docs/style_guide.rst

.. _chapter-style:

.. default-domain:: cpp

.. highlight:: sh

===========================
Style Guide and Conventions
===========================

.. tip::
  Pigweed runs ``pw format`` as part of ``pw presubmit`` to perform some code
  formatting checks. To speed up the review process, consider adding ``pw
  presubmit`` as a git push hook using the following command:
  ``pw presubmit --install``

---------
C++ style
---------

The Pigweed C++ style guide is closely based on Google's external C++ Style
Guide, which is found on the web at
https://google.github.io/styleguide/cppguide.html. The Google C++ Style Guide
applies to Pigweed except as described in this document.

The Pigweed style guide only applies to Pigweed itself. It does not apply to
projects that use Pigweed or to the third-party code included with Pigweed.
Non-Pigweed code is free to use features restricted by Pigweed, such as dynamic
memory allocation and the entirety of the C++ Standard Library.

Recommendations in the :doc:`embedded_cpp_guide` are considered part of the
Pigweed style guide, but are separated out since it covers more general
embedded development beyond just C++ style.

Automatic formatting
====================
Pigweed uses `clang-format <https://clang.llvm.org/docs/ClangFormat.html>`_ to
automatically format Pigweed source code. A ``.clang-format`` configuration is
provided with the Pigweed repository.

Automatic formatting is essential to facilitate large-scale, automated changes
in Pigweed. Therefore, all code in Pigweed is expected to be formatted with
``clang-format`` prior to submission. Existing code may be reformatted at any
time.

If ``clang-format`` formats code in an undesirable or incorrect way, it can be
disabled for the affected lines by adding ``// clang-format off``.
``clang-format`` must then be re-enabled with a ``// clang-format on`` comment.

.. code-block:: cpp

  // clang-format off
  constexpr int kMyMatrix[] = {
      100,  23,   0,
        0, 542,  38,
        1,   2, 201,
  };
  // clang-format on

C Standard Library
==================
In C++ headers, always use the C++ versions of C Standard Library headers (e.g.
``<cstdlib>`` instead of ``<stdlib.h>``). If the header is used by both C and
C++ code, only the C header should be used.

In C++ code, it is preferred to use C functions from the ``std`` namespace. For
example, use ``std::memcpy`` instead of ``memcpy``. The C++ standard does not
require the global namespace versions of the functions to be provided. Using
``std::`` is more consistent with the C++ Standard Library and makes it easier
to distinguish Pigweed functions from library functions.

Within core Pigweed, do not use C standard library functions that allocate
memory, such as ``std::malloc``. There are exceptions to this for when dynamic
allocation is enabled for a system; Pigweed modules are allowed to add extra
functionality when a heap is present; but this must be optional.

C++ Standard Library
====================
Much of the C++ Standard Library is not a good fit for embedded software. Many
of the classes and functions were not designed with the RAM, flash, and
performance constraints of a microcontroller in mind. For example, simply
adding the line ``#include <iostream>`` can increase the binary size by 150 KB!
This is larger than many microcontrollers' entire internal storage.

However, with appropriate caution, a limited set of standard C++ libraries can
be used to great effect. Developers can leverage familiar, well-tested
abstractions instead of writing their own. C++ library algorithms and classes
can give equivalent or better performance than hand-written C code.

A limited subset of the C++ Standard Library is permitted in Pigweed. To keep
Pigweed small, flexible, and portable, functions that allocate dynamic memory
must be avoided. Care must be exercised when using multiple instantiations of a
template function, which can lead to code bloat.

The following C++ Standard Library headers are always permitted:

  * ``<array>``
  * ``<complex>``
  * ``<initializer_list>``
  * ``<iterator>``
  * ``<limits>``
  * ``<optional>``
  * ``<random>``
  * ``<ratio>``
  * ``<span>``
  * ``<string_view>``
  * ``<tuple>``
  * ``<type_traits>``
  * ``<utility>``
  * ``<variant>``
  * C Standard Library headers (``<c*>``)

With caution, parts of the following headers can be used:

  * ``<algorithm>`` -- be wary of potential memory allocation
  * ``<atomic>`` -- not all MCUs natively support atomic operations
  * ``<bitset>`` -- conversions to or from strings are disallowed
  * ``<functional>`` -- do **not** use ``std::function``
  * ``<new>`` -- for placement new
  * ``<numeric>`` -- be wary of code size with multiple template instantiations

Never use any of these headers:

  * Dynamic containers (``<list>``, ``<map>``, ``<set>``, ``<vector>``, etc.)
  * Streams (``<iostream>``, ``<ostream>``, ``<fstream>``, etc.)
  * ``<exception>``
  * ``<future>``, ``<mutex>``, ``<thread>``
  * ``<memory>``
  * ``<regex>``
  * ``<scoped_allocator>``
  * ``<sstream>``
  * ``<stdexcept>``
  * ``<string>``
  * ``<valarray>``

Headers not listed here should be carefully evaluated before they are used.

