# C++ style guide

The Fuchsia project follows the public [Google C++ style guide][google-guide], with a
few exceptions.

Using [clang-format][clang-format] is a good practice as it ensures your code is in
compliance with the style guide. Tricium checks in Gerrit also use clang-format as a
non-gating linter. However, you may still manually format code as long as it complies
with these guidelines.

## Fuchsia-specific style

### TODO comments

The Google C++ style guide requires referencing a bug number in TODO comments.
On Fuchsia this is done in the format: `TODO(https://fxbug.dev/42074368)`.

### Compilation flags

Please do not add `-Wall` or `-Wextra`.

```gn
source_set("my_sources") {
  ...
  cflags = [
    # Don't do this!
    "-Wall",
  ]
}
```

This flag is already added in a centralized place in the build, followed by
additional flags that suppress certain warnings globally.

Occasionally new compiler warnings are added upstream. In order to roll the
latest compiler, global maintainers may opt to temporarily suppress a
newly-introduced warning in a centralized place while we work through a backlog
of fixing instances of this warning throughout the codebase. If your project
uses `-Wall` then it may break with a Clang roll.

Do feel free to enable/disable specific warnings that are not set globally.
If these warnings are set globally later in a manner that's congruent with your
own preferences then it's a good idea to remove any local overrides.

```gn
source_set("my_sources") {
  ...
  cflags = [
    # We don't want any write-only params/vars in our code.
    # TODO(https://fxbug.dev/42133916): delete the below when these warnings are
    # enabled globally.
    "-Wunused-but-set-parameter",
    "-Wunused-but-set-variable",
  ]
}
```

## Exceptions

### Line length

Fuchsia uses 100 columns instead of 80.

### Braces

Always use braces `{ }` when the contents of a block are more than one line.
This is something you need to watch since Clang-format doesn't know to add
these.

```cpp
// Don't do this.
while (!done)
  doSomethingWithAReallyLongLine(
       wrapped_arg);

// Correct.
while (!done) {
  doSomethingWithAReallyLongLine(
       wrapped_arg);
}
```


### Conditionals and loops

Do not use spaces inside parentheses (the Google style guide discourages but
permits this).

Do not use the single-line form for short conditionals and loops (the Google
style guide permits both forms):

```cpp
// Don't do this:
if (x == kFoo) return new Foo();

// Correct.
if (x == kFoo) {
  return new Foo;
}
```

### Namespace names

* Nested namespaces are forbidden, with the following exceptions:
  - `internal` (when required to hide implementation details of templated code)
  - code generated by the FIDL compiler
* The following top-level namespaces are forbidden:
  - `internal`
  - `fuchsia` (except in code generated by the FIDL compiler)
* Namespaces in IDK libraries must be kept to as short a list as possible.
  A later document will provide an explicit list of allowed namespaces; in the
  meantime, new namespaces should be introduced thoughtfully.
* Namespaces in non-IDK libraries should also be chosen so as to reduce the risk
  of clashes. Very generic nouns (eg. `media`) are best avoided.

Rationale: [Tip of the Week #130: Namespace Naming][totw-130]

[clang-format]: https://clang.llvm.org/docs/ClangFormat.html
[google-guide]: https://google.github.io/styleguide/cppguide.html
[totw-130]: https://abseil.io/tips/130

### Includes

* If the header being included is a system, global, or library header (see
  [Naming C/C++ objects](naming.md) for precise definitions), use
  `<angle brackets>` and the complete name of the header. These headers are
  considered "C library headers" for the purposes of the Google C++ Style
  Guide:

  ```cpp
  #include <zircon/syscalls.h>           // System header
  #include <fuchsia/io/cpp/fidl.h>       // Global header
  #include <lib/fdio/fd.h>               // Library header
  ```

* If the header being included is a implementation header, use `"quotes"` and
  use the full path to the header from the root of the source tree. These
  headers are considered "your project's headers" for the purposes of the
  Google C++ Style Guide:

  ```cpp
  #include "src/ui/scenic/bin/app.h"     // Implementation header
  ```

* Third-party headers can be included using the root-relative path (e.g.
  `#include "third_party/skia/include/core/SkPaint.h"`) or using their canonical header
  names (e.g. `#include <gtest/gtest.h>`).