std::move` and `std::forward` do return `&&`, but they are just casts -- used by convention only in expression contexts where a reference to a temporary object is passed along within the same expression before the temporary is destroyed. We don't know of any other good examples of returning `&&`.
##### Enforcement
Flag any use of `&&` as a return type, except in `std::move` and `std::forward`.
### <a name="rf-main"></a>F.46: `int` is the return type for `main()`
##### Reason
It's a language rule, but violated through "language extensions" so often that it is worth mentioning.
Declaring `main` (the one global `main` of a program) `void` limits portability.
##### Example
void main() { /* ... */ }; // bad, not C++
int main()
{
std::cout << "This is the way to do it\n";
}
##### Note
We mention this only because of the persistence of this error in the community.
Note that despite its non-void return type, the main function does not require an explicit return statement.
##### Enforcement
* The compiler should do it
* If the compiler doesn't do it, let tools flag it
### <a name="rf-assignment-op"></a>F.47: Return `T&` from assignment operators
##### Reason
The convention for operator overloads (especially on concrete types) is for
`operator=(const T&)` to perform the assignment and then return (non-`const`)
`*this`. This ensures consistency with standard-library types and follows the
principle of "do as the ints do."
##### Note
Historically there was some guidance to make the assignment operator return `const T&`.
This was primarily to avoid code of the form `(a = b) = c` -- such code is not common enough to warrant violating consistency with standard types.
##### Example
class Foo
{
public:
...
Foo& operator=(const Foo& rhs)
{
// Copy members.
...
return *this;
}
};
##### Enforcement
This should be enforced by tooling by checking the return type (and return
value) of any assignment operator.
### <a name="rf-return-move-local"></a>F.48: Don't `return std::move(local)`
##### Reason
Returning a local variable implicitly moves it anyway.
An explicit `std::move` is always a pessimization, because it prevents Return Value Optimization (RVO),
which can eliminate the move completely.
##### Example, bad
S bad()
{
S result;
return std::move(result);
}
##### Example, good
S good()
{
S result;
// Named RVO: move elision at best, move construction at worst
return result;
}
##### Enforcement
This should be enforced by tooling by checking the return expression.
### <a name="rf-return-const"></a>F.49: Don't return `const T`
##### Reason
It is not recommended to return a `const` value.
Such older advice is now obsolete; it does not add value, and it interferes with move semantics.
##### Example
const vector<int> fct(); // bad: that "const" is more trouble than it is worth
void g(vector<int>& vx)
{
// ...
fct() = vx; // prevented by the "const"
// ...
vx = fct(); // expensive copy: move semantics suppressed by the "const"
// ...
}
The argument for adding `const` to a return value is that it prevents (very rare) accidental access to a temporary.
The argument against is that it prevents (very frequent) use of move semantics.
**See also**: [F.20, the general item about "out" output values](#rf-out)
##### Enforcement
* Flag returning a `const` value. To fix: Remove `const` to return a non-`const` value instead.
### <a name="rf-capture-vs-overload"></a>F.50: Use a lambda when a function won't do (to capture local variables, or to write a local function)
##### Reason
Functions can't capture local variables or be defined at local scope; if you need those things, prefer a lambda where possible, and a handwritten function object where not. On the other hand, lambdas and function objects don't overload; if you need to overload, prefer a function (the workarounds to make lambdas overload are ornate). If either will work, prefer writing a function; use the simplest tool necessary.
##### Example
// writing a function that should only take an int or a string
// -- overloading is natural
void f(int);
void f(const string&);
// writing a function object that needs to capture local state and appear
// at statement or expression scope -- a lambda is natural
vector<work> v = lots_of_work();
for (int tasknum = 0; tasknum < max; ++tasknum) {
pool.run([=, &v] {
/*
...
... process (1/max)-th of v, the tasknum-th chunk
...
*/
});
}
pool.join();
##### Exception
Generic lambdas offer a concise way to write function templates and so can be useful even when a normal function template would do equally well with a little more syntax. This advantage will probably disappear in the future once all functions gain the ability to have Concept parameters.
##### Enforcement
* Warn on use of a named non-generic lambda (e.g., `auto x = [](int i) { /*...*/; };`) that captures nothing and appears at global scope. Write an ordinary function instead.
### <a name="rf-default-args"></a>F.51: Where there is a choice, prefer default arguments over overloading
##### Reason
Default arguments simply provide alternative interfaces to a single implementation.
There is no guarantee that a set of overloaded functions all implement the same semantics.
The use of default arguments can avoid code replication.
##### Note
There is a choice between using default argument and overloading when the alternatives are from a set of arguments of the same types.
For example:
void print(const string& s, format f = {});
as opposed to
void print(const string& s); // use default format
void print(const string& s, format f);
There is not a choice when a set of functions are used to do a semantically equivalent operation to a set of types. For example:
void print(const char&);
void print(int);
void print(zstring);
##### See also
[Default arguments for virtual functions](#rh-virtual-default-arg)
##### Enforcement
* Warn on an overload set where the overloads have a common prefix of parameters (e.g., `f(int)`, `f(int, const string&)`, `f(int, const string&, double)`). (Note: Review this enforcement if it's too noisy in practice.)
### <a name="rf-reference-capture"></a>F.52: Prefer capturing by reference in lambdas that will be used locally, including passed to algorithms
##### Reason
For efficiency and correctness, you nearly always want to capture by reference when using the lambda locally. This includes when writing or calling parallel algorithms that are local because they join before returning.
##### Discussion
The efficiency consideration is that most types are cheaper to pass by reference than by value.
The correctness consideration is that many calls want to perform side effects on the original object at the call site (see example below). Passing by value prevents this.
##### Note
Unfortunately, there is no simple way to capture by reference to `const` to get the efficiency for a local call but also prevent side effects.
##### Example
Here, a large object (a network message) is passed to an iterative algorithm, and it is not efficient or correct to copy the message (which might not be copyable):
std::for_each(begin(sockets), end(sockets), [&message](auto& socket)
{
socket.send(message);
});
##### Example
This is a simple three-stage parallel pipeline. Each `stage` object encapsulates a worker thread and a queue, has a `process` function to enqueue work, and in its destructor automatically blocks waiting for the queue to empty before ending the thread.
void send_packets(buffers& bufs)
{
stage encryptor([](buffer& b) { encrypt(b); });
stage compressor([&](buffer& b) { compress(b); encryptor.process(b); });
stage decorator([&](buffer& b) { decorate(b); compressor.process(b); });
for (auto& b : bufs) { decorator.process(b); }
} // automatically blocks waiting for pipeline to finish
##### Enforcement
Flag a lambda that captures by reference, but is used other than locally within the function scope or passed to a function by reference. (Note: This rule is an approximation, but does flag passing by pointer as those are more likely to be stored by the callee, writing to a heap location accessed via a parameter, returning the lambda, etc. The Lifetime rules will also provide general rules that flag escaping pointers and references including via lambdas.)
### <a name="rf-value-capture"></a>F.53: Avoid capturing by reference in lambdas that will be used non-locally, including returned, stored on the heap, or passed to another thread
##### Reason
Pointers and references to locals shouldn't outlive their scope. Lambdas that capture by reference are just another place to store a reference to a local object, and shouldn't do so if they (or a copy) outlive the scope.
##### Example, bad
int local = 42;
// Want a reference to local.
// Note that after program exits this scope,
// local no longer exists, therefore
// process() call will have undefined behavior!
thread_pool.queue_work([&] { process(local); });
##### Example, good
int local = 42;
// Want a copy of local.