These restrictions do not apply to third party code or to projects that use
Pigweed.

Combining C and C++
===================
Prefer to write C++ code over C code, using ``extern "C"`` for symbols that must
have C linkage. ``extern "C"`` functions should be defined within C++
namespaces to simplify referring to other code.

C++ functions with no parameters do not include ``void`` in the parameter list.
C functions with no parameters must include ``void``.

.. code-block:: cpp

  namespace pw {

  bool ThisIsACppFunction() { return true; }

  extern "C" int pw_ThisIsACFunction(void) { return -1; }

  extern "C" {

  int pw_ThisIsAlsoACFunction(void) {
    return ThisIsACppFunction() ? 100 : 0;
  }

  }  // extern "C"

  }  // namespace pw

Comments
========
Prefer C++-style (``//``) comments over C-style commments (``/* */``). C-style
comments should only be used for inline comments.

.. code-block:: cpp

  // Use C++-style comments, except where C-style comments are necessary.
  // This returns a random number using an algorithm I found on the internet.
  #define RANDOM_NUMBER() [] {                \
    return 4;  /* chosen by fair dice roll */ \
  }()

Indent code in comments with two additional spaces, making a total of three
spaces after the ``//``. All code blocks must begin and end with an empty
comment line, even if the blank comment line is the last line in the block.

.. code-block:: cpp

  // Here is an example of code in comments.
  //
  //   int indentation_spaces = 2;
  //   int total_spaces = 3;
  //
  //   engine_1.thrust = RANDOM_NUMBER() * indentation_spaces + total_spaces;
  //
  bool SomeFunction();

Control statements
==================
All loops and conditional statements must use braces.

The syntax ``while (true)`` is preferred over ``for (;;)`` for infinite loops.

Include guards
==============
The first non-comment line of every header file must be ``#pragma once``. Do
not use traditional macro include guards. The ``#pragma once`` should come
directly after the Pigweed copyright block, with no blank line, followed by a
blank, like this:

.. code-block:: cpp

  // Copyright 2020 The Pigweed Authors
  //
  // Licensed under the Apache License, Version 2.0 (the "License"); you may not
  // use this file except in compliance with the License. You may obtain a copy of
  // the License at
  //
  //     https://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
  // WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
  // License for the specific language governing permissions and limitations under
  // the License.
  #pragma once

  // Header file-level comment goes here...

Memory allocation
=================
Dynamic memory allocation can be problematic. Heap allocations and deallocations
occupy valuable CPU cycles. Memory usage becomes nondeterministic, which can
result in a system crashing without a clear culprit.

To keep Pigweed portable, core Pigweed code is not permitted to dynamically
(heap) allocate memory, such as with ``malloc`` or ``new``. All memory should be
allocated with automatic (stack) or static (global) storage duration. Pigweed
must not use C++ libraries that use dynamic allocation.

Projects that use Pigweed are free to use dynamic allocation, provided they
have selected a target that enables the heap.

Naming
======
Entities shall be named according to the `Google style guide
<https://google.github.io/styleguide/cppguide.html>`_, with the following
additional requirements.

**C++ code**
  * All Pigweed C++ code must be in the ``pw`` namespace. Namespaces for
    modules should be nested under ``pw``. For example,
    ``pw::string::Format()``.
  * Whenever possible, private code should be in a source (.cc) file and placed
    in anonymous namespace nested under ``pw``.
  * If private code must be exposed in a header file, it must be in a namespace
    nested under ``pw``. The namespace may be named for its subsystem or use a
    name that designates it as private, such as ``internal``.
  * Template arguments for non-type names (e.g. ``template <int foo_bar>``)
    should follow the variable naming convention, which means snake case (e.g.
    ``foo_bar``). This matches the Google C++ style, however the wording in the
    official style guide isn't explicit and could be interpreted to use
    ``kFooBar`` style naming. Wide practice establishes that the naming
    convention is ``snake_case``, and so that is the style we use in Pigweed.

    **Note:** At time of writing much of Pigweed incorrectly follows the
    ``kCamelCase`` naming for non-type template arguments. This is a bug that
    will be fixed eventually.

