# Chromium C++ style guide
_For other languages, please see the
[Chromium style guides](https://chromium.googlesource.com/chromium/src/+/main/styleguide/styleguide.md)._
Chromium follows the [Google C++ Style
Guide](https://google.github.io/styleguide/cppguide.html) unless an exception
is listed below.
A checkout should give you
[clang-format](https://chromium.googlesource.com/chromium/src/+/main/docs/clang_format.md)
to automatically format C++ code. By policy, Clang's formatting of code should
always be accepted in code reviews.
You can propose changes to this style guide by sending an email to
`[email protected]`. Ideally, the list will arrive at some consensus and you can
request review for a change to this file. If there's no consensus,
`src/styleguide/c++/OWNERS` get to decide.
Blink code in `third_party/blink` uses [Blink style](blink-c++.md).
## Modern C++ features
Google and Chromium style
[targets C++20](https://google.github.io/styleguide/cppguide.html#C++_Version).
Additionally, some features of supported C++ versions remain forbidden. The
status of Chromium's C++ support is covered in more detail in
[Modern C++ use in Chromium](c++-features.md).
## Naming
* "Chromium" is the name of the project, not the product, and should never
appear in code, variable names, API names etc. Use "Chrome" instead.
## Tests and Test-only Code
* Functions used only for testing should be restricted to test-only usages
with the testing suffixes supported by
[PRESUBMIT.py](https://chromium.googlesource.com/chromium/src/+/main/PRESUBMIT.py).
`ForTesting` is the conventional suffix although similar patterns, such as
`ForTest`, are also accepted. These suffixes are checked at presubmit time
to ensure the functions are called only by test files. In the rare case of
adding a test-only code path to an area where a testing suffix is not
possible, CHECK_IS_TEST() may be appropriate.
* Classes used only for testing should be in a GN build target that is
marked `testonly=true`. Tests can depend on such targets, but production
code can not.
* While test files generally appear alongside the production code they test,
support code for `testonly` targets should be placed in a `test/` subdirectory.
For example, see `//mojo/core/core_unittest.cc` and
`//mojo/core/test/mojo_test_base.cc`. For test classes used across multiple
directories, it might make sense to move them into a nested `test` namespace for
clarity.
* Despite the Google C++ style guide
[deprecating](https://google.github.io/styleguide/cppguide.html#File_Names)
the `_unittest.cc` suffix for unit test files, in Chromium we still use this
suffix to distinguish unit tests from browser tests, which are written in
files with the `_browsertest.cc` suffix.
## Code formatting
* Put `*` and `&` by the type rather than the variable name.
* In class declarations, group function overrides together within each access
control section, with one labeled group per parent class.
* Prefer `(foo == 0)` to `(0 == foo)`.
* Use `{}` on all conditionals/loops.
## Unnamed namespaces
Items local to a .cc file should be wrapped in an unnamed namespace. While some
such items are already file-scope by default in C++, not all are; also, shared
objects on Linux builds export all symbols, so unnamed namespaces (which
restrict these symbols to the compilation unit) improve function call cost and
reduce the size of entry point tables.
## Exporting symbols
Symbols can be exported (made visible outside of a shared library/DLL) by
annotating with a `<COMPONENT>_EXPORT` macro name (where `<COMPONENT>` is the
name of the component being built, e.g. BASE, NET, CONTENT, etc.). Class
annotations should precede the class name:
```c++
class FOO_EXPORT Foo {
void Bar();
void Baz();
// ...
};
```
Function annotations should precede the return type:
```c++
class FooSingleton {
FOO_EXPORT Foo& GetFoo();
FOO_EXPORT Foo& SetFooForTesting(Foo* foo);
void SetFoo(Foo* foo); // Not exported.
};
```
## Multiple inheritance
Multiple inheritance and virtual inheritance are permitted in Chromium code,
but discouraged (beyond the "interface" style of inheritance allowed by the
Google style guide, for which we do not require classes to have the "Interface"
suffix). Consider whether composition could solve the problem instead.
## Inline functions
Simple accessors should generally be the only inline functions. These should be
named using `snake_case()`. Virtual functions should never be declared this way.