// Since a copy of local is made, it will
// always be available for the call.
thread_pool.queue_work([=] { process(local); });
##### Note
If a non-local pointer must be captured, consider using `unique_ptr`; this handles both lifetime and synchronization.
If the `this` pointer must be captured, consider using `[*this]` capture, which creates a copy of the entire object.
##### Enforcement
* (Simple) Warn when capture-list contains a reference to a locally declared variable
* (Complex) Flag when capture-list contains a reference to a locally declared variable and the lambda is passed to a non-`const` and non-local context
### <a name="rf-this-capture"></a>F.54: When writing a lambda that captures `this` or any class data member, don't use `[=]` default capture
##### Reason
It's confusing. Writing `[=]` in a member function appears to capture by value, but actually captures data members by reference because it actually captures the invisible `this` pointer by value. If you meant to do that, write `this` explicitly.
##### Example
class My_class {
int x = 0;
// ...
void f()
{
int i = 0;
// ...
auto lambda = [=] { use(i, x); }; // BAD: "looks like" copy/value capture
x = 42;
lambda(); // calls use(0, 42);
x = 43;
lambda(); // calls use(0, 43);
// ...
auto lambda2 = [i, this] { use(i, x); }; // ok, most explicit and least confusing
// ...
}
};
##### Note
If you intend to capture a copy of all class data members, consider C++17 `[*this]`.
##### Enforcement
* Flag any lambda capture-list that specifies a capture-default of `[=]` and also captures `this` (whether explicitly or via the default capture and a use of `this` in the body)
### <a name="f-varargs"></a>F.55: Don't use `va_arg` arguments
##### Reason
Reading from a `va_arg` assumes that the correct type was actually passed.
Passing to varargs assumes the correct type will be read.
This is fragile because it cannot generally be enforced to be safe in the language and so relies on programmer discipline to get it right.
##### Example
int sum(...)
{
// ...
while (/*...*/)
result += va_arg(list, int); // BAD, assumes it will be passed ints
// ...
}
sum(3, 2); // ok
sum(3.14159, 2.71828); // BAD, undefined
template<class ...Args>
auto sum(Args... args) // GOOD, and much more flexible
{
return (... + args); // note: C++17 "fold expression"
}
sum(3, 2); // ok: 5
sum(3.14159, 2.71828); // ok: ~5.85987
##### Alternatives
* overloading
* variadic templates
* `variant` arguments
* `initializer_list` (homogeneous)
##### Note
Declaring a `...` parameter is sometimes useful for techniques that don't involve actual argument passing, notably to declare "take-anything" functions so as to disable "everything else" in an overload set or express a catchall case in a template metaprogram.
##### Enforcement
* Issue a diagnostic for using `va_list`, `va_start`, or `va_arg`.
* Issue a diagnostic for passing an argument to a vararg parameter of a function that does not offer an overload for a more specific type in the position of the vararg. To fix: Use a different function, or `[[suppress("type")]]`.
### <a name="f-nesting"></a>F.56: Avoid unnecessary condition nesting
##### Reason
Shallow nesting of conditions makes the code easier to follow. It also makes the intent clearer.
Strive to place the essential code at outermost scope, unless this obscures intent.
##### Example
Use a guard-clause to take care of exceptional cases and return early.
// Bad: Deep nesting
void foo() {
...
if (x) {
computeImportantThings(x);
}
}
// Bad: Still a redundant else.
void foo() {
...
if (!x) {
return;
}
else {
computeImportantThings(x);
}
}
// Good: Early return, no redundant else
void foo() {
...
if (!x)
return;
computeImportantThings(x);
}
##### Example
// Bad: Unnecessary nesting of conditions
void foo() {
...
if (x) {
if (y) {
computeImportantThings(x);
}
}
}
// Good: Merge conditions + return early
void foo() {
...
if (!(x && y))
return;
computeImportantThings(x);
}
##### Enforcement
Flag a redundant `else`.
Flag a function whose body is simply a conditional statement enclosing a block.
# <a name="s-class"></a>C: Classes and class hierarchies
A class is a user-defined type, for which a programmer can define the representation, operations, and interfaces.
Class hierarchies are used to organize related classes into hierarchical structures.
Class rule summary:
* [C.1: Organize related data into structures (`struct`s or `class`es)](#rc-org)
* [C.2: Use `class` if the class has an invariant; use `struct` if the data members can vary independently](#rc-struct)
* [C.3: Represent the distinction between an interface and an implementation using a class](#rc-interface)
* [C.4: Make a function a member only if it needs direct access to the representation of a class](#rc-member)
* [C.5: Place helper functions in the same namespace as the class they support](#rc-helper)
* [C.7: Don't define a class or enum and declare a variable of its type in the same statement](#rc-standalone)
* [C.8: Use `class` rather than `struct` if any member is non-public](#rc-class)
* [C.9: Minimize exposure of members](#rc-private)
Subsections:
* [C.concrete: Concrete types](#ss-concrete)
* [C.ctor: Constructors, assignments, and destructors](#s-ctor)
* [C.con: Containers and other resource handles](#ss-containers)
* [C.lambdas: Function objects and lambdas](#ss-lambdas)
* [C.hier: Class hierarchies (OOP)](#ss-hier)
* [C.over: Overloading and overloaded operators](#ss-overload)
* [C.union: Unions](#ss-union)
### <a name="rc-org"></a>C.1: Organize related data into structures (`struct`s or `class`es)
##### Reason
Ease of comprehension.
If data is related (for fundamental reasons), that fact should be reflected in code.
##### Example
void draw(int x, int y, int x2, int y2); // BAD: unnecessary implicit relationships
void draw(Point from, Point to); // better
##### Note
A simple class without virtual functions implies no space or time overhead.
##### Note
From a language perspective `class` and `struct` differ only in the default visibility of their members.
##### Enforcement
Probably impossible. Maybe a heuristic looking for data items used together is possible.
### <a name="rc-struct"></a>C.2: Use `class` if the class has an invariant; use `struct` if the data members can vary independently
##### Reason
Readability.
Ease of comprehension.
The use of `class` alerts the programmer to the need for an invariant.
This is a useful convention.
##### Note
An invariant is a logical condition for the members of an object that a constructor must establish for the public member functions to assume.
After the invariant is established (typically by a constructor) every member function can be called for the object.
An invariant can be stated informally (e.g., in a comment) or more formally using `Expects`.
If all data members can vary independently of each other, no invariant is possible.
##### Example
struct Pair { // the members can vary independently
string name;
int volume;
};
but:
class Date {
public:
// validate that {yy, mm, dd} is a valid date and initialize
Date(int yy, Month mm, char dd);
// ...
private:
int y;
Month m;
char d; // day
};
##### Note
If a class has any `private` data, a user cannot completely initialize an object without the use of a constructor.
Hence, the class definer will provide a constructor and must specify its meaning.
This effectively means the definer needs to define an invariant.
**See also**:
* [define a class with private data as `class`](#rc-class)
* [Prefer to place the interface first in a class](#rl-order)
* [minimize exposure of members](#rc-private)
* [Avoid `protected` data](#rh-protected)
##### Enforcement
Look for `struct`s with all data private and `class`es with public members.
### <a name="rc-interface"></a>C.3: Represent the distinction between an interface and an implementation using a class
##### Reason
An explicit distinction between interface and implementation improves readability and simplifies maintenance.
