.. _module-pw_preprocessor:

---------------
pw_preprocessor
---------------
The preprocessor module provides various helpful preprocessor macros.

Compatibility
=============
C and C++

Headers
=======
The preprocessor module provides several headers.

pw_preprocessor/arguments.h
---------------------------------
Defines macros for handling variadic arguments to function-like macros. Macros
include the following:

.. c:macro:: PW_DELEGATE_BY_ARG_COUNT(name, ...)

  Selects and invokes a macro based on the number of arguments provided. Expands
  to ``<name><arg_count>(...)``. For example,
  ``PW_DELEGATE_BY_ARG_COUNT(foo_, 1, 2, 3)`` expands to ``foo_3(1, 2, 3)``.

  This example shows how ``PW_DELEGATE_BY_ARG_COUNT`` could be used to log a
  customized message based on the number of arguments provided.

  .. code-block:: cpp

      #define ARG_PRINT(...)  PW_DELEGATE_BY_ARG_COUNT(_ARG_PRINT, __VA_ARGS__)
      #define _ARG_PRINT0(a)        LOG_INFO("nothing!")
      #define _ARG_PRINT1(a)        LOG_INFO("1 arg: %s", a)
      #define _ARG_PRINT2(a, b)     LOG_INFO("2 args: %s, %s", a, b)
      #define _ARG_PRINT3(a, b, c)  LOG_INFO("3 args: %s, %s, %s", a, b, c)

  When used, ``ARG_PRINT`` expands to the ``_ARG_PRINT#`` macro corresponding
  to the number of arguments.

  .. code-block:: cpp

      ARG_PRINT();               // Outputs: nothing!
      ARG_PRINT("a");            // Outputs: 1 arg: a
      ARG_PRINT("a", "b");       // Outputs: 2 args: a, b
      ARG_PRINT("a", "b", "c");  // Outputs: 3 args: a, b, c

.. c:macro:: PW_COMMA_ARGS(...)

  Expands to a comma followed by the arguments if any arguments are provided.
  Otherwise, expands to nothing. If the final argument is empty, it is omitted.
  This is useful when passing ``__VA_ARGS__`` to a variadic function or template
  parameter list, since it removes the extra comma when no arguments are
  provided. ``PW_COMMA_ARGS`` must NOT be used when invoking a macro from
  another macro.

  For example. ``PW_COMMA_ARGS(1, 2, 3)``, expands to ``, 1, 2, 3``, while
  ``PW_COMMA_ARGS()`` expands to nothing. ``PW_COMMA_ARGS(1, 2, )`` expands to
  ``, 1, 2``.

pw_preprocessor/boolean.h
-------------------------
Defines macros for boolean logic on literal 1s and 0s. This is useful for
situations when a literal is needed to build the name of a function or macro.

pw_preprocessor/compiler.h
--------------------------
Macros for compiler-specific features, such as attributes or builtins.

Modifying compiler diagnostics
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
``pw_preprocessor/compiler.h`` provides macros for enabling or disabling
compiler diagnostics (warnings or errors).

.. c:macro:: PW_MODIFY_DIAGNOSTICS_PUSH()

  Starts a new group of :c:macro:`PW_MODIFY_DIAGNOSTIC` statements. A
  :c:macro:`PW_MODIFY_DIAGNOSTICS_POP` statement must follow.

.. c:macro:: PW_MODIFY_DIAGNOSTICS_POP()

  :c:macro:`PW_MODIFY_DIAGNOSTIC` statements since the most recent
  :c:macro:`PW_MODIFY_DIAGNOSTICS_PUSH` no longer apply after this statement.

.. c:macro:: PW_MODIFY_DIAGNOSTIC(kind, option)

  Changes how a diagnostic (warning or error) is handled. Most commonly used to
  disable warnings. ``PW_MODIFY_DIAGNOSTIC`` should be used between
  :c:macro:`PW_MODIFY_DIAGNOSTICS_PUSH` and :c:macro:`PW_MODIFY_DIAGNOSTICS_POP`
  statements to avoid applying the modifications too broadly.

  ``kind`` may be ``warning``, ``error``, or ``ignored``.

These macros can be used to disable warnings for precise sections of code, even
a single line if necessary.

.. code-block:: c

  PW_MODIFY_DIAGNOSTICS_PUSH();
  PW_MODIFY_DIAGNOSTIC(ignored, "-Wunused-variable");

  static int this_variable_is_never_used;

  PW_MODIFY_DIAGNOSTICS_POP();

.. tip::

  :c:macro:`PW_MODIFY_DIAGNOSTIC` and related macros should rarely be used.
  Whenever possible, fix the underlying issues about which the compiler is
  warning, rather than silencing the diagnostics.

pw_preprocessor/concat.h
------------------------
Defines the ``PW_CONCAT(...)`` macro, which expands its arguments if they are
macros and token pastes the results. This can be used for building names of
classes, variables, macros, etc.

pw_preprocessor/util.h
----------------------
General purpose, useful macros.

* ``PW_ARRAY_SIZE(array)`` -- calculates the size of a C array
* ``PW_STRINGIFY(...)`` -- expands its arguments as macros and converts them to
  a string literal
* ``PW_EXTERN_C`` -- declares a name to be ``extern "C"`` in C++; expands to
  nothing in C
* ``PW_EXTERN_C_START`` / ``PW_EXTERN_C_END`` -- declares an ``extern "C" { }``
  block in C++; expands to nothing in C