## Logging
Remove all logging before checking in code. The exception is temporary logging
to track down a specific bug. This should be a rare exception, and you should
have a plan for how to manually collect/use the logged data. Afterwards you
should remove the logging. Note that logs are not present in crashes. Use
`base::debug::ScopedCrashKeyString`
([link](https://chromium.googlesource.com/chromium/src/+/main/base/debug/crash_logging.h))
for that.
For the rare case when logging needs to stay in the codebase for a while,
prefer `DVLOG(1)` to other logging methods. This avoids bloating the release
executable and in debug can be selectively enabled at runtime by command-line
arguments:
* `--v=n` sets the global log level to n (default 0). All log statements with
a log level less than or equal to the global level will be printed.
* `--vmodule=mod=n[,mod=n,...]` overrides the global log level for the module
mod. Supplying the string foo for mod will affect all files named foo.cc,
while supplying a wildcard like `*bar/baz*` will affect all files with
`bar/baz` in their full pathnames.
Rationale:
* Logging is expensive: binary size, runtime.
* Logging quickly loses utility as more components emit logs: too much noise,
not enough signal.
* Logging is often used to document impossible edge cases which should be
enforced with CHECKs. The latter makes it easier to reason about the code, and
can result in more performant binaries.
## Platform-specific code
To `#ifdef` code for specific platforms, use the macros defined in
`build/build_config.h` and in the Chromium build config files, not other macros
set by specific compilers or build environments (e.g. `WIN32`).
Place platform-specific #includes in their own section below the "normal"
`#includes`. Repeat the standard `#include` order within this section:
```c++
#include "foo/foo.h"
#include <stdint.h>
#include <algorithm>
#include "base/strings/utf_string_conversions.h"
#include "build/build_config.h"
#include "chrome/common/render_messages.h"
#if BUILDFLAG(IS_WIN)
#include <windows.h>
#include "base/win/com_init_util.h"
#elif BUILDFLAG(IS_POSIX)
#include "base/posix/global_descriptors.h"
#endif
```
## Types
* Refer to the [Mojo style
guide](https://chromium.googlesource.com/chromium/src/+/main/docs/security/mojo.md)
when working with types that will be passed across network or process
boundaries. For example, explicitly-sized integral types must be used for
safety, since the sending and receiving ends may not have been compiled
with the same sizes for things like `int` and `size_t`.
* Use `size_t` for object and allocation sizes, object counts, array and
pointer offsets, vector indices, and so on. This prevents casts when
dealing with STL APIs, and if followed consistently across the codebase,
minimizes casts elsewhere.
* Occasionally classes may have a good reason to use a type other than
`size_t` for one of these concepts, e.g. as a storage space optimization. In
these cases, continue to use `size_t` in public-facing function
declarations, and continue to use unsigned types internally (e.g.
`uint32_t`).
* Follow the [integer semantics
guide](https://chromium.googlesource.com/chromium/src/+/main/docs/security/integer-semantics.md)
for all arithmetic conversions and calculations used in memory management
or passed across network or process boundaries. In other circumstances,
follow [Google C++ casting
conventions](https://google.github.io/styleguide/cppguide.html#Casting)
to convert arithmetic types when you know the conversion is safe. Use
`checked_cast<T>` (from `base/numerics/safe_conversions.h`) when you need to
`CHECK` that the source value is in range for the destination type. Use
`saturated_cast<T>` if you instead wish to clamp out-of-range values.
`CheckedNumeric` is an ergonomic way to perform safe arithmetic and casting
in many cases.
* The Google Style Guide [bans
UTF-16](https://google.github.io/styleguide/cppguide.html#Non-ASCII_Characters).
For various reasons, Chromium uses UTF-16 extensively. Use `std::u16string`
and `char16_t*` for 16-bit strings, `u"..."` to declare UTF-16 literals, and
either the actual characters or the `\uXXXX` or `\UXXXXXXXX` escapes for
Unicode characters. Avoid `\xXX...`-style escapes, which can cause subtle
problems if someone attempts to change the type of string that holds the
literal. In code used only on Windows, it may be necessary to use
`std::wstring` and `wchar_t*`; these are legal, but note that they are
distinct types and are often not 16-bit on other platforms.