##### Example
class Date {
public:
Date();
// validate that {yy, mm, dd} is a valid date and initialize
Date(int yy, Month mm, char dd);
int day() const;
Month month() const;
// ...
private:
// ... some representation ...
};
For example, we can now change the representation of a `Date` without affecting its users (recompilation is likely, though).
##### Note
Using a class in this way to represent the distinction between interface and implementation is of course not the only way.
For example, we can use a set of declarations of freestanding functions in a namespace, an abstract base class, or a function template with concepts to represent an interface.
The most important issue is to explicitly distinguish between an interface and its implementation "details."
Ideally, and typically, an interface is far more stable than its implementation(s).
##### Enforcement
???
### <a name="rc-member"></a>C.4: Make a function a member only if it needs direct access to the representation of a class
##### Reason
Less coupling than with member functions, fewer functions that can cause trouble by modifying object state, reduces the number of functions that need to be modified after a change in representation.
##### Example
class Date {
// ... relatively small interface ...
};
// helper functions:
Date next_weekday(Date);
bool operator==(Date, Date);
The "helper functions" have no need for direct access to the representation of a `Date`.
##### Note
This rule becomes even better if C++ gets ["uniform function call"](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0251r0.pdf).
##### Exception
The language requires `virtual` functions to be members, and not all `virtual` functions directly access data.
In particular, members of an abstract class rarely do.
Note [multi-methods](https://web.archive.org/web/20200605021759/https://parasol.tamu.edu/~yuriys/papers/OMM10.pdf).
##### Exception
The language requires operators `=`, `()`, `[]`, and `->` to be members.
##### Exception
An overload set could have some members that do not directly access `private` data:
class Foobar {
public:
void foo(long x) { /* manipulate private data */ }
void foo(double x) { foo(std::lround(x)); }
// ...
private:
// ...
};
##### Exception
Similarly, a set of functions could be designed to be used in a chain:
x.scale(0.5).rotate(45).set_color(Color::red);
Typically, some but not all of such functions directly access `private` data.
##### Enforcement
* Look for non-`virtual` member functions that do not touch data members directly.
The snag is that many member functions that do not need to touch data members directly do.
* Ignore `virtual` functions.
* Ignore functions that are part of an overload set out of which at least one function accesses `private` members.
* Ignore functions returning `this`.
### <a name="rc-helper"></a>C.5: Place helper functions in the same namespace as the class they support
##### Reason
A helper function is a function (usually supplied by the writer of a class) that does not need direct access to the representation of the class, yet is seen as part of the useful interface to the class.
Placing them in the same namespace as the class makes their relationship to the class obvious and allows them to be found by argument dependent lookup.
##### Example
namespace Chrono { // here we keep time-related services
class Time { /* ... */ };
class Date { /* ... */ };
// helper functions:
bool operator==(Date, Date);
Date next_weekday(Date);
// ...
}
##### Note
This is especially important for [overloaded operators](#ro-namespace).
##### Enforcement
* Flag global functions taking argument types from a single namespace.
### <a name="rc-standalone"></a>C.7: Don't define a class or enum and declare a variable of its type in the same statement
##### Reason
Mixing a type definition and the definition of another entity in the same declaration is confusing and unnecessary.
##### Example, bad
struct Data { /*...*/ } data{ /*...*/ };
##### Example, good
struct Data { /*...*/ };
Data data{ /*...*/ };
##### Enforcement
* Flag if the `}` of a class or enumeration definition is not followed by a `;`. The `;` is missing.
### <a name="rc-class"></a>C.8: Use `class` rather than `struct` if any member is non-public
##### Reason
Readability.
To make it clear that something is being hidden/abstracted.
This is a useful convention.
##### Example, bad
struct Date {
int d, m;
Date(int i, Month m);
// ... lots of functions ...
private:
int y; // year
};
There is nothing wrong with this code as far as the C++ language rules are concerned,
but nearly everything is wrong from a design perspective.
The private data is hidden far from the public data.
The data is split in different parts of the class declaration.
Different parts of the data have different access.
All of this decreases readability and complicates maintenance.
##### Note
Prefer to place the interface first in a class, [see NL.16](#rl-order).
##### Enforcement
Flag classes declared with `struct` if there is a `private` or `protected` member.
### <a name="rc-private"></a>C.9: Minimize exposure of members
##### Reason
Encapsulation.
Information hiding.
Minimize the chance of unintended access.
This simplifies maintenance.
##### Example
template<typename T, typename U>
struct pair {
T a;
U b;
// ...
};
Whatever we do in the `//`-part, an arbitrary user of a `pair` can arbitrarily and independently change its `a` and `b`.
In a large code base, we cannot easily find which code does what to the members of `pair`.
This might be exactly what we want, but if we want to enforce a relation among members, we need to make them `private`
and enforce that relation (invariant) through constructors and member functions.
For example:
class Distance {
public:
// ...
double meters() const { return magnitude*unit; }
void set_unit(double u)
{
// ... check that u is a factor of 10 ...
// ... change magnitude appropriately ...
unit = u;
}
// ...
private:
double magnitude;
double unit; // 1 is meters, 1000 is kilometers, 0.001 is millimeters, etc.
};
##### Note
If the set of direct users of a set of variables cannot be easily determined, the type or usage of that set cannot be (easily) changed/improved.
For `public` and `protected` data, that's usually the case.
##### Example
A class can provide two interfaces to its users.
One for derived classes (`protected`) and one for general users (`public`).
For example, a derived class might be allowed to skip a run-time check because it has already guaranteed correctness:
class Foo {
public:
int bar(int x) { check(x); return do_bar(x); }
// ...
protected:
int do_bar(int x); // do some operation on the data
// ...
private:
// ... data ...
};
class Dir : public Foo {
//...
int mem(int x, int y)
{
/* ... do something ... */
return do_bar(x + y); // OK: derived class can bypass check
}
};
void user(Foo& x)
{
int r1 = x.bar(1); // OK, will check
int r2 = x.do_bar(2); // error: would bypass check
// ...
}
##### Note
[`protected` data is a bad idea](#rh-protected).
##### Note
Prefer the order `public` members before `protected` members before `private` members; see [NL.16](#rl-order).
##### Enforcement
* [Flag protected data](#rh-protected).
* Flag mixtures of `public` and `private` data
## <a name="ss-concrete"></a>C.concrete: Concrete types
Concrete type rule summary:
* [C.10: Prefer concrete types over class hierarchies](#rc-concrete)
* [C.11: Make concrete types regular](#rc-regular)
* [C.12: Don't make data members `const` or references in a copyable or movable type](#rc-constref)
### <a name="rc-concrete"></a>C.10: Prefer concrete types over class hierarchies
##### Reason
A concrete type is fundamentally simpler than a type in a class hierarchy:
easier to design, easier to implement, easier to use, easier to reason about, smaller, and faster.
You need a reason (use cases) for using a hierarchy.
##### Example
class Point1 {
int x, y;
// ... operations ...
// ... no virtual functions ...
};
class Point2 {
int x, y;
// ... operations, some virtual ...
virtual ~Point2();
};
void use()
{
Point1 p11 {1, 2}; // make an object on the stack
Point1 p12 {p11}; // a copy
auto p21 = make_unique<Point2>(1, 2); // make an object on the free store
auto p22 = p21->clone(); // make a copy
// ...
}
If a class is part of a hierarchy, we (in real code if not necessarily in small examples) must manipulate its objects through pointers or references.