**C code**
  * Public names used by C code must be prefixed with ``pw_``.
  * If private code must be exposed in a header, private names used by C code
    must be prefixed with ``_pw_``.
  * Avoid writing C source (.c) files in Pigweed. Prefer to write C++ code with
    C linkage using ``extern "C"``. Within C source, private C functions and
    variables must be named with the ``_pw_`` prefix and should be declared
    ``static`` whenever possible; for example, ``_pw__MyPrivateFunction``.
  * The C prefix rules apply to

    * C functions (``int pw_FunctionName(void);``),
    * variables used by C code (``int pw_variable_name;``),
    * constant variables used by C code (``int pw_kConstantName;``), and
    * structs used by C code (``typedef struct {} pw_StructName;``).

    The prefix does not apply to struct members, which use normal Google style.

**Preprocessor macros**
  * Public Pigweed macros must be prefixed with ``PW_``.
  * Private Pigweed macros must be prefixed with ``_PW_``.

**Example**

.. code-block:: cpp

  namespace pw {
  namespace nested_namespace {

  // C++ names (types, variables, functions) must be in the pw namespace.
  // They are named according to the Google style guide.
  constexpr int kGlobalConstant = 123;

  // Prefer using functions over extern global variables.
  extern int global_variable;

  class Class {};

  void Function();

  extern "C" {

  // Public Pigweed code used from C must be prefixed with pw_.
  extern const int pw_kGlobalConstant;

  extern int pw_global_variable;

  void pw_Function(void);

  typedef struct {
    int member_variable;
  } pw_Struct;

  // Private Pigweed code used from C must be prefixed with _pw_.
  extern const int _pw_kPrivateGlobalConstant;

  extern int _pw_private_global_variable;

  void _pw_PrivateFunction(void);

  typedef struct {
    int member_variable;
  } _pw_PrivateStruct;

  }  // extern "C"

  // Public macros must be prefixed with PW_.
  #define PW_PUBLIC_MACRO(arg) arg

  // Private macros must be prefixed with _PW_.
  #define _PW_PRIVATE_MACRO(arg) arg

  }  // namespace nested_namespace
  }  // namespace pw

Namespace scope formatting
==========================
All non-indented blocks (namespaces, ``extern "C"`` blocks, and preprocessor
conditionals) must have a comment on their closing line with the
contents of the starting line.

All nested namespaces should be declared together with no blank lines between
them.

.. code-block:: cpp

  #include "some/header.h"

  namespace pw::nested {
  namespace {

  constexpr int kAnonConstantGoesHere = 0;

  }  // namespace

  namespace other {

  const char* SomeClass::yes = "no";

  bool ThisIsAFunction() {
  #if PW_CONFIG_IS_SET
    return true;
  #else
    return false;
  #endif  // PW_CONFIG_IS_SET
  }

  extern "C" {

  const int pw_kSomeConstant = 10;
  int pw_some_global_variable = 600;

  void pw_CFunction() { ... }

  }  // extern "C"

  }  // namespace
  }  // namespace pw::nested

Pointers and references
=======================
For pointer and reference types, place the asterisk or ampersand next to the
type.

.. code-block:: cpp

  int* const number = &that_thing;
  constexpr const char* kString = "theory!"

  bool FindTheOneRing(const Region& where_to_look) { ... }

Prefer storing references over storing pointers. Pointers are required when the
pointer can change its target or may be ``nullptr``. Otherwise, a reference or
const reference should be used. In accordance with the Google C++ style guide,
only const references are permitted as function arguments; pointers must be used
in place of mutable references when passed as function arguments.

Preprocessor macros
===================
Macros should only be used when they significantly improve upon the C++ code
they replace. Macros should make code more readable, robust, and safe, or
provide features not possible with standard C++, such as stringification, line
number capturing, or conditional compilation. When possible, use C++ constructs
like constexpr variables in place of macros. Never use macros as constants,
except when a string literal is needed or the value must be used by C code.

When macros are needed, the macros should be accompanied with extensive tests
to ensure the macros are hard to use wrong.

Stand-alone statement macros
----------------------------
Macros that are standalone statements must require the caller to terminate the
macro invocation with a semicolon. For example, the following does *not* conform
to Pigweed's macro style:

.. code-block:: cpp

  // BAD! Definition has built-in semicolon.
  #define PW_LOG_IF_BAD(mj) \
    CallSomeFunction(mj);

  // BAD! Compiles without error; semicolon is missing.
  PW_LOG_IF_BAD("foo")

Here's how to do this instead:

.. code-block:: cpp

  // GOOD; requires semicolon to compile.
  #define PW_LOG_IF_BAD(mj) \
    CallSomeFunction(mj)

  // GOOD; fails to compile due to lacking semicolon.
  PW_LOG_IF_BAD("foo")

For macros in function scope that do not already require a semicolon, the
contents can be placed in a ``do { ... } while (0)`` loop.

