CMake/Help/manual/cmake-compile-features.7.rst

.. cmake-manual-description: CMake Compile Features Reference

cmake-compile-features(7)
*************************

.. only:: html

   .. contents::

Introduction
============

Project source code may depend on, or be conditional on, the availability
of certain features of the compiler.  There are three use-cases which arise:
`Compile Feature Requirements`_, `Optional Compile Features`_
and `Conditional Compilation Options`_.

While features are typically specified in programming language standards,
CMake provides a primary user interface based on granular handling of
the features, not the language standard that introduced the feature.

The :prop_gbl:`CMAKE_C_KNOWN_FEATURES` and
:prop_gbl:`CMAKE_CXX_KNOWN_FEATURES` global properties contain all the
features known to CMake, regardless of compiler support for the feature.
The :variable:`CMAKE_C_COMPILE_FEATURES` and
:variable:`CMAKE_CXX_COMPILE_FEATURES` variables contain all features
CMake knows are known to the compiler, regardless of language standard
or compile flags needed to use them.

Features known to CMake are named mostly following the same convention
as the Clang feature test macros.  The are some execptions, such as
CMake using ``cxx_final`` and ``cxx_override`` instead of the single
``cxx_override_control`` used by Clang.

Compile Feature Requirements
============================

Compile feature requirements may be specified with the
:command:`target_compile_features` command.  For example, if a target must
be compiled with compiler support for the
:prop_gbl:`cxx_constexpr <CMAKE_CXX_KNOWN_FEATURES>` feature:

.. code-block:: cmake

  add_library(mylib requires_constexpr.cpp)
  target_compile_features(mylib PRIVATE cxx_constexpr)

In processing the requirement for the ``cxx_constexpr`` feature,
:manual:`cmake(1)` will ensure that the in-use C++ compiler is capable
of the feature, and will add any necessary flags such as ``-std=gnu++11``
to the compile lines of C++ files in the ``mylib`` target.  A
``FATAL_ERROR`` is issued if the compiler is not capable of the
feature.

The exact compile flags and language standard are deliberately not part
of the user interface for this use-case.  CMake will compute the
appropriate compile flags to use by considering the features specified
for each target.

Such compile flags are added even if the compiler supports the
particular feature without the flag. For example, the GNU compiler
supports variadic templates (with a warning) even if ``-std=gnu++98`` is
used.  CMake adds the ``-std=gnu++11`` flag if ``cxx_variadic_templates``
is specified as a requirement.

In the above example, ``mylib`` requires ``cxx_constexpr`` when it
is built itself, but consumers of ``mylib`` are not required to use a
compiler which supports ``cxx_constexpr``.  If the interface of
``mylib`` does require the ``cxx_constexpr`` feature (or any other
known feature), that may be specified with the ``PUBLIC`` or
``INTERFACE`` signatures of :command:`target_compile_features`:

.. code-block:: cmake

  add_library(mylib requires_constexpr.cpp)
  # cxx_constexpr is a usage-requirement
  target_compile_features(mylib PUBLIC cxx_constexpr)

  # main.cpp will be compiled with -std=gnu++11 on GNU for cxx_constexpr.
  add_executable(myexe main.cpp)
  target_link_libraries(myexe mylib)

Feature requirements are evaluated transitively by consuming the link
implementation.  See :manual:`cmake-buildsystem(7)` for more on
transitive behavior of build properties and usage requirements.

Because the :prop_tgt:`CXX_EXTENSIONS` target property is ``ON`` by default,
CMake uses extended variants of language dialects by default, such as
``-std=gnu++11`` instead of ``-std=c++11``.  That target property may be
set to ``OFF`` to use the non-extended variant of the dialect flag.  Note
that because most compilers enable extensions by default, this could
expose cross-platform bugs in user code or in the headers of third-party
dependencies.

Optional Compile Features
=========================

Compile features may be preferred if available, without creating a hard
requirement.  For example, a library may provides alternative
implementations depending on whether the ``cxx_variadic_templates``
feature is available:

.. code-block:: c++

  #if Foo_COMPILER_CXX_VARIADIC_TEMPLATES
  template<int I, int... Is>
  struct Interface;

  template<int I>
  struct Interface<I>
  {
    static int accumulate()
    {
      return I;
    }
  };

  template<int I, int... Is>
  struct Interface
  {
    static int accumulate()
    {
      return I + Interface<Is...>::accumulate();
    }
  };
  #else
  template<int I1, int I2 = 0, int I3 = 0, int I4 = 0>
  struct Interface
  {
    static int accumulate() { return I1 + I2 + I3 + I4; }
  };
  #endif

Such an interface depends on using the correct preprocessor defines for the
compiler features.  CMake can generate a header file containing such
defines using the :module:`WriteCompilerDetectionHeader` module.  The
module contains the ``write_compiler_detection_header`` function which
accepts parameters to control the content of the generated header file:

.. code-block:: cmake

  write_compiler_detection_header(
    FILE "${CMAKE_CURRENT_BINARY_DIR}/foo_compiler_detection.h"
    PREFIX Foo
    COMPILERS GNU
    FEATURES
      cxx_variadic_templates
  )

Such a header file may be used internally in the source code of a project,
and it may be installed and used in the interface of library code.