That implies more memory overhead, more allocations and deallocations, and more run-time overhead to perform the resulting indirections.
##### Note
Concrete types can be stack-allocated and be members of other classes.
##### Note
The use of indirection is fundamental for run-time polymorphic interfaces.
The allocation/deallocation overhead is not (that's just the most common case).
We can use a base class as the interface of a scoped object of a derived class.
This is done where dynamic allocation is prohibited (e.g. hard-real-time) and to provide a stable interface to some kinds of plug-ins.
##### Enforcement
???
### <a name="rc-regular"></a>C.11: Make concrete types regular
##### Reason
Regular types are easier to understand and reason about than types that are not regular (irregularities require extra effort to understand and use).
The C++ built-in types are regular, and so are standard-library classes such as `string`, `vector`, and `map`. Concrete classes without assignment and equality can be defined, but they are (and should be) rare.
##### Example
struct Bundle {
string name;
vector<Record> vr;
};
bool operator==(const Bundle& a, const Bundle& b)
{
return a.name == b.name && a.vr == b.vr;
}
Bundle b1 { "my bundle", {r1, r2, r3}};
Bundle b2 = b1;
if (!(b1 == b2)) error("impossible!");
b2.name = "the other bundle";
if (b1 == b2) error("No!");
In particular, if a concrete type is copyable, prefer to also give it an equality comparison operator, and ensure that `a = b` implies `a == b`.
##### Note
For structs intended to be shared with C code, defining `operator==` may not be feasible.
##### Note
Handles for resources that cannot be cloned, e.g., a `scoped_lock` for a `mutex`, are concrete types but typically cannot be copied (instead, they can usually be moved),
so they can't be regular; instead, they tend to be move-only.
##### Enforcement
???
### <a name="rc-constref"></a>C.12: Don't make data members `const` or references in a copyable or movable type
##### Reason
`const` and reference data members are not useful in a copyable or movable type, and make such types difficult to use by making them at least partly uncopyable/unmovable for subtle reasons.
##### Example; bad
class bad {
const int i; // bad
string& s; // bad
// ...
};
The `const` and `&` data members make this class "only-sort-of-copyable" -- copy-constructible but not copy-assignable.
##### Note
If you need a member to point to something, use a pointer (raw or smart, and `gsl::not_null` if it should not be null) instead of a reference.
##### Enforcement
Flag a data member that is `const`, `&`, or `&&` in a type that has any copy or move operation.
## <a name="s-ctor"></a>C.ctor: Constructors, assignments, and destructors
These functions control the lifecycle of objects: creation, copy, move, and destruction.
Define constructors to guarantee and simplify initialization of classes.
These are *default operations*:
* a default constructor: `X()`
* a copy constructor: `X(const X&)`
* a copy assignment: `operator=(const X&)`
* a move constructor: `X(X&&)`
* a move assignment: `operator=(X&&)`
* a destructor: `~X()`
By default, the compiler defines each of these operations if it is used, but the default can be suppressed.
The default operations are a set of related operations that together implement the lifecycle semantics of an object.
By default, C++ treats classes as value-like types, but not all types are value-like.
Set of default operations rules:
* [C.20: If you can avoid defining any default operations, do](#rc-zero)
* [C.21: If you define or `=delete` any copy, move, or destructor function, define or `=delete` them all](#rc-five)
* [C.22: Make default operations consistent](#rc-matched)
Destructor rules:
* [C.30: Define a destructor if a class needs an explicit action at object destruction](#rc-dtor)
* [C.31: All resources acquired by a class must be released by the class's destructor](#rc-dtor-release)
* [C.32: If a class has a raw pointer (`T*`) or reference (`T&`), consider whether it might be owning](#rc-dtor-ptr)
* [C.33: If a class has an owning pointer member, define a destructor](#rc-dtor-ptr2)
* [C.35: A base class destructor should be either public and virtual, or protected and non-virtual](#rc-dtor-virtual)
* [C.36: A destructor must not fail](#rc-dtor-fail)
* [C.37: Make destructors `noexcept`](#rc-dtor-noexcept)
Constructor rules:
* [C.40: Define a constructor if a class has an invariant](#rc-ctor)
* [C.41: A constructor should create a fully initialized object](#rc-complete)
* [C.42: If a constructor cannot construct a valid object, throw an exception](#rc-throw)
* [C.43: Ensure that a copyable class has a default constructor](#rc-default0)
* [C.44: Prefer default constructors to be simple and non-throwing](#rc-default00)
* [C.45: Don't define a default constructor that only initializes data members; use member initializers instead](#rc-default)
* [C.46: By default, declare single-argument constructors `explicit`](#rc-explicit)
* [C.47: Define and initialize data members in the order of member declaration](#rc-order)
* [C.48: Prefer default member initializers to member initializers in constructors for constant initializers](#rc-in-class-initializer)
* [C.49: Prefer initialization to assignment in constructors](#rc-initialize)
* [C.50: Use a factory function if you need "virtual behavior" during initialization](#rc-factory)
* [C.51: Use delegating constructors to represent common actions for all constructors of a class](#rc-delegating)
* [C.52: Use inheriting constructors to import constructors into a derived class that does not need further explicit initialization](#rc-inheriting)
Copy and move rules:
* [C.60: Make copy assignment non-`virtual`, take the parameter by `const&`, and return by non-`const&`](#rc-copy-assignment)
* [C.61: A copy operation should copy](#rc-copy-semantic)
* [C.62: Make copy assignment safe for self-assignment](#rc-copy-self)
* [C.63: Make move assignment non-`virtual`, take the parameter by `&&`, and return by non-`const&`](#rc-move-assignment)
* [C.64: A move operation should move and leave its source in a valid state](#rc-move-semantic)
* [C.65: Make move assignment safe for self-assignment](#rc-move-self)
* [C.66: Make move operations `noexcept`](#rc-move-noexcept)
* [C.67: A polymorphic class should suppress public copy/move](#rc-copy-virtual)
Other default operations rules:
* [C.80: Use `=default` if you have to be explicit about using the default semantics](#rc-eqdefault)
* [C.81: Use `=delete` when you want to disable default behavior (without wanting an alternative)](#rc-delete)
* [C.82: Don't call virtual functions in constructors and destructors](#rc-ctor-virtual)
* [C.83: For value-like types, consider providing a `noexcept` swap function](#rc-swap)
* [C.84: A `swap` must not fail](#rc-swap-fail)
* [C.85: Make `swap` `noexcept`](#rc-swap-noexcept)
* [C.86: Make `==` symmetric with respect of operand types and `noexcept`](#rc-eq)
* [C.87: Beware of `==` on base classes](#rc-eq-base)
* [C.89: Make a `hash` `noexcept`](#rc-hash)
* [C.90: Rely on constructors and assignment operators, not memset and memcpy](#rc-memset)
## <a name="ss-defop"></a>C.defop: Default Operations
By default, the language supplies the default operations with their default semantics.
However, a programmer can disable or replace these defaults.
### <a name="rc-zero"></a>C.20: If you can avoid defining default operations, do
##### Reason
It's the simplest and gives the cleanest semantics.
##### Example
struct Named_map {
public:
explicit Named_map(const string& n) : name(n) {}
// no copy/move constructors
// no copy/move assignment operators
// no destructor
private:
string name;
map<int, int> rep;
};
Named_map nm("map"); // construct
Named_map nm2 {nm}; // copy construct
Since `std::map` and `string` have all the special functions, no further work is needed.
