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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Google C++ Style Guide for Drake</title>
<link rel="stylesheet" href="include/styleguide.css">
<script src="include/styleguide.js"></script>
<link rel="shortcut icon" href="https://drake.mit.edu/_static/favicon.ico">
</head>
<body onload="initStyleGuide();">
<div id="content">
<h1>Google C++ Style Guide for Drake</h1>
<div class="horizontal_toc" id="tocDiv"></div>
<h2 id="Background" class="ignoreLink">Background</h2>
<p class="drake">Forked and adapted from the
<a href="https://google.github.io/styleguide/cppguide.html">Google style
guide</a>. Many references to Google are deliberately left in to minimize
merge conflicts.</p>
<p>C++ is one of the main development languages used by
many of Google's open-source projects. As every C++
programmer knows, the language has many powerful features, but
this power brings with it complexity, which in turn can make
code more bug-prone and harder to read and maintain.</p>
<p>The goal of this guide is to manage this complexity by
describing in detail the dos and don'ts of writing C++
code. These rules exist to
keep the codebase manageable while still allowing
coders to use C++ language features productively.</p>
<p><em>Style</em>, also known as readability, is what we call
the conventions that govern our C++ code. The term Style is a
bit of a misnomer, since these conventions cover far more than
just source file formatting.</p>
<p>
Most code in Drake conforms to the requirements in this guide. Where
Drake-specific rules contradict previous Google conventions, the text is
retained for reference but otherwise marked <span class="nondrake">with a
strikethrough</span>. The style is intentionally difficult to read in the
normal course of perusal. Copying-and-pasting, or even simply highlighting the
text, will facilitate reading if necessary.
Rules specific to Drake <span class="drake">are highlighted like this</span> for
clarity and easier maintainability.
</p>
<p class="drake">
Drake uses <a href="https://github.com/abseil/abseil-cpp">Abseil</a> only in
<code>*.cc</code> files and never in headers; please ignore any suggestions or
rules that regard Abseil.
</p>
<p>Note that this guide is not a C++ tutorial: we assume that
the reader is familiar with the language. </p>
<h3 id="Goals">Goals of the Style Guide</h3>
<p>Why do we have this document?</p>
<p>There are a few core goals that we believe this guide should
serve. These are the fundamental <b>why</b>s that
underlie all of the individual rules. By bringing these ideas to
the fore, we hope to ground discussions and make it clearer to our
broader community why the rules are in place and why particular
decisions have been made. If you understand what goals each rule is
serving, it should be clearer to everyone when a rule may be waived
(some can be), and what sort of argument or alternative would be
necessary to change a rule in the guide.</p>
<p>The goals of the style guide as we currently see them are as follows:</p>
<dl>
<dt>Style rules should pull their weight</dt>
<dd>The benefit of a style rule
must be large enough to justify asking all of our engineers to
remember it. The benefit is measured relative to the codebase we would
get without the rule, so a rule against a very harmful practice may
still have a small benefit if people are unlikely to do it
anyway. This principle mostly explains the rules we don't have, rather
than the rules we do: for example, <code>goto</code> contravenes many
of the following principles, but is already vanishingly rare, so the Style
Guide doesn't discuss it.</dd>
<dt>Optimize for the reader, not the writer</dt>
<dd>Our codebase (and most individual components submitted to it) is
expected to continue for quite some time. As a result, more time will
be spent reading most of our code than writing it. We explicitly
choose to optimize for the experience of our average software engineer
reading, maintaining, and debugging code in our codebase rather than
ease when writing said code. "Leave a trace for the reader" is a
particularly common sub-point of this principle: When something
surprising or unusual is happening in a snippet of code (for example,
transfer of pointer ownership), leaving textual hints for the reader
at the point of use is valuable (<code>std::unique_ptr</code>
demonstrates the ownership transfer unambiguously at the call
site). </dd>
<dt>Be consistent with existing code</dt>
<dd>Using one style consistently through our codebase lets us focus on
other (more important) issues. Consistency also allows for automation:
tools that format your code or adjust your <code>#include</code>s only
work properly when your code is consistent with the expectations of
the tooling. In many cases, rules that are attributed to "Be