For each feature listed in ``FEATURES``, a preprocessor definition
is created in the header file, and defined to either ``1`` or ``0``.

Additionally, some features call for additional defines, such as the
``cxx_final`` and ``cxx_override`` features. Rather than being used in
``#ifdef`` code, the ``final`` keyword is abstracted by a symbol
which is defined to either ``final``, a compiler-specific equivalent, or
to empty.  That way, C++ code can be written to unconditionally use the
symbol, and compiler support determines what it is expanded to:

.. code-block:: c++

  struct Interface {
    virtual void Execute() = 0;
  };

  struct Concrete Foo_FINAL {
    void Execute() Foo_OVERRIDE;
  };

In this case, ``Foo_FINAL`` will expand to ``final`` if the
compiler supports the keyword, or to empty otherwise.

In this use-case, the CMake code will wish to enable a particular language
standard if available from the compiler. The :prop_tgt:`CXX_STANDARD`
target property variable may be set to the desired language standard
for a particular target, and the :variable:`CMAKE_CXX_STANDARD` may be
set to influence all following targets:

.. code-block:: cmake

  write_compiler_detection_header(
    FILE "${CMAKE_CURRENT_BINARY_DIR}/foo_compiler_detection.h"
    PREFIX Foo
    COMPILERS GNU
    FEATURES
      cxx_final cxx_override
  )

  # Includes foo_compiler_detection.h and uses the Foo_FINAL symbol
  # which will expand to 'final' if the compiler supports the requested
  # CXX_STANDARD.
  add_library(foo foo.cpp)
  set_property(TARGET foo PROPERTY CXX_STANDARD 11)

  # Includes foo_compiler_detection.h and uses the Foo_FINAL symbol
  # which will expand to 'final' if the compiler supports the feature,
  # even though CXX_STANDARD is not set explicitly.  The requirement of
  # cxx_constexpr causes CMake to set CXX_STANDARD internally, which
  # affects the compile flags.
  add_library(foo_impl foo_impl.cpp)
  target_compile_features(foo_impl PRIVATE cxx_constexpr)

The ``write_compiler_detection_header`` function also creates compatibility
code for other features which have standard equivalents.  For example, the
``cxx_static_assert`` feature is emulated with a template and abstracted
via the ``<PREFIX>_STATIC_ASSERT`` and ``<PREFIX>_STATIC_ASSERT_MSG``
function-macros.

Conditional Compilation Options
===============================

Libraries may provide entirely different header files depending on
requested compiler features.

For example, a header at ``with_variadics/interface.h`` may contain:

.. code-block:: c++

  template<int I, int... Is>
  struct Interface;

  template<int I>
  struct Interface<I>
  {
    static int accumulate()
    {
      return I;
    }
  };

  template<int I, int... Is>
  struct Interface
  {
    static int accumulate()
    {
      return I + Interface<Is...>::accumulate();
    }
  };

while a header at ``no_variadics/interface.h`` may contain:

.. code-block:: c++

  template<int I1, int I2 = 0, int I3 = 0, int I4 = 0>
  struct Interface
  {
    static int accumulate() { return I1 + I2 + I3 + I4; }
  };

It would be possible to write a abstraction ``interface.h`` header
containing something like:

.. code-block:: c++

  #include "foo_compiler_detection.h"
  #if Foo_COMPILER_CXX_VARIADIC_TEMPLATES
  #include "with_variadics/interface.h"
  #else
  #include "no_variadics/interface.h"
  #endif

However this could be unmaintainable if there are many files to
abstract. What is needed is to use alternative include directories
depending on the compiler capabilities.

CMake provides a ``COMPILE_FEATURES``
:manual:`generator expression <cmake-generator-expressions(7)>` to implement
such conditions.  This may be used with the build-property commands such as
:command:`target_include_directories` and :command:`target_link_libraries`
to set the appropriate :manual:`buildsystem <cmake-buildsystem(7)>`
properties:

.. code-block:: cmake

  add_library(foo INTERFACE)
  set(with_variadics ${CMAKE_CURRENT_SOURCE_DIR}/with_variadics)
  set(no_variadics ${CMAKE_CURRENT_SOURCE_DIR}/no_variadics)
  target_link_libraries(foo
    INTERFACE
      "$<$<COMPILE_FEATURES:cxx_variadic_templates>:${with_variadics}>"
      "$<$<NOT:$<COMPILE_FEATURES:cxx_variadic_templates>>:${no_variadics}>"
    )

Consuming code then simply links to the ``foo`` target as usual and uses
the feature-appropriate include directory

.. code-block:: cmake

  add_executable(consumer_with consumer_with.cpp)
  target_link_libraries(consumer_with foo)
  set_property(TARGET consumer_with CXX_STANDARD 11)

  add_executable(consumer_no consumer_no.cpp)
  target_link_libraries(consumer_no foo)