## Object ownership and calling conventions
When functions need to take raw or smart pointers as parameters, use the
following conventions. Here we refer to the parameter type as `T` and name as
`t`.
* If the function does not modify `t`'s ownership, declare the param as `T*`.
The caller is expected to ensure `t` stays alive as long as necessary,
generally through the duration of the call. Exception: In rare cases (e.g.
using lambdas with STL algorithms over containers of `unique_ptr<>`s), you
may be forced to declare the param as `const std::unique_ptr<T>&`. Do this
only when required.
* If the function takes ownership of a non-refcounted object, declare the
param as `std::unique_ptr<T>`.
* If the function (at least sometimes) takes a ref on a refcounted object,
declare the param as `scoped_refptr<T>`. The caller can decide
whether it wishes to transfer ownership (by calling `std::move(t)` when
passing `t`) or retain its ref (by simply passing t directly).
* In short, functions should never take ownership of parameters passed as raw
pointers, and there should rarely be a need to pass smart pointers by const
ref.
Conventions for return values are similar with an important distinction:
* Return raw pointers if-and-only-if the caller does not take ownership.
* Return `std::unique_ptr<T>` or `scoped_refptr<T>` by value when the impl is
handing off ownership.
* **Distinction**: Return `const scoped_refptr<T>&` when the impl retains
ownership so the caller isn't required to take a ref: this avoids bumping
the reference count if the caller doesn't need ownership and also
[helps binary size](https://crrev.com/c/1435627)).
A great deal of Chromium code predates the above rules. In particular, some
functions take ownership of params passed as `T*`, or take `const
scoped_refptr<T>&` instead of `T*`, or return `T*` instead of
`scoped_refptr<T>` (to avoid refcount churn pre-C++11). Try to clean up such
code when you find it, or at least not make such usage any more widespread.
## Non-owning pointers in class fields
Use `const raw_ref<T>` or `raw_ptr<T>` for class and struct fields in place of a
raw C++ reference `T&` or pointer `T*` whenever possible, except in paths that include
`/renderer/` or `blink/public/web/`. These are non-owning smart pointers that
have improved memory-safety over raw pointers and references, and can prevent
exploitation of a significant percentage of Use-after-Free bugs.
Prefer `const raw_ref<T>` whenever the held pointer will never be null, and it's
ok to drop the `const` if the internal reference can be reassigned to point to a
different `T`. Use `raw_ptr<T>` in order to express that the pointer _can_ be
null. Only `raw_ptr<T>` can be default-constructed, since `raw_ref<T>` disallows
nullness.
Using `raw_ref<T>` or `raw_ptr<T>` may not be possible in rare cases for
[performance reasons](../../base/memory/raw_ptr.md#Performance). Additionally,
`raw_ptr<T>` doesn’t support some C++ scenarios (e.g. `constexpr`, ObjC
pointers). Tooling will help to encourage use of these types in the future. See
[raw_ptr.md](../../base/memory/raw_ptr.md#When-to-use-raw_ptr_T) for how to add
exclusions.
## thread_local variables
Much code in Chrome needs to be "sequence-aware" rather than "thread-aware". If
you need a sequence-local variable, see
[`base::SequenceLocalStorageSlot`](../../base/threading/sequence_local_storage_slot.h).
If you truly need a thread-local variable, then you can use a `thread_local`, as
long as it complies with the following requirements:
* Its type must satisfy `std::is_trivially_destructible_v<T>`, due to past
problems with "spooky action at a distance" during destruction. Note that
`raw_ptr<T>` is not a trivially-destructible type and may not be contained
in `thread_locals`.
* It must not be exported (e.g. via `COMPONENT_EXPORT`), since this may result
in codegen bugs on Mac; and at least on Windows, this probably won't compile
in the component build anyway. As a workaround, create an exported getter
function that creates a `thread_local` internally and returns a ref to it.
* If it lives at class/namespace scope, it must be marked `ABSL_CONST_INIT`,
as specified in
[the Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html#thread_local).
* It must not be constructed inside OOM handlers or any other code that cannot
allocate memory, since on POSIX, construction may alloc.
If you can't comply with these requirements, consider
[`base::ThreadLocalOwnedPointer`](../../base/threading/thread_local.h) or
another nearby low-level utility.
