WriteCompilerDetectionHeader

This module provides the function write_compiler_detection_header().

This function can be used to generate a file suitable for preprocessor inclusion which contains macros to be used in source code:

write_compiler_detection_header(
          FILE <file>
          PREFIX <prefix>
          [OUTPUT_FILES_VAR <output_files_var> OUTPUT_DIR <output_dir>]
          COMPILERS <compiler> [...]
          FEATURES <feature> [...]
          [BARE_FEATURES <feature> [...]]
          [VERSION <version>]
          [PROLOG <prolog>]
          [EPILOG <epilog>]
          [ALLOW_UNKNOWN_COMPILERS]
          [ALLOW_UNKNOWN_COMPILER_VERSIONS]
)

This generates the file <file> with macros which all have the prefix <prefix>.

By default, all content is written directly to the <file>. The OUTPUT_FILES_VAR may be specified to cause the compiler-specific content to be written to separate files. The separate files are then available in the <output_files_var> and may be consumed by the caller for installation for example. The OUTPUT_DIR specifies a relative path from the main <file> to the compiler-specific files. For example:

write_compiler_detection_header(
  FILE climbingstats_compiler_detection.h
  PREFIX ClimbingStats
  OUTPUT_FILES_VAR support_files
  OUTPUT_DIR compilers
  COMPILERS GNU Clang MSVC Intel
  FEATURES cxx_variadic_templates
)
install(FILES
  ${CMAKE_CURRENT_BINARY_DIR}/climbingstats_compiler_detection.h
  DESTINATION include
)
install(FILES
  ${support_files}
  DESTINATION include/compilers
)

VERSION may be used to specify the API version to be generated. Future versions of CMake may introduce alternative APIs. A given API is selected by any <version> value greater than or equal to the version of CMake that introduced the given API and less than the version of CMake that introduced its succeeding API. The value of the CMAKE_MINIMUM_REQUIRED_VERSION variable is used if no explicit version is specified. (As of CMake version 3.17.5 there is only one API version.)

PROLOG may be specified as text content to write at the start of the header. EPILOG may be specified as text content to write at the end of the header

At least one <compiler> and one <feature> must be listed. Compilers which are known to CMake, but not specified are detected and a preprocessor #error is generated for them. A preprocessor macro matching <PREFIX>_COMPILER_IS_<compiler> is generated for each compiler known to CMake to contain the value 0 or 1.

Possible compiler identifiers are documented with the CMAKE_<LANG>_COMPILER_ID variable. Available features in this version of CMake are listed in the CMAKE_C_KNOWN_FEATURES and CMAKE_CXX_KNOWN_FEATURES global properties. The {c,cxx}_std_* meta-features are ignored if requested.

See the cmake-compile-features(7) manual for information on compile features.

BARE_FEATURES will define the compatibility macros with the name used in newer versions of the language standard, so the code can use the new feature name unconditionally.

ALLOW_UNKNOWN_COMPILERS and ALLOW_UNKNOWN_COMPILER_VERSIONS cause the module to generate conditions that treat unknown compilers as simply lacking all features. Without these options the default behavior is to generate a #error for unknown compilers and versions.

Feature Test Macros

For each compiler, a preprocessor macro is generated matching <PREFIX>_COMPILER_IS_<compiler> which has the content either 0 or 1, depending on the compiler in use. Preprocessor macros for compiler version components are generated matching <PREFIX>_COMPILER_VERSION_MAJOR <PREFIX>_COMPILER_VERSION_MINOR and <PREFIX>_COMPILER_VERSION_PATCH containing decimal values for the corresponding compiler version components, if defined.

A preprocessor test is generated based on the compiler version denoting whether each feature is enabled. A preprocessor macro matching <PREFIX>_COMPILER_<FEATURE>, where <FEATURE> is the upper-case <feature> name, is generated to contain the value 0 or 1 depending on whether the compiler in use supports the feature:

write_compiler_detection_header(
  FILE climbingstats_compiler_detection.h
  PREFIX ClimbingStats
  COMPILERS GNU Clang AppleClang MSVC Intel
  FEATURES cxx_variadic_templates
)

#if ClimbingStats_COMPILER_CXX_VARIADIC_TEMPLATES
template<typename... T>
void someInterface(T t...) { /* ... */ }
#else
// Compatibility versions
template<typename T1>
void someInterface(T1 t1) { /* ... */ }
template<typename T1, typename T2>
void someInterface(T1 t1, T2 t2) { /* ... */ }
template<typename T1, typename T2, typename T3>
void someInterface(T1 t1, T2 t2, T3 t3) { /* ... */ }
#endif

Symbol Macros

Some additional symbol-defines are created for particular features for use as symbols which may be conditionally defined empty:

class MyClass ClimbingStats_FINAL
{
    ClimbingStats_CONSTEXPR int someInterface() { return 42; }
};

The ClimbingStats_FINAL macro will expand to final if the compiler (and its flags) support the cxx_final feature, and the ClimbingStats_CONSTEXPR macro will expand to constexpr if cxx_constexpr is supported.

If BARE_FEATURES cxx_final was given as argument the final keyword will be defined for old compilers, too.

The following features generate corresponding symbol defines and if they are available as BARE_FEATURES:

Feature	Define	Symbