Provided by: cmake-data_3.24.2-1ubuntu1_all 
NAME
cmake-compile-features - CMake Compile Features Reference
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 CMAKE_C_KNOWN_FEATURES, CMAKE_CUDA_KNOWN_FEATURES, and CMAKE_CXX_KNOWN_FEATURES global
properties contain all the features known to CMake, regardless of compiler support for the
feature. The CMAKE_C_COMPILE_FEATURES, CMAKE_CUDA_COMPILE_FEATURES , and
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. There are some exceptions, such as CMake using cxx_final and
cxx_override instead of the single cxx_override_control used by Clang.
Note that there are no separate compile features properties or variables for the OBJC or
OBJCXX languages. These are based off C or C++ respectively, so the properties and
variables for their corresponding base language should be used instead.
COMPILE FEATURE REQUIREMENTS
Compile feature requirements may be specified with the target_compile_features() command.
For example, if a target must be compiled with compiler support for the cxx_constexpr
feature:
add_library(mylib requires_constexpr.cpp)
target_compile_features(mylib PRIVATE cxx_constexpr)
In processing the requirement for the cxx_constexpr feature, 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
target_compile_features():
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
cmake-buildsystem(7) for more on transitive behavior of build properties and usage
requirements.
Requiring Language Standards
In projects that use a large number of commonly available features from a particular
language standard (e.g. C++ 11) one may specify a meta-feature (e.g. cxx_std_11) that
requires use of a compiler mode that is at minimum aware of that standard, but could be
greater. This is simpler than specifying all the features individually, but does not
guarantee the existence of any particular feature. Diagnosis of use of unsupported
features will be delayed until compile time.
For example, if C++ 11 features are used extensively in a project's header files, then
clients must use a compiler mode that is no less than C++ 11. This can be requested with
the code:
target_compile_features(mylib PUBLIC cxx_std_11)
In this example, CMake will ensure the compiler is invoked in a mode of at-least C++ 11
(or C++ 14, C++ 17, ...), adding flags such as -std=gnu++11 if necessary. This applies to
sources within mylib as well as any dependents (that may include headers from mylib).
NOTE:
If the compiler's default standard level is at least that of the requested feature,
CMake may omit the -std= flag. The flag may still be added if the compiler's default
extensions mode does not match the <LANG>_EXTENSIONS target property, or if the
<LANG>_STANDARD target property is set.
Availability of Compiler Extensions
The <LANG>_EXTENSIONS target property defaults to the compiler's default (see
CMAKE_<LANG>_EXTENSIONS_DEFAULT). Note that because most compilers enable extensions by
default, this may expose portability bugs in user code or in the headers of third-party
dependencies.
<LANG>_EXTENSIONS used to default to ON. See CMP0128.
OPTIONAL COMPILE FEATURES
Compile features may be preferred if available, without creating a hard requirement.
This can be achieved by not specifying features with target_compile_features() and instead
checking the compiler capabilities with preprocessor conditions in project code.
In this use-case, the project may wish to establish a particular language standard if
available from the compiler, and use preprocessor conditions to detect the features
actually available. A language standard may be established by Requiring Language
Standards using target_compile_features() with meta-features like cxx_std_11, or by
setting the CXX_STANDARD target property or CMAKE_CXX_STANDARD variable.
See also policy CMP0120 and legacy documentation on Example Usage of the deprecated
WriteCompilerDetectionHeader module.
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:
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:
template<int I1, int I2 = 0, int I3 = 0, int I4 = 0>
struct Interface
{
static int accumulate() { return I1 + I2 + I3 + I4; }
};
It may be possible to write an abstraction interface.h header containing something like:
#ifdef HAVE_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 generator expression to implement such conditions. This
may be used with the build-property commands such as target_include_directories() and
target_link_libraries() to set the appropriate buildsystem properties:
add_library(foo INTERFACE)
set(with_variadics ${CMAKE_CURRENT_SOURCE_DIR}/with_variadics)
set(no_variadics ${CMAKE_CURRENT_SOURCE_DIR}/no_variadics)
target_include_directories(foo
INTERFACE
"$<$<COMPILE_FEATURES:cxx_variadic_templates>:${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
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)
SUPPORTED COMPILERS
CMake is currently aware of the C++ standards and compile features available from the
following compiler ids as of the versions specified for each:
• AppleClang: Apple Clang for Xcode versions 4.4+.
• Clang: Clang compiler versions 2.9+.
• GNU: GNU compiler versions 4.4+.
• MSVC: Microsoft Visual Studio versions 2010+.
• SunPro: Oracle SolarisStudio versions 12.4+.
• Intel: Intel compiler versions 12.1+.
CMake is currently aware of the C standards and compile features available from the
following compiler ids as of the versions specified for each:
• all compilers and versions listed above for C++.
• GNU: GNU compiler versions 3.4+
CMake is currently aware of the C++ standards and their associated meta-features (e.g.
cxx_std_11) available from the following compiler ids as of the versions specified for
each:
• Cray: Cray Compiler Environment version 8.1+.
• Fujitsu: Fujitsu HPC compiler 4.0+.
• PGI: PGI version 12.10+.
• NVHPC: NVIDIA HPC compilers version 11.0+.
• TI: Texas Instruments compiler.
• XL: IBM XL version 10.1+.
CMake is currently aware of the C standards and their associated meta-features (e.g.
c_std_99) available from the following compiler ids as of the versions specified for each:
• all compilers and versions listed above with only meta-features for C++.
CMake is currently aware of the CUDA standards and their associated meta-features (e.g.
cuda_std_11) available from the following compiler ids as of the versions specified for
each:
• Clang: Clang compiler 5.0+.
• NVIDIA: NVIDIA nvcc compiler 7.5+.
COPYRIGHT
2000-2022 Kitware, Inc. and Contributors