Consistent" boil down to "Just pick one and stop worrying about it";
the potential value of allowing flexibility on these points is
outweighed by the cost of having people argue over them. However,
there are limits to consistency; it is a good tie breaker when there
is no clear technical argument, nor a long-term direction. It applies
more heavily locally (per file, or for a tightly-related set of
interfaces). Consistency should not generally be used as a
justification to do things in an old style without considering the
benefits of the new style, or the tendency of the codebase to converge
on newer styles over time.</dd>
<dt>Be consistent with the broader C++ community when appropriate</dt>
<dd>Consistency with the way other organizations use C++ has value for
the same reasons as consistency within our codebase. If a feature in
the C++ standard solves a problem, or if some idiom is widely known
and accepted, that's an argument for using it. However, sometimes
standard features and idioms are flawed, or were just designed without
our codebase's needs in mind. In those cases (as described below) it's
appropriate to constrain or ban standard features. In some cases we
prefer a homegrown or third-party library over a library defined in
the C++ Standard, either out of perceived superiority or insufficient
value to transition the codebase to the standard interface.</dd>
<dt>Avoid surprising or dangerous constructs</dt>
<dd>C++ has features that are more surprising or dangerous than one
might think at a glance. Some style guide restrictions are in place to
prevent falling into these pitfalls. There is a high bar for style
guide waivers on such restrictions, because waiving such rules often
directly risks compromising program correctness.
</dd>
<dt>Avoid constructs that our average C++ programmer would find tricky
or hard to maintain</dt>
<dd>C++ has features that may not be generally appropriate because of
the complexity they introduce to the code. In widely used
code, it may be more acceptable to use
trickier language constructs, because any benefits of more complex
implementation are multiplied widely by usage, and the cost in understanding
the complexity does not need to be paid again when working with new
portions of the codebase. When in doubt, waivers to rules of this type
can be sought by asking
your project leads. This is specifically
important for our codebase because code ownership and team membership
changes over time: even if everyone that works with some piece of code
currently understands it, such understanding is not guaranteed to hold a
few years from now.</dd>
<dt>Be mindful of our scale</dt>
<dd>With a codebase of 100+ million lines and thousands of engineers,
some mistakes and simplifications for one engineer can become costly
for many. For instance it's particularly important to
avoid polluting the global namespace: name collisions across a
codebase of hundreds of millions of lines are difficult to work with
and hard to avoid if everyone puts things into the global
namespace.</dd>
<dt>Concede to optimization when necessary</dt>
<dd>Performance optimizations can sometimes be necessary and
appropriate, even when they conflict with the other principles of this
document.</dd>
</dl>
<p>The intent of this document is to provide maximal guidance with
reasonable restriction. As always, common sense and good taste should
prevail. By this we specifically refer to the established conventions
of the entire Google C++ community, not just your personal preferences
or those of your team. Be skeptical about and reluctant to use
clever or unusual constructs: the absence of a prohibition is not the
same as a license to proceed. Use your judgment, and if you are
unsure, please don't hesitate to ask your project leads to get additional
input.</p>
<h2 id="C++_Version">C++ Version</h2>
<p class="drake">Code may use any C++ features supported by Drake's continuous
integration test suite. See <a href="/installation.html">Installation</a> for
a basic overview of the targeted compiler and C++ language versions.</p>
<div class="nondrake">
<p>Currently, code should target C++20, i.e., should not use C++23
features. The C++ version targeted by this guide will advance
(aggressively) over time.</p>
<p>Do not use
<a href="#Nonstandard_Extensions">non-standard extensions</a>.</p>
<div>
<p>Consider portability to other environments before using features
from C++17 and C++20 in your project.</p>
</div>
</div>
<h2 id="Header_Files">Header Files</h2>
<p>In general, every <code>.cc</code> file should have an
associated <code>.h</code> file. There are some common
exceptions, such as unit tests and small <code>.cc</code> files containing
just a <code>main()</code> function.</p>
<p>Correct use of header files can make a huge difference to