## Forward declarations vs. #includes
Unlike the Google style guide, Chromium style prefers forward declarations to
`#includes` where possible. This can reduce compile times and result in fewer
files needing recompilation when a header changes.
You can and should use forward declarations for most types passed or returned
by value, reference, or pointer, or types stored as pointer members or in most
STL containers. However, if it would otherwise make sense to use a type as a
member by-value, don't convert it to a pointer just to be able to
forward-declare the type.
## File headers
All files in Chromium start with a common license header. That header should
look like this:
```c++
// Copyright $YEAR The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
```
Some important notes about this header:
* `$YEAR` should be set to the current year at the time a file is created, and
not changed thereafter.
* For files specific to ChromiumOS, replace the word Chromium with the phrase
ChromiumOS.
* The Chromium project hosts mirrors of some upstream open-source projects.
When contributing to these portions of the repository, retain the existing
file headers.
Use standard `#include` guards in all header files (see the Google style guide
sections on these for the naming convention). Do not use `#pragma once`;
historically it was not supported on all platforms, and it does not seem to
outperform #include guards even on platforms which do support it.
## CHECK(), DCHECK() and NOTREACHED()
Use the `CHECK()` family of macros to both document and verify invariants.
* Exception: If the invariant is known to be too expensive to verify in
production, you may fall back to `DCHECK()`. Do not do this unless
necessary.
* Exception: If your pre-stable coverage is too small to prevent a stability
risk once `CHECK()`s hit stable, and failure doesn't obviously result in a
crash or security risk, you may use `CHECK(Foo(),
base::NotFatalUntil::M120)` with a future milestone to gather non-fatal
diagnostics in stable before automatically turning fatal in a later
milestone.
* Historically, Chromium code used `DCHECK()` in most cases, so a great deal
of existing code uses `DCHECK()` instead of `CHECK()`. You are encouraged
to migrate to `CHECK()`s with a trailing `base::NotFatalUntil::M120`
argument, as there's stability risk given the under-tested invariant, or add
a comment explaining why DCHECK is appropriate given the current guidance.
Use `NOTREACHED()` to indicate a piece of code is unreachable. Control flow does
not leave this call, so there should be no executable statements after it (even
return statements from non-void functions). The compiler will issue dead-code
warnings.
* Prefer to unconditionally `CHECK()` instead of conditionally hitting a
`NOTREACHED()`, where feasible.
* Exception: If your pre-stable coverage is too small to prevent a stability
risk once `NOTREACHED()`s hit stable, and failure doesn't obviously
result in a crash or security risk, you may use `NOTREACHED(
base::NotFatalUntil::M120)` with a future milestone to gather non-fatal
diagnostics in stable before automatically turning fatal in a later
milestone.
Use `base::ImmediateCrash()` in the rare case where it's necessary to terminate
the current process for reasons outside its control, that are not violations of
our invariants.
Use `base::debug::DumpWithoutCrashing()` to generate a crash report but keep
running in the case where you are investigating some failure but know that it's
safe to continue execution.
Use `DLOG(FATAL)` (does nothing in production) or `LOG(DFATAL)` (logs an error
and continues running in production) if you need to log an error in tests from
production code. From test code, use `ADD_FAILURE()` directly. Do not use these
for invariant failures. Those should use `CHECK()` or `NOTREACHED()` as noted
above.
For more details, see [checks.md](checks.md).
## Test-only code paths in production code
Try to avoid test-only code paths in production code. Such code paths make
production code behave differently in tests. This makes both tests and
production code hard to reason about. Consider dependency injection, fake
classes, etc to avoid such code paths.
However, if a test-only path in production code cannot be avoided, instrument
that code path with `CHECK_IS_TEST();` to assert that the code is only run in
tests.
```c++
// `profile_manager` may not be available in tests.
if (!profile_manager) {
CHECK_IS_TEST();
return std::string();
}
```
`CHECK_IS_TEST();` will crash outside of tests. This asserts that the test-only
code path is not accidentally or maliciously taken in production.
## Miscellany
* Use UTF-8 file encodings and LF line endings.
* Unit tests and performance tests should be placed in the same directory as
the functionality they're testing.
* The [C++ Dos and Don'ts](c++-dos-and-donts.md) page has more helpful
information.
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