##### Note
This is known as "the rule of zero".
##### Enforcement
(Not enforceable) While not enforceable, a good static analyzer can detect patterns that indicate a possible improvement to meet this rule.
For example, a class with a (pointer, size) pair of members and a destructor that `delete`s the pointer could probably be converted to a `vector`.
### <a name="rc-five"></a>C.21: If you define or `=delete` any copy, move, or destructor function, define or `=delete` them all
##### Reason
The semantics of copy, move, and destruction are closely related, so if one needs to be declared, the odds are that others need consideration too.
Declaring any copy/move/destructor function,
even as `=default` or `=delete`, will suppress the implicit declaration
of a move constructor and move assignment operator.
Declaring a move constructor or move assignment operator, even as
`=default` or `=delete`, will cause an implicitly generated copy constructor
or implicitly generated copy assignment operator to be defined as deleted.
So as soon as any of these are declared, the others should
all be declared to avoid unwanted effects like turning all potential moves
into more expensive copies, or making a class move-only.
##### Example, bad
struct M2 { // bad: incomplete set of copy/move/destructor operations
public:
// ...
// ... no copy or move operations ...
~M2() { delete[] rep; }
private:
pair<int, int>* rep; // zero-terminated set of pairs
};
void use()
{
M2 x;
M2 y;
// ...
x = y; // the default assignment
// ...
}
Given that "special attention" was needed for the destructor (here, to deallocate), the likelihood that the implicitly-defined copy and move assignment operators will be correct is low (here, we would get double deletion).
##### Note
This is known as "the rule of five."
##### Note
If you want a default implementation (while defining another), write `=default` to show you're doing so intentionally for that function.
If you don't want a generated default function, suppress it with `=delete`.
##### Example, good
When a destructor needs to be declared just to make it `virtual`, it can be
defined as defaulted.
class AbstractBase {
public:
virtual void foo() = 0; // at least one abstract method to make the class abstract
virtual ~AbstractBase() = default;
// ...
};
To prevent slicing as per [C.67](#rc-copy-virtual),
make the copy and move operations protected or `=delete`d, and add a `clone`:
class CloneableBase {
public:
virtual unique_ptr<CloneableBase> clone() const;
virtual ~CloneableBase() = default;
CloneableBase() = default;
CloneableBase(const CloneableBase&) = delete;
CloneableBase& operator=(const CloneableBase&) = delete;
CloneableBase(CloneableBase&&) = delete;
CloneableBase& operator=(CloneableBase&&) = delete;
// ... other constructors and functions ...
};
Defining only the move operations or only the copy operations would have the
same effect here, but stating the intent explicitly for each special member
makes it more obvious to the reader.
##### Note
Compilers enforce much of this rule and ideally warn about any violation.
##### Note
Relying on an implicitly generated copy operation in a class with a destructor is deprecated.
##### Note
Writing these functions can be error-prone.
Note their argument types:
class X {
public:
// ...
virtual ~X() = default; // destructor (virtual if X is meant to be a base class)
X(const X&) = default; // copy constructor
X& operator=(const X&) = default; // copy assignment
X(X&&) noexcept = default; // move constructor
X& operator=(X&&) noexcept = default; // move assignment
};
A minor mistake (such as a misspelling, leaving out a `const`, using `&` instead of `&&`, or leaving out a special function) can lead to errors or warnings.
To avoid the tedium and the possibility of errors, try to follow the [rule of zero](#rc-zero).
##### Enforcement
(Simple) A class should have a declaration (even a `=delete` one) for either all or none of the copy/move/destructor functions.
### <a name="rc-matched"></a>C.22: Make default operations consistent
##### Reason
The default operations are conceptually a matched set. Their semantics are interrelated.
Users will be surprised if copy/move construction and copy/move assignment do logically different thin
gitextract_y8p6tgzy/
├── .github/
│ └── workflows/
│ ├── build.yml
│ └── jekyll-gh-pages.yml
├── .gitignore
├── .travis.yml
├── CONTRIBUTING.md
├── CppCoreGuidelines.md
├── LICENSE
├── README.md
├── SECURITY.md
├── _config.yml
├── _includes/
│ ├── head.html
│ └── sidebar.html
├── _layouts/
│ └── default.html
├── docs/
│ ├── dyn_array.md
│ └── gsl-intro.md
├── index.html
├── params.json
├── public/
│ └── css/
│ ├── custom.css
│ ├── hyde.css
│ └── poole.css
├── scripts/
│ ├── Makefile
│ ├── hunspell/
│ │ ├── en_US.aff
│ │ ├── en_US.dic
│ │ └── isocpp.dic
│ ├── nodejs/
│ │ ├── package.json
│ │ └── remark/
│ │ └── .remarkrc
│ └── python/
│ ├── Makefile.in
│ ├── cpplint.py
│ ├── cpplint_wrap.py
│ └── md-split.py
└── talks/
└── README.md
SYMBOL INDEX (191 symbols across 3 files)
FILE: scripts/python/cpplint.py
function GetHeaderExtensions (line 69) | def GetHeaderExtensions():
function GetAllExtensions (line 76) | def GetAllExtensions():
function GetNonHeaderExtensions (line 81) | def GetNonHeaderExtensions():
function unicode_escape_decode (line 670) | def unicode_escape_decode(x):
function ParseNolintSuppressions (line 683) | def ParseNolintSuppressions(filename, raw_line, linenum, error):
function ProcessGlobalSuppresions (line 715) | def ProcessGlobalSuppresions(lines):
function ResetNolintSuppressions (line 733) | def ResetNolintSuppressions():
function IsErrorSuppressedByNolint (line 739) | def IsErrorSuppressedByNolint(category, linenum):
function Match (line 757) | def Match(pattern, s):
function ReplaceAll (line 767) | def ReplaceAll(pattern, rep, s):
function Search (line 785) | def Search(pattern, s):
function _IsSourceExtension (line 792) | def _IsSourceExtension(s):
class _IncludeState (line 797) | class _IncludeState(object):
method __init__ (line 832) | def __init__(self):
method FindHeader (line 838) | def FindHeader(self, header):
method ResetSection (line 853) | def ResetSection(self, directive):
method SetLastHeader (line 871) | def SetLastHeader(self, header_path):
method CanonicalizeAlphabeticalOrder (line 874) | def CanonicalizeAlphabeticalOrder(self, header_path):
method IsInAlphabeticalOrder (line 889) | def IsInAlphabeticalOrder(self, clean_lines, linenum, header_path):
method CheckNextIncludeOrder (line 910) | def CheckNextIncludeOrder(self, header_type):
class _CppLintState (line 964) | class _CppLintState(object):
method __init__ (line 967) | def __init__(self):
method SetOutputFormat (line 989) | def SetOutputFormat(self, output_format):
method SetVerboseLevel (line 993) | def SetVerboseLevel(self, level):
method SetCountingStyle (line 999) | def SetCountingStyle(self, counting_style):
method SetFilters (line 1003) | def SetFilters(self, filters):
method AddFilters (line 1021) | def AddFilters(self, filters):
method BackupFilters (line 1032) | def BackupFilters(self):
method RestoreFilters (line 1036) | def RestoreFilters(self):
method ResetErrorCounts (line 1040) | def ResetErrorCounts(self):
method IncrementErrorCount (line 1045) | def IncrementErrorCount(self, category):
method PrintErrorCounts (line 1055) | def PrintErrorCounts(self):
method PrintInfo (line 1063) | def PrintInfo(self, message):
method PrintError (line 1067) | def PrintError(self, message):
method AddJUnitFailure (line 1073) | def AddJUnitFailure(self, filename, linenum, message, category, confid...