the readability, size and performance of your code.</p>
<p>The following rules will guide you through the various
pitfalls of using header files.</p>
<a id="The_-inl.h_Files"></a>
<h3 id="Self_contained_Headers">Self-contained Headers</h3>
<p>Header files should be self-contained (compile on their own) and
end in <code>.h</code>. Non-header files that are meant for inclusion
should end in <code>.inc</code> and be used sparingly.</p>
<p>All header files should be self-contained. Users and refactoring
tools should not have to adhere to special conditions to include the
header. Specifically, a header should
have <a href="#The__define_Guard">header guards</a> and include all
other headers it needs.</p>
<p>When a header declares inline functions or templates that clients of the
header will instantiate, the inline functions and templates must also have
definitions in the header, either directly or in files it includes. Do not move
these definitions to separately included header (<code>-inl.h</code>) files;
this practice was common in the past, but is no longer allowed. When all
instantiations of a template occur in one <code>.cc</code> file, either because
they're <a href="https://en.cppreference.com/w/cpp/language/class_template#Explicit_instantiation">
explicit</a> or because the definition is accessible to only
the <code>.cc</code> file, the template definition can be kept in that file.</p>
<p>There are rare cases where a file designed to be included is not
self-contained. These are typically intended to be included at unusual
locations, such as the middle of another file. They might not
use <a href="#The__define_Guard">header guards</a>, and might not include
their prerequisites. Name such files with the <code>.inc</code>
extension. Use sparingly, and prefer self-contained headers when
possible.</p>
<h3 id="The__pragma_once_Guard">The #pragma once Guard</h3>
<p>All header files should have <code class="drake">#pragma once</code><code class="nondrake">#define</code> guards to
prevent multiple inclusion.
<span class="nondrake">The format of the symbol name
should be
<code><i><PROJECT></i>_<i><PATH></i>_<i><FILE></i>_H_</code>.</p>
</span>
<p>To guarantee uniqueness, they should
be based on the full path in a project's source tree. For
example, the file <code>foo/src/bar/baz.h</code> in
project <code>foo</code> should have the following
guard:</p>
<pre class="goodcode">#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_
...
#endif // FOO_BAR_BAZ_H_
</pre>
<p class="drake">
The advantages of using #pragma once are (1) it does not need to be updated
each time the name of the header file changes, and (2) it prevents silly
mistakes that occur when a developer copy-pastes a header file and forgets
to update its #define guard. The known drawbacks of using #pragma once,
namely the possibility of lack of compiler support and
compiler-dependent-behavior, is mitigated since Drake has
a <a href="/developers.html#supported-configurations">limited
set of officially supported platform configurations</a> on which correct
behavior will be
guaranteed. (<a href="https://github.com/RobotLocomotion/drake/issues/2104">#2104</a>)
</p>
<h3 id="Include_What_You_Use">Include What You Use</h3>
<p>If a source or header file refers to a symbol defined elsewhere,
the file should
<span class="nondrake">directly</span>
include a header file which properly intends
to provide a declaration or definition of that symbol. It should not
include header files for any other reason.
</p>
<div class="nondrake">
<p>Do not rely on transitive inclusions. This allows people to remove
no-longer-needed <code>#include</code> statements from their headers without
breaking clients. This also applies to related headers
- <code>foo.cc</code> should include <code>bar.h</code> if it uses a
symbol from it even if <code>foo.h</code>
includes <code>bar.h</code>.</p>
</div>
<p class="drake"><a name="iwyu_exception"/>
While we encourage you to Include What You Use (IWYU) since it improves code
transparency and readability, it will not be strictly enforced. Instead, we
enforce a “weak include” style that simply requires every symbol referenced
within a file be covered by the transitive closure of all #include
statements in the file. We decided to make this exception because (1) we can
always adopt an IWYU rule later, (2) to reduce verbosity, and (3) we don’t
have a tool to enforce IWYU at this time.
</p>
<p class="drake">
In particular, we do not expect a <code>foo.cc</code> file to repeat any
<code>#include</code> statements from its associated <code>foo.h</code>
header file.
</p>
<h3 id="Forward_Declarations">Forward Declarations</h3>
<p>Avoid using forward declarations where possible.
Instead, <a href="#Include_What_You_Use">include the headers you need</a>.