.. code-block:: cpp

  #define PW_LOG_IF_BAD(mj)  \
    do {                     \
      if (mj.Bad()) {        \
        Log(#mj " is bad")   \
      }                      \
    } while (0)

Standalone macros at global scope that do not already require a semicolon can
add a ``static_assert`` or throwaway struct declaration statement as their
last line.

.. code-block:: cpp

  #define PW_NEAT_THING(thing)             \
    bool IsNeat_##thing() { return true; } \
    static_assert(true, "Macros must be terminated with a semicolon")

Private macros in public headers
--------------------------------
Private macros in public headers must be prefixed with ``_PW_``, even if they
are undefined after use; this prevents collisions with downstream users. For
example:

.. code-block:: cpp

  #define _PW_MY_SPECIAL_MACRO(op) ...
  ...
  // Code that uses _PW_MY_SPECIAL_MACRO()
  ...
  #undef _PW_MY_SPECIAL_MACRO

Macros in private implementation files (.cc)
--------------------------------------------
Macros within .cc files that should only used within one file should be
undefined after their last use; for example:

.. code-block:: cpp

  #define DEFINE_OPERATOR(op) \
    T operator ## op(T x, T y) { return x op y; } \
    static_assert(true, "Macros must be terminated with a semicolon") \

  DEFINE_OPERATOR(+);
  DEFINE_OPERATOR(-);
  DEFINE_OPERATOR(/);
  DEFINE_OPERATOR(*);

  #undef DEFINE_OPERATOR

Preprocessor conditional statements
===================================
When using macros for conditional compilation, prefer to use ``#if`` over
``#ifdef``. This checks the value of the macro rather than whether it exists.

 * ``#if`` handles undefined macros equivalently to ``#ifdef``. Undefined
   macros expand to 0 in preprocessor conditional statements.
 * ``#if`` evaluates false for macros defined as 0, while ``#ifdef`` evaluates
   true.
 * Macros defined using compiler flags have a default value of 1 in GCC and
   Clang, so they work equivalently for ``#if`` and ``#ifdef``.
 * Macros defined to an empty statement cause compile-time errors in ``#if``
   statements, which avoids ambiguity about how the macro should be used.

All ``#endif`` statements should be commented with the expression from their
corresponding ``#if``. Do not indent within preprocessor conditional statements.

.. code-block:: cpp

  #if USE_64_BIT_WORD
  using Word = uint64_t;
  #else
  using Word = uint32_t;
  #endif  // USE_64_BIT_WORD

Unsigned integers
=================
Unsigned integers are permitted in Pigweed. Aim for consistency with existing
code and the C++ Standard Library. Be very careful mixing signed and unsigned
integers.

------------
Python style
------------
Pigweed uses the standard Python style: PEP8, which is available on the web at
https://www.python.org/dev/peps/pep-0008/. All Pigweed Python code should pass
``yapf`` when configured for PEP8 style.

Python 3
========
Pigweed uses Python 3. Some modules may offer limited support for Python 2, but
Python 3.6 or newer is required for most Pigweed code.

---------------
Build files: GN
---------------

Each Pigweed source module will require a build file named BUILD.gn which
encapsulates the build targets and specifies their sources and dependencies.
The format of this file is simlar in structure to the
`Bazel/Blaze format <https://docs.bazel.build/versions/3.2.0/build-ref.html>`_
(Googlers may also review `go/build-style <go/build-style>`_), but with
nomenclature specific to Pigweed. For each target specified within the build
file there are a list of depdency fields. Those fields, in their expected
order, are:

  * ``<public_config>`` -- external build configuration
  * ``<public_deps>`` -- necessary public dependencies (ie: Pigweed headers)
  * ``<public>`` -- exposed package public interface header files
  * ``<config>`` -- package build configuration
  * ``<sources>`` -- package source code
  * ``<deps>`` -- package necessary local dependencies

Assets within each field must be listed in alphabetical order

.. code-block:: cpp

  # Here is a brief example of a GN build file.

  import("$dir_pw_unit_test/test.gni")

  config("default_config") {
    include_dirs = [ "public" ]
  }

  source_set("pw_sample_module") {
    public_configs = [ ":default_config" ]
    public_deps = [
      dir_pw_span,
      dir_pw_status,
    ]
    public = [ "public/pw_sample_module/sample_module.h" ]
    sources = [
      "public/pw_sample_module/internal/sample_module.h",
      "sample_module.cc",
      "used_by_sample_module.cc",
    ]
    deps = [ dir_pw_varint ]
  }

  pw_test_group("tests") {
    tests = [ ":sample_module_test" ]
  }

  pw_test("sample_module_test") {
    sources = [ "sample_module_test.cc" ]
    deps = [ ":sample_module" ]
  }

  pw_doc_group("docs") {
    sources = [ "docs.rst" ]
  }