cmake-compile-features(7) CMake cmake-compile-features(7)
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 ${CMAKE_CURRENT_SOURCE_DIR}/no_variadics)
target_include_directories(foo
INTERFACE
"$<$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:
o AppleClang: Apple Clang for Xcode versions 4.4+.
o Clang: Clang compiler versions 2.9+.
o GNU: GNU compiler versions 4.4+.
o MSVC: Microsoft Visual Studio versions 2010+.
o SunPro: Oracle SolarisStudio versions 12.4+.
o 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:
o all compilers and versions listed above for C++.
o 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:
o Cray: Cray Compiler Environment version 8.1+.
o Fujitsu: Fujitsu HPC compiler 4.0+.
o PGI: PGI version 12.10+.
o NVHPC: NVIDIA HPC compilers version 11.0+.
o TI: Texas Instruments compiler.
o TIClang: Texas Instruments Clang-based compilers.
o 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:
o 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:
o Clang: Clang compiler 5.0+.
o NVIDIA: NVIDIA nvcc compiler 7.5+.
LANGUAGE STANDARD FLAGS
In order to satisfy requirements specified by the
target_compile_features() command or the CMAKE_<LANG>_STANDARD
variable, CMake may pass a language standard flag to the compiler, such
as -std=c++11.
For Visual Studio Generators, CMake cannot precisely control the
placement of the language standard flag on the compiler command line.
For Ninja Generators, Makefile Generators, and Xcode, CMake places the
language standard flag just after the language-wide flags from
CMAKE_<LANG>_FLAGS and CMAKE_<LANG>_FLAGS_<CONFIG>.
Changed in version 3.26: The language standard flag is placed before
flags specified by other abstractions such as the
target_compile_options() command. Prior to CMake 3.26, the language
standard flag was placed after them.
COPYRIGHT
2000-2024 Kitware, Inc. and Contributors
3.29.3 May 15, 2024 cmake-compile-features(7)
cmake 3.29.3 - Generated Wed May 15 16:23:46 CDT 2024