</p>
<p class="definition"></p>
<p>A "forward declaration" is a declaration of an entity
without an associated definition.</p>
<pre class="neutralcode">// In a C++ source file:
class B;
void FuncInB();
extern int variable_in_b;
ABSL_DECLARE_FLAG(flag_in_b);
</pre>
<p class="pros"></p>
<ul>
<li>Forward declarations can save compile time, as
<code>#include</code>s force the compiler to open
more files and process more input.</li>
<li>Forward declarations can save on unnecessary
recompilation. <code>#include</code>s can force
your code to be recompiled more often, due to unrelated
changes in the header.</li>
</ul>
<p class="cons"></p>
<ul>
<li>Forward declarations can hide a dependency, allowing
user code to skip necessary recompilation when headers
change.</li>
<li>A forward declaration as opposed to an <code>#include</code> statement
makes it difficult for automatic tooling to discover the module
defining the symbol.</li>
<li>A forward declaration may be broken by subsequent
changes to the library. Forward declarations of functions
and templates can prevent the header owners from making
otherwise-compatible changes to their APIs, such as
widening a parameter type, adding a template parameter
with a default value, or migrating to a new namespace.</li>
<li>Forward declaring symbols from namespace
<code>std::</code> yields undefined behavior.</li>
<li>It can be difficult to determine whether a forward
declaration or a full <code>#include</code> is needed.
Replacing an <code>#include</code> with a forward
declaration can silently change the meaning of
code:
<pre class="neutralcode">// b.h:
struct B {};
struct D : B {};
// good_user.cc:
#include "b.h"
void f(B*);
void f(void*);
void test(D* x) { f(x); } // Calls f(B*)
</pre>
If the <code>#include</code> was replaced with forward
decls for <code>B</code> and <code>D</code>,
<code>test()</code> would call <code>f(void*)</code>.
</li>
<li>Forward declaring multiple symbols from a header
can be more verbose than simply
<code>#include</code>ing the header.</li>
<li>Structuring code to enable forward declarations
(e.g., using pointer members instead of object members)
can make the code slower and more complex.</li>
</ul>
<p class="decision"></p>
<p>Try to avoid forward declarations of entities
defined in another project.</p>
<a id="Inline_Functions"></a>
<h3 id="Defining_Functions_in_Header_Files">Defining Functions in Header Files</h3>
<p>Include the definition of a function at its point of declaration in a header file only
when the definition is short. If the definition otherwise has a reason be in the header,
put it in an internal part of the file. If necessary to make the definition ODR-safe, mark
it with an <code>inline</code> specifier.</p>
<p class="definition"></p>
<p>Functions defined in header files are sometimes referred to as "inline functions",
which is a somewhat overloaded term that refers to several distinct but overlapping
situations:</p>
<ol>
<li>A <em>textually inline</em> symbol's definition is exposed to the reader at
the point of declaration.</li>
<li>A function or variable defined in a header file is <em>expandable inline</em>
since its definition is available for
<a href="https://en.wikipedia.org/wiki/Inline_expansion">inline expansion</a>
by the compiler, which can lead to more efficient object code.</li>
<li><em>ODR-safe</em> entities do not violate the
<a href="https://en.cppreference.com/w/cpp/language/definition">"One Definition Rule"</a>,
which often requires the <a href="https://en.cppreference.com/w/cpp/language/inline">inline</a>
keyword for things defined in header files
.</li>
</ol>
<p>While functions tend to be a more common source of confusion, these definitions apply to
variables as well, and so do the rules here.</p>
<p class="pros"></p>
<ul>
<li>Defining a function textually in-line reduces boilerplate code for simple functions
like accessors and mutators.</li>
<li> As noted above, function definitions in header files <i>can</i> lead to
more efficient object code for small functions due to inline expansion by the
compiler.</li>
<li>Function templates and <code>constexpr</code> functions generally need to
be defined in the header file that declares them (but not necessarily the public part).
<p class="drake">Note that
<a href="https://en.cppreference.com/w/cpp/language/class_template#Explicit_instantiation">
explicit template instantiation</a> enables the use of templated classes or
functions without defining them in the header file.</p>
</li>
</ul>
<p class="cons"></p>
<ul>
<li>Embedding a function definition in the public API makes the API harder to skim,
and incurs cognitive overhead for readers of that API- the more complex the function
the higher the cost.</li>
<li>Public definitions expose implementation details that are at best harmless and
often extraneous<span class="drake">, and can lead to bloated compilation times</span>.</li>
</ul>
<p class="decision"></p>
<p>Only define a function at its public declaration if it is short, say, 10 lines or fewer. Put
longer function bodies in the <code>.cc</code> file unless they must be in the header for
performance or technical reasons.</p>
<p>Even if a definition must be in the header, this is not a sufficient reason to put it within
the public part. Instead, the definition can be in an internal part of the header, such as the
<a href="#Access_Control">private</a> section of a class, within a namespace that includes the word
<code>internal</code>, or below a comment like <code>// Implementation details only
below here</code>.</p>
<p>Once a definition is in a header file, it must be ODR-safe by having the <code>inline</code>
specifier or being implicitly specified inline by being a function template or defined in a class
body when first declared.</p>
<pre class="goodcode">template <typename T>
class Foo {
public:
int bar() { return bar_; }
void MethodWithHugeBody();
private:
int bar_;
};
// Implementation details only below here
template <typename T>
void Foo<T>::MethodWithHugeBody() {
...
}
</pre>
<div class="drake">
<p>The fundamental premise of this section is that function definitions should
customarily be placed into <code>*.cc</code> files; only if there is an acute
reason to put it in a header may it appear there.
</p>
<p>You are permitted to define a function in a header file only:
<ol>
<li>if the function is a
<a href="#Function_Names"><code>snake_case</code></a>
getter or setter, especially in cases where the function generates no
object code; or</li>
<li>for constructors if all of the member fields themselves are cheap to
construct (e.g., primitive types, strings, structs of primitives, etc.);
or</li>
<li>for destructors if all of the member fields themselves are cheap to
destruct (note that the <code>std::</code> collection class templates
are <em>not</em> cheap to destruct),
<ol type="a">
<li>and the destructor is non-virtual; or</li>
</ol></li>
<li>if required by an open-ended template (e.g., the template class
<code>drake::Value<T></code> can wrap any kind of <code>T</code>,
so its definitions cannot move to the <code>*.cc</code> file); or
<li>if the function call overhead is plausibly performance-sensitive when
measured against the cost of the function as a whole,
<ol type="a">
<li>and the function is non-virtual,</li>
<li>and the function is not recursive,</li>
<li>and the function is "small" (~10 lines).</li>
</ol></li>
</ol></p>
<p>For cases (1)..(4) our custom <em>is</em> to inline the function in the
header file.</p>
<p>For inline constructors and destructors (2)..(3), it's often simpler to
allow the implicitly-generated
<a href="https://en.cppreference.com/w/cpp/language/default_constructor#Implicitly-declared_default_constructor">constructor</a>
or
<a href="https://en.cppreference.com/w/cpp/language/destructor#Implicitly-declared_destructor">destructor</a>
be used, without writing it out.
</p>
<p>Drake's assertion-like macros
<code>DRAKE_DEMAND</code>,
<code>DRAKE_ASSERT</code>, and
<code>DRAKE_THROW_UNLESS</code>
are carefully crafted with an eye to object code size and therefore are
valid to call within an inline function. Use of those macros is not a
sufficient reason on its own to force a function to be non-inline.
In contrast, throwing an exception with
<code>if (condition) { throw std::logic_error(fmt::format(...)); }</code>
is usually a huge amount of object code and should not appear inline.
</p>
<p>Drake's macro for copyable classes
<code>DRAKE_DEFAULT_COPY_AND_MOVE_AND_ASSIGN</code>
might bend the rules on function inlining when applied to classes with
non-trivial member fields (e.g., <code>std::</code> collection types).
That is not a sufficient reason to avoid the macro and declare the
special operations one by one. If we find that the macro is too bloaty,
we will provide alternative macro; declaring the operations by hand is
too brittle.
</p>
</div>
<h4 id="Inline_Functions_Scalars">Inline functions for classes templated on
a scalar type</h4>
<p class="drake">The use of a numerical scalar class template is not a
sufficient reason on its own to place a method definition in a header file.