method FormatJUnitXML (line 1077) | def FormatJUnitXML(self):
function _OutputFormat (line 1124) | def _OutputFormat():
function _SetOutputFormat (line 1129) | def _SetOutputFormat(output_format):
function _VerboseLevel (line 1134) | def _VerboseLevel():
function _SetVerboseLevel (line 1139) | def _SetVerboseLevel(level):
function _SetCountingStyle (line 1144) | def _SetCountingStyle(level):
function _Filters (line 1149) | def _Filters():
function _SetFilters (line 1154) | def _SetFilters(filters):
function _AddFilters (line 1166) | def _AddFilters(filters):
function _BackupFilters (line 1178) | def _BackupFilters():
function _RestoreFilters (line 1182) | def _RestoreFilters():
class _FunctionState (line 1186) | class _FunctionState(object):
method __init__ (line 1192) | def __init__(self):
method Begin (line 1197) | def Begin(self, function_name):
method Count (line 1207) | def Count(self):
method Check (line 1212) | def Check(self, error, filename, linenum):
method End (line 1240) | def End(self):
class _IncludeError (line 1245) | class _IncludeError(Exception):
class FileInfo (line 1250) | class FileInfo(object):
method __init__ (line 1257) | def __init__(self, filename):
method FullName (line 1260) | def FullName(self):
method RepositoryName (line 1264) | def RepositoryName(self):
method Split (line 1324) | def Split(self):
method BaseName (line 1338) | def BaseName(self):
method Extension (line 1342) | def Extension(self):
method NoExtension (line 1346) | def NoExtension(self):
method IsSource (line 1350) | def IsSource(self):
function _ShouldPrintError (line 1355) | def _ShouldPrintError(category, confidence, linenum):
function Error (line 1383) | def Error(filename, linenum, category, confidence, message):
function IsCppString (line 1441) | def IsCppString(line):
function CleanseRawStrings (line 1458) | def CleanseRawStrings(raw_lines):
function FindNextMultiLineCommentStart (line 1534) | def FindNextMultiLineCommentStart(lines, lineix):
function FindNextMultiLineCommentEnd (line 1545) | def FindNextMultiLineCommentEnd(lines, lineix):
function RemoveMultiLineCommentsFromRange (line 1554) | def RemoveMultiLineCommentsFromRange(lines, begin, end):
function RemoveMultiLineComments (line 1562) | def RemoveMultiLineComments(filename, lines, error):
function CleanseComments (line 1578) | def CleanseComments(line):
class CleansedLines (line 1594) | class CleansedLines(object):
method __init__ (line 1605) | def __init__(self, lines):
method NumLines (line 1617) | def NumLines(self):
method _CollapseStrings (line 1622) | def _CollapseStrings(elided):
function FindEndOfExpressionInLine (line 1689) | def FindEndOfExpressionInLine(line, startpos, stack):
function CloseExpression (line 1767) | def CloseExpression(clean_lines, linenum, pos):
function FindStartOfExpressionInLine (line 1811) | def FindStartOfExpressionInLine(line, endpos, stack):
function ReverseCloseExpression (line 1888) | def ReverseCloseExpression(clean_lines, linenum, pos):
function CheckForCopyright (line 1926) | def CheckForCopyright(filename, lines, error):
function GetIndentLevel (line 1939) | def GetIndentLevel(line):
function GetHeaderGuardCPPVariable (line 1955) | def GetHeaderGuardCPPVariable(filename):
function CheckForHeaderGuard (line 1986) | def CheckForHeaderGuard(filename, clean_lines, error):
function CheckHeaderFileIncluded (line 2089) | def CheckHeaderFileIncluded(filename, include_state, error):
function CheckForBadCharacters (line 2116) | def CheckForBadCharacters(filename, lines, error):
function CheckForNewlineAtEOF (line 2141) | def CheckForNewlineAtEOF(filename, lines, error):
function CheckForMultilineCommentsAndStrings (line 2159) | def CheckForMultilineCommentsAndStrings(filename, clean_lines, linenum, ...
function CheckPosixThreading (line 2227) | def CheckPosixThreading(filename, clean_lines, linenum, error):
function CheckVlogArguments (line 2253) | def CheckVlogArguments(filename, clean_lines, linenum, error):
function CheckInvalidIncrement (line 2277) | def CheckInvalidIncrement(filename, clean_lines, linenum, error):
function IsMacroDefinition (line 2299) | def IsMacroDefinition(clean_lines, linenum):
function IsForwardClassDeclaration (line 2309) | def IsForwardClassDeclaration(clean_lines, linenum):
class _BlockInfo (line 2313) | class _BlockInfo(object):
method __init__ (line 2316) | def __init__(self, linenum, seen_open_brace):
method CheckBegin (line 2323) | def CheckBegin(self, filename, clean_lines, linenum, error):
method CheckEnd (line 2338) | def CheckEnd(self, filename, clean_lines, linenum, error):
method IsBlockInfo (line 2351) | def IsBlockInfo(self):
class _ExternCInfo (line 2363) | class _ExternCInfo(_BlockInfo):
method __init__ (line 2366) | def __init__(self, linenum):
class _ClassInfo (line 2370) | class _ClassInfo(_BlockInfo):
method __init__ (line 2373) | def __init__(self, name, class_or_struct, clean_lines, linenum):
method CheckBegin (line 2403) | def CheckBegin(self, filename, clean_lines, linenum, error):
method CheckEnd (line 2408) | def CheckEnd(self, filename, clean_lines, linenum, error):
class _NamespaceInfo (line 2439) | class _NamespaceInfo(_BlockInfo):
method __init__ (line 2442) | def __init__(self, name, linenum):
method CheckEnd (line 2447) | def CheckEnd(self, filename, clean_lines, linenum, error):
class _PreprocessorInfo (line 2500) | class _PreprocessorInfo(object):
method __init__ (line 2503) | def __init__(self, stack_before_if):
class NestingState (line 2514) | class NestingState(object):
method __init__ (line 2517) | def __init__(self):
method SeenOpenBrace (line 2540) | def SeenOpenBrace(self):
method InNamespaceBody (line 2549) | def InNamespaceBody(self):
method InExternC (line 2557) | def InExternC(self):
method InClassDeclaration (line 2565) | def InClassDeclaration(self):
method InAsmBlock (line 2573) | def InAsmBlock(self):
method InTemplateArgumentList (line 2581) | def InTemplateArgumentList(self, clean_lines, linenum, pos):
method UpdatePreprocessor (line 2633) | def UpdatePreprocessor(self, line):
method Update (line 2690) | def Update(self, filename, clean_lines, linenum, error):
method InnermostClass (line 2854) | def InnermostClass(self):
method CheckCompletedBlocks (line 2866) | def CheckCompletedBlocks(self, filename, error):
function CheckForNonStandardConstructs (line 2888) | def CheckForNonStandardConstructs(filename, clean_lines, linenum,
function CheckSpacingForFunctionCall (line 3051) | def CheckSpacingForFunctionCall(filename, clean_lines, linenum, error):
function IsBlankLine (line 3128) | def IsBlankLine(line):
function CheckForNamespaceIndentation (line 3143) | def CheckForNamespaceIndentation(filename, nesting_state, clean_lines, l...
