# 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.

### 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 SDK 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-SDK 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/ledger/bin/filesystem/detached_path.h"
  ```

* 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>`).

4. *Exception:* In `//zircon`, headers in the same target can use relative
   paths (e.g., `#include "private.h"`). This exception exists because Zircon's
   previous build system did not distinguish between public and private headers.
   We might revise this rule if we start distinguishing public and private
   headers in Zircon in the future.