When a class is templated only on its numerical scalar type, Drake provides the
<a href="https://drake.mit.edu/doxygen_cxx/group__default__scalars.html">
default scalars</a> macros to declare and define the template instantiations
that allow placing definitions into the <tt>*.cc</tt> file. These macros should
be used whenever practical, and the class's method definitions should be placed
into the <tt>*.cc</tt> file in most cases, consistent with the above rules of
thumb.</p>
<p class="drake">Example header file:</p>
<pre class="drake">#include "drake/common/default_scalars.h"
namespace sample {
template <typename T>
class MySystem final : public LeafSystem<T> {
public:
// Expensive functions such as this constructor or destructor are not inlined.
MySystem();
~MySystem() final;
// Short and inexpensive accessors can be inlined.
int size() { return get_output_port(0).size(); }
private:
// This calculation function is also expensive, and therefore isn't inlined.
void CalcFoo(const Context<T>& context, BasicVector<T>* output);
};
} // namespace sample
DRAKE_DECLARE_CLASS_TEMPLATE_INSTANTIATIONS_ON_DEFAULT_SCALARS(
class ::sample::MySystem)
</pre>
<p class="drake">Example cc file:</p>
<pre class="drake">#include "my_system.h"
namespace sample {
// Even though the constructor appears to be short, by virtue of implicitly
// calling the LeafSystem base class constructor it fails to meet the 10-line
// rule of thumb.
template <typename T>
MySystem<T>::MySystem() = default;
// Ditto for the destructor.
template <typename T>
MySystem<T>::~MySystem() = default;
template <typename T>
void MySystem<T>::CalcFoo(
const Context<T>& context,
BasicVector<T>* output) {
// ... (10+ lines long, etc.) ...
}
} // namespace sample
DRAKE_DEFINE_CLASS_TEMPLATE_INSTANTIATIONS_ON_DEFAULT_SCALARS(
class ::sample::MySystem)
</pre>
<h3 id="Names_and_Order_of_Includes">Names and Order of Includes</h3>
<p>Include headers in the following order: Related header, C system headers,
C++ standard library headers,
other libraries' headers, your project's
headers.</p>
<p class="drake">Separate each category of #include statements with a blank
line. Then, run <a href="#clang-format">clang-format</a>.</p>
<p>
All of a project's header files should be
listed as descendants of the project's source
directory without use of UNIX directory aliases
<code>.</code> (the current directory) or <code>..</code>
(the parent directory). For example,
<code>google-awesome-project/src/base/logging.h</code>
should be included as:</p>
<pre class="goodcode">#include "base/logging.h"
</pre>
<div class="nondrake">
<p>Headers should only be included using an angle-bracketed path if the library
requires you to do so. In particular, the following headers require angle
brackets:</p>
<ul>
<li>C and C++ standard library headers (e.g., <code><stdlib.h></code>
and <code><string></code>).</li>
<li>POSIX, Linux, and Windows system headers (e.g., <code><unistd.h></code>
and <code><windows.h></code>).</li>
<li>In rare cases, third_party libraries (e.g., <code><Python.h></code>).</li>
</ul>
</div>
<p>In <code><var>dir/foo</var>.cc</code> or
<code><var>dir/foo_test</var>.cc</code>, whose main
purpose is to implement or test the stuff in
<code><var>dir2/foo2</var>.h</code>, order your includes
as follows:</p>
<ol>
<li><code><var>dir2/foo2</var>.h</code>.</li>
<li>A blank line</li>
<li>C system headers, and any other headers in angle brackets with the
<code>.h</code> extension, e.g., <code><unistd.h></code>,
<code><stdlib.h></code>, <code><Python.h></code>.</li>
<li>A blank line</li>
<li>C++ standard library headers (without file extension), e.g.,
<code><algorithm></code>, <code><cstddef></code>.</li>
<li>A blank line</li>
<div>
<li>Other libraries' <code>.h</code> files.</li>
<li>A blank line</li>
</div>
<li>
Your project's <code>.h</code>
files.</li>
</ol>
<p>Separate each non-empty group with one blank line.</p>
<p>With the preferred ordering, if the related header
<code><var>dir2/foo2</var>.h</code> omits any necessary
includes, the build of <code><var>dir/foo</var>.cc</code>
or <code><var>dir/foo</var>_test.cc</code> will break.