function CheckForFunctionLengths (line 3157) | def CheckForFunctionLengths(filename, clean_lines, linenum,
function CheckComment (line 3228) | def CheckComment(line, filename, linenum, next_line_start, error):
function CheckAccess (line 3282) | def CheckAccess(filename, clean_lines, linenum, nesting_state, error):
function CheckSpacing (line 3312) | def CheckSpacing(filename, clean_lines, linenum, nesting_state, error):
function CheckOperatorSpacing (line 3440) | def CheckOperatorSpacing(filename, clean_lines, linenum, error):
function CheckParenthesisSpacing (line 3555) | def CheckParenthesisSpacing(filename, clean_lines, linenum, error):
function CheckCommaSpacing (line 3593) | def CheckCommaSpacing(filename, clean_lines, linenum, error):
function _IsType (line 3629) | def _IsType(clean_lines, nesting_state, expr):
function CheckBracesSpacing (line 3692) | def CheckBracesSpacing(filename, clean_lines, linenum, nesting_state, er...
function IsDecltype (line 3781) | def IsDecltype(clean_lines, linenum, column):
function CheckSectionSpacing (line 3798) | def CheckSectionSpacing(filename, clean_lines, class_info, linenum, error):
function GetPreviousNonBlankLine (line 3853) | def GetPreviousNonBlankLine(clean_lines, linenum):
function CheckBraces (line 3876) | def CheckBraces(filename, clean_lines, linenum, error):
function CheckTrailingSemicolon (line 3995) | def CheckTrailingSemicolon(filename, clean_lines, linenum, error):
function CheckEmptyBlockBody (line 4143) | def CheckEmptyBlockBody(filename, clean_lines, linenum, error):
function FindCheckMacro (line 4247) | def FindCheckMacro(line):
function CheckCheck (line 4270) | def CheckCheck(filename, clean_lines, linenum, error):
function CheckAltTokens (line 4388) | def CheckAltTokens(filename, clean_lines, linenum, error):
function GetLineWidth (line 4420) | def GetLineWidth(line):
function CheckStyle (line 4442) | def CheckStyle(filename, clean_lines, linenum, file_extension, nesting_s...
function _DropCommonSuffixes (line 4578) | def _DropCommonSuffixes(filename):
function _ClassifyInclude (line 4608) | def _ClassifyInclude(fileinfo, include, is_system):
function CheckIncludeLine (line 4674) | def CheckIncludeLine(filename, clean_lines, linenum, include_state, error):
function _GetTextInside (line 4753) | def _GetTextInside(text, start_pattern):
function CheckLanguage (line 4838) | def CheckLanguage(filename, clean_lines, linenum, file_extension,
function CheckGlobalStatic (line 4999) | def CheckGlobalStatic(filename, clean_lines, linenum, error):
function CheckPrintf (line 5060) | def CheckPrintf(filename, clean_lines, linenum, error):
function IsDerivedFunction (line 5089) | def IsDerivedFunction(clean_lines, linenum):
function IsOutOfLineMethodDefinition (line 5111) | def IsOutOfLineMethodDefinition(clean_lines, linenum):
function IsInitializerList (line 5127) | def IsInitializerList(clean_lines, linenum):
function CheckForNonConstReference (line 5169) | def CheckForNonConstReference(filename, clean_lines, linenum,
function CheckCasts (line 5308) | def CheckCasts(filename, clean_lines, linenum, error):
function CheckCStyleCast (line 5427) | def CheckCStyleCast(filename, clean_lines, linenum, cast_type, pattern, ...
function ExpectingFunctionArgs (line 5480) | def ExpectingFunctionArgs(clean_lines, linenum):
function FilesBelongToSameModule (line 5572) | def FilesBelongToSameModule(filename_cc, filename_h):
function UpdateIncludeState (line 5630) | def UpdateIncludeState(filename, include_dict, io=codecs):
function CheckForIncludeWhatYouUse (line 5657) | def CheckForIncludeWhatYouUse(filename, clean_lines, include_state, error,
function CheckMakePairUsesDeduction (line 5761) | def CheckMakePairUsesDeduction(filename, clean_lines, linenum, error):
function CheckRedundantVirtual (line 5782) | def CheckRedundantVirtual(filename, clean_lines, linenum, error):
function CheckRedundantOverrideOrFinal (line 5846) | def CheckRedundantOverrideOrFinal(filename, clean_lines, linenum, error):
function IsBlockInNameSpace (line 5879) | def IsBlockInNameSpace(nesting_state, is_forward_declaration):
function ShouldCheckNamespaceIndentation (line 5898) | def ShouldCheckNamespaceIndentation(nesting_state, is_namespace_indent_i...
function CheckItemIndentationInNamespace (line 5931) | def CheckItemIndentationInNamespace(filename, raw_lines_no_comments, lin...
function ProcessLine (line 5939) | def ProcessLine(filename, file_extension, clean_lines, line,
function FlagCxx11Features (line 5984) | def FlagCxx11Features(filename, clean_lines, linenum, error):
function FlagCxx14Features (line 6036) | def FlagCxx14Features(filename, clean_lines, linenum, error):
function ProcessFileData (line 6055) | def ProcessFileData(filename, file_extension, lines, error,
function ProcessConfigOverrides (line 6106) | def ProcessConfigOverrides(filename):
function ProcessFile (line 6202) | def ProcessFile(filename, vlevel, extra_check_functions=None):
function PrintUsage (line 6290) | def PrintUsage(message):
function PrintCategories (line 6304) | def PrintCategories():
function ParseArguments (line 6313) | def ParseArguments(args):
function _ExpandDirectories (line 6414) | def _ExpandDirectories(filenames):
function _FilterExcludedFiles (line 6446) | def _FilterExcludedFiles(filenames):
function main (line 6453) | def main():
FILE: scripts/python/cpplint_wrap.py
function main (line 5) | def main():
function write_code_lines (line 28) | def write_code_lines(filename):
FILE: scripts/python/md-split.py
function main (line 17) | def main():
function process_code (line 82) | def process_code(read_filehandle, text_filehandle, line, linenum, source...
function clean_trailing_newlines (line 132) | def clean_trailing_newlines(linebuffer):
function write_with_harness (line 145) | def write_with_harness(codefile, sourcefile, start_linenum, linebuffer):
function is_code (line 172) | def is_code(line, indent_depth = 4):
function is_inside_code (line 178) | def is_inside_code(line, indent_depth):
function stripped (line 181) | def stripped(line):
function dedent (line 187) | def dedent(line, indent_depth):
function get_marker (line 194) | def get_marker(line):
function line_length (line 203) | def line_length(filename):
Condensed preview — 31 files, each showing path, character count, and a content snippet. Download the .json file or copy for the full structured content (1,978K chars).
