Provided by: cmake-data_3.5.1-1ubuntu3_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 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 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. The are some exceptions, 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 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.
Because the 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:
#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
WriteCompilerDetectionHeader module. The module contains the
write_compiler_detection_header function which accepts parameters to control the content
of the generated header file:
write_compiler_detection_header(
FILE "${CMAKE_CURRENT_BINARY_DIR}/foo_compiler_detection.h"
PREFIX Foo
COMPILERS GNU
FEATURES
cxx_variadic_templates
)
Such a header file may be used internally in the source code of a project, and it may be
installed and used in the interface of library code.
For each feature listed in FEATURES, a preprocessor definition is created in the header
file, and defined to either 1 or 0.
Additionally, some features call for additional defines, such as the cxx_final and
cxx_override features. Rather than being used in #ifdef code, the final keyword is
abstracted by a symbol which is defined to either final, a compiler-specific equivalent,
or to empty. That way, C++ code can be written to unconditionally use the symbol, and
compiler support determines what it is expanded to:
struct Interface {
virtual void Execute() = 0;
};
struct Concrete Foo_FINAL {
void Execute() Foo_OVERRIDE;
};
In this case, Foo_FINAL will expand to final if the compiler supports the keyword, or to
empty otherwise.
In this use-case, the CMake code will wish to enable a particular language standard if
available from the compiler. The CXX_STANDARD target property variable may be set to the
desired language standard for a particular target, and the CMAKE_CXX_STANDARD may be set
to influence all following targets:
write_compiler_detection_header(
FILE "${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:
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 would be possible to write a abstraction interface.h header containing something like:
#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 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 language 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 though 6.2.
• Clang: Clang compiler versions 2.9 through 3.4.
• GNU: GNU compiler versions 4.4 through 5.0.
• MSVC: Microsoft Visual Studio versions 2010 through 2015.
• SunPro: Oracle SolarisStudio version 12.4.
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