Thus, this rule ensures that build breaks show up first
for the people working on these files, not for innocent
people in other packages.</p>
<p><code><var>dir/foo</var>.cc</code> and
<code><var>dir2/foo2</var>.h</code> are usually in the same
directory (e.g., <code>base/basictypes_test.cc</code> and
<code>base/basictypes.h</code>), but may sometimes be in different
directories too.</p>
<p>Note that the C headers such as <code>stddef.h</code>
are essentially interchangeable with their C++ counterparts
(<code>cstddef</code>).
<span class="nondrake">Either style is acceptable</span>
<span class="drake">Prefer <code>cstddef</code></span>, but prefer consistency
with existing code.</p>
<p>Within each section the includes should be ordered
alphabetically. Note that older code might not conform to
this rule and should be fixed when convenient.</p>
<p>For example, the includes in
<code>google-awesome-project/src/foo/internal/fooserver.cc</code>
might look like this:</p>
<pre class="goodcode">#include "foo/server/fooserver.h"
#include <sys/types.h>
#include <unistd.h>
#include <string>
#include <vector>
#include "base/basictypes.h"
#include "foo/server/bar.h"
#include <third_party/absl/flags/flag.h>
</pre>
<p><b>Exception:</b></p>
<p>Sometimes, system-specific code needs
conditional includes. Such code can put conditional
includes after other includes. Of course, keep your
system-specific code small and localized. Example:</p>
<pre class="goodcode">#include "foo/public/fooserver.h"
#ifdef _WIN32
#include <windows.h>
#endif // _WIN32
</pre>
<h2 id="Scoping">Scoping</h2>
<h3 id="Namespaces">Namespaces</h3>
<p>With few exceptions, place code in a namespace. Namespaces
should have unique names based on the project name, and possibly
its path. Do not use <i>using-directives</i> (e.g.,
<code>using namespace foo</code>). Do not use
inline namespaces. For unnamed namespaces, see
<a href="#Internal_Linkage">Internal Linkage</a>.
</p><p class="definition"></p>
<p>Namespaces subdivide the global scope
into distinct, named scopes, and so are useful for preventing
name collisions in the global scope.</p>
<p class="pros"></p>
<p>Namespaces provide a method for preventing name conflicts
in large programs while allowing most code to use reasonably
short names.</p>
<p>For example, if two different projects have a class
<code>Foo</code> in the global scope, these symbols may
collide at compile time or at runtime. If each project
places their code in a namespace, <code>project1::Foo</code>
and <code>project2::Foo</code> are now distinct symbols that
do not collide, and code within each project's namespace
can continue to refer to <code>Foo</code> without the prefix.</p>
<p>Inline namespaces automatically place their names in
the enclosing scope. Consider the following snippet, for
example:</p>
<pre class="neutralcode">namespace outer {
inline namespace inner {
void foo();
} // namespace inner
} // namespace outer
</pre>
<p>The expressions <code>outer::inner::foo()</code> and
<code>outer::foo()</code> are interchangeable. Inline
namespaces are primarily intended for ABI compatibility
across versions.</p>
<p class="cons"></p>
<p>Namespaces can be confusing, because they complicate
the mechanics of figuring out what definition a name refers
to.</p>
<p>Inline namespaces, in particular, can be confusing
because names aren't actually restricted to the namespace
where they are declared. They are only useful as part of
some larger versioning policy.</p>
<p>In some contexts, it's necessary to repeatedly refer to
symbols by their fully-qualified names. For deeply-nested
namespaces, this can add a lot of clutter.</p>
<p class="decision"></p>
<p>Namespaces should be used as follows:</p>
<ul>
<li>Follow the rules on <a href="#Namespace_Names">Namespace Names</a>.
</li><li>Terminate multi-line namespaces with comments as shown in the given examples.
</li><li>
<p>Namespaces wrap the entire source file after
includes,
<a href="https://gflags.github.io/gflags/">
gflags</a> definitions/declarations
and forward declarations of classes from other namespaces.</p>
<pre class="goodcode">// In the .h file
namespace mynamespace {
// All declarations are within the namespace scope.
// Notice the lack of indentation.
class MyClass {
public:
...
void Foo();
};
} // namespace mynamespace
</pre>
<pre class="goodcode">// In the .cc file
namespace mynamespace {
// Definition of functions is within scope of the namespace.
void MyClass::Foo() {
...