[
{
"path": ".github/workflows/build.yml",
"chars": 381,
"preview": "name: build\non: [push, pull_request]\n\njobs:\n build:\n runs-on: ubuntu-latest\n\n steps:\n - uses: actions/checko"
},
{
"path": ".github/workflows/jekyll-gh-pages.yml",
"chars": 1411,
"preview": "# Sample workflow for building and deploying a Jekyll site to GitHub Pages\nname: Deploy Jekyll with GitHub Pages depende"
},
{
"path": ".gitignore",
"chars": 146,
"preview": "scripts/build\nscripts/nodesjs/build\nnode_modules\n_site\nscripts/python/__pycache__\nscripts/python/*.pyc\n\n# VS Code\n.vs/\n\n"
},
{
"path": ".travis.yml",
"chars": 391,
"preview": "# This is the configuration for Travis CI, a free github-integrated service that runs this script for each pull request "
},
{
"path": "CONTRIBUTING.md",
"chars": 6369,
"preview": "## Contributing to the C++ Core Guidelines\n\n>\"Within C++ is a smaller, simpler, safer language struggling to get out.\" \n"
},
{
"path": "CppCoreGuidelines.md",
"chars": 838416,
"preview": "# <a name=\"main\"></a>C++ Core Guidelines\n\nJul 8, 2025\n\nEditors:\n\n* [Bjarne Stroustrup](https://www.stroustrup.com)\n* [He"
},
{
"path": "LICENSE",
"chars": 2451,
"preview": "Copyright (c) Standard C++ Foundation and its contributors\n\nStandard C++ Foundation grants you a worldwide, nonexclusive"
},
{
"path": "README.md",
"chars": 4135,
"preview": "[](http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines)"
},
{
"path": "SECURITY.md",
"chars": 116,
"preview": "# Security Policy\n\n## Reporting a Vulnerability\n\nPlease report vulnerabilities, if any, to cppcg-editors@isocpp.org\n"
},
{
"path": "_config.yml",
"chars": 77,
"preview": "include: [CppCoreGuidelines.md]\nexclude: [docs, talks, Gemfile, params.json]\n"
},
{
"path": "_includes/head.html",
"chars": 2502,
"preview": "<head>\n <link href=\"http://gmpg.org/xfn/11\" rel=\"profile\">\n <meta http-equiv=\"X-UA-Compatible\" content=\"IE=edge\">\n <m"
},
{
"path": "_includes/sidebar.html",
"chars": 3858,
"preview": "<div class=\"sidebar\">\n <div class=\"container sidebar-sticky\">\n <div class=\"sidebar-about\">\n <img src=\"cpp_core_"
},
{
"path": "_layouts/default.html",
"chars": 674,
"preview": "<!DOCTYPE html>\n<html lang=\"en-us\">\n\n {% include head.html %}\n\n <body>\n\n {% include sidebar.html %}\n\n <div class"
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{
"path": "docs/dyn_array.md",
"chars": 4110,
"preview": "<!-- $ pandoc -V geometry:margin=1in -V colorlinks=true -o dyn_array.pdf dyn_array.md --> \n\n# `gsl::dyn_array<T, Allocat"
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{
"path": "docs/gsl-intro.md",
"chars": 10015,
"preview": "\n# Using the Guidelines Support Library (GSL): A Tutorial and FAQ\n\nby Herb Sutter\n\nupdated 2018-01-08\n\n\n## Overview: \"Is"
},
{
"path": "index.html",
"chars": 938,
"preview": "---\nlayout: default\n---\n\n<p class=\"sidebar-about\">This site is copy of the C++ Core Guidelines rendered for easier brow"
},
{
"path": "params.json",
"chars": 3030,
"preview": "{\"name\":\"Cppcoreguidelines\",\"tagline\":\"The C++ Core Guidelines are a set of tried-and-true guidelines, rules, and best p"
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{
"path": "public/css/custom.css",
"chars": 6751,
"preview": ".outer {\n width: 100%;\n}\n\n.inner {\n position: relative;\n max-width: 1280px;\n padding: 20px 10px;\n margin: 0 auto;\n}"
},
{
"path": "public/css/hyde.css",
"chars": 4322,
"preview": "/*\n * __ __\n * /\\ \\ /\\ \\\n * \\ \\ \\___ __ __ \\_\\ \\ __\n * \\ \\ _ `\\/\\ \\/\\ \\ /'_"
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"path": "public/css/poole.css",
"chars": 6920,
"preview": "/*\n * ___\n * /\\_ \\\n * _____ ___ ___\\//\\ \\ __\n * /\\ '__`\\ / _"
},
{
"path": "scripts/Makefile",
"chars": 5396,
"preview": "# This Makefile is supposed to run on the Travis CI server and also locally\n# it assumes the nodejs package managaer npm"
},
{
"path": "scripts/hunspell/en_US.aff",
"chars": 11344,
"preview": "SET ISO8859-1\nKEY qwertyuiop|asdfghjkl|zxcvbnm\nTRY esianrtolcdugmphbyfvkwzESIANRTOLCDUGMPHBYFVKWZ'-\nNOSUGGEST !\n\n# ordin"
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{
"path": "scripts/hunspell/en_US.dic",
"chars": 696329,
"preview": "62154\n0/nm\n0s/pt\n0th/pt\n1/n1\n1990s\n1st/p\n1th/tc\n2/nm\n2nd/p\n2th/tc\n3/nm\n3rd/p\n3th/tc\n4/nm\n4th/pt\n5/nm\n5th/pt\n6/nm\n6th/pt\n"
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{
"path": "scripts/hunspell/isocpp.dic",
"chars": 4246,
"preview": "'\n10'000\n0xFF0000\n0b0101'0101\n10x\n20x\n2D\n2K\n2ndEdition\n2RDU00001\n3rdEdition\n98's\nà\na1\na2\naa\nABA\nabi\nABIs\nAbrahams\nAbraha"
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{
"path": "scripts/nodejs/package.json",
"chars": 314,
"preview": "{\n \"name\": \"CppCoreGuidelinesCheck\",\n \"version\": \"0.0.0\",\n \"description\": \"CppCoreGuidelines Check\",\n \"priva"
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{
"path": "scripts/nodejs/remark/.remarkrc",
"chars": 999,
"preview": "{\n \"presets\": [\"lint-recommended\", \"lint-consistent\"],\n \"plugins\": {\n \"remark-lint\": {\n \"unorder"
},
{
"path": "scripts/python/Makefile.in",
"chars": 266,
"preview": ".PHONY: default\ndefault: all\n\n.PHONY: all\nall: \\\ncpplint-all\n\nCXX_SRCS := $(wildcard *.cpp)\n\n#### cpplint, check extract"
},
{
"path": "scripts/python/cpplint.py",
"chars": 249647,
"preview": "#!/usr/bin/env python\n#\n# Copyright (c) 2009 Google Inc. All rights reserved.\n#\n# Redistribution and use in source and b"
},
{
"path": "scripts/python/cpplint_wrap.py",
"chars": 1382,
"preview": "## wraps local cpplint to produce verbose output without code harness\nimport cpplint\nimport sys\n\ndef main():\n FILTERS"
},
{
"path": "scripts/python/md-split.py",
"chars": 7648,
"preview": "#! /usr/bin/env python\n\n# A script that splits a Markdown file into plain text (for spell checking) and c++ files.\n\n\nfro"
},
{
"path": "talks/README.md",
"chars": 973,
"preview": "The guidelines were introduced during a number of talks at [CppCon](http://cppcon.org) 2015.\nHere are video recordings o"
}
]
About this extraction
This page contains the full source code of the isocpp/CppCoreGuidelines GitHub repository, extracted and formatted as plain text for AI agents and large language models (LLMs). The extraction includes 31 files (1.8 MB), approximately 575.3k tokens, and a symbol index with 191 extracted functions, classes, methods, constants, and types. Use this with OpenClaw, Claude, ChatGPT, Cursor, Windsurf, or any other AI tool that accepts text input. You can copy the full output to your clipboard or download it as a .txt file.
Extracted by GitExtract — free GitHub repo to text converter for AI. Built by Nikandr Surkov.