}
} // namespace mynamespace
</pre>
<p>More complex <code>.cc</code> files might have additional details,
like flags or using-declarations.</p>
<pre class="goodcode">#include "a.h"
ABSL_FLAG(bool, someflag, false, "a flag");
namespace mynamespace {
using ::foo::Bar;
...code for mynamespace... // Code goes against the left margin.
} // namespace mynamespace
</pre>
</li>
<li>To place generated protocol
message code in a namespace, use the
<code>package</code> specifier in the
<code>.proto</code> file. See
<a href="https://protobuf.dev/reference/cpp/cpp-generated/#package">
Protocol Buffer Packages</a>
for details.</li>
<li>Do not declare anything in namespace
<code>std</code>, including forward declarations of
standard library classes. Declaring entities in
namespace <code>std</code> is undefined behavior, i.e.,
not portable. To declare entities from the standard
library, include the appropriate header file.</li>
<li><p>You may not use a <i>using-directive</i>
to make all names from a namespace available.</p>
<pre class="badcode">// Forbidden -- This pollutes the namespace.
using namespace foo;
</pre>
</li>
<li><p>Do not use <i>Namespace aliases</i> at namespace scope
in header files except in explicitly marked
internal-only namespaces, because anything imported into a namespace
in a header file becomes part of the public API exported by that file.
Namespace aliases can be used when those conditions don't apply, but
they must have <a href="#Naming_Aliases">appropriate names</a>.</p>
<pre class="goodcode">// In a .h file, an alias must not be a separate API, or must be hidden in an
// implementation detail.
namespace librarian {
namespace internal { // Internal, not part of the API.
namespace sidetable = ::pipeline_diagnostics::sidetable;
} // namespace internal
inline void my_inline_function() {
// Local to a function.
namespace baz = ::foo::bar::baz;
...
}
} // namespace librarian
</pre>
<pre class="goodcode">// Remove uninteresting parts of some commonly used names in .cc files.
namespace sidetable = ::pipeline_diagnostics::sidetable;
</pre>
</li><li>Do not use inline namespaces.</li>
<li><p>Use namespaces with "internal" in the name to document parts of an API that
should not be mentioned by users of the API.
</p>
<pre class="badcode">// We shouldn't use this internal name in non-absl code.
using ::absl::container_internal::ImplementationDetail;
</pre>
<p>Note that there is still a risk of collision between libraries within a
nested <code>internal</code> namespace, so give each library within a
namespace a unique internal namespace by adding the library's
filename. For example, <code>gshoe/widget.h</code> would use
<code>gshoe::internal_widget</code> as opposed to just
<code>gshoe::internal</code>.</p>
</li>
<li><p>Single-line nested namespace declarations
are preferred in new code, but are not required.</p>
</li>
</ul>
<a id="Unnamed_Namespaces_and_Static_Variables"></a>
<h3 id="Internal_Linkage">Internal Linkage</h3>
<p>When definitions in a <code>.cc</code> file do not need to be
referenced outside that file, give them internal linkage by placing
them in an unnamed namespace or declaring them <code>static</code>.
Do not use either of these constructs in <code>.h</code> files.
</p><p class="definition"></p>
<p>All declarations can be given internal linkage by placing them in unnamed
namespaces. Functions and variables can also be given internal linkage by
declaring them <code>static</code>. This means that anything you're declaring
can't be accessed from another file. If a different file declares something with
the same name, then the two entities are completely independent.</p>
<p class="decision"></p>
<p>Use of internal linkage in <code>.cc</code> files is encouraged
for all code that does not need to be referenced elsewhere.
Do not use internal linkage in <code>.h</code> files.</p>
<p>Format unnamed namespaces like named namespaces. In the
terminating comment, leave the namespace name empty:</p>
<pre class="goodcode">namespace {
...
} // namespace
</pre>
<h3 id="Nonmember,_Static_Member,_and_Global_Functions">Nonmember, Static Member, and Global Functions</h3>
<p>Prefer placing nonmember functions in a namespace; use completely global
functions rarely. Do not use a class simply to group static members. Static
methods of a class should generally be closely related to instances of the
class or the class's static data.</p>
<p class="pros"></p>
<p>Nonmember and static member functions can be useful in
some situations. Putting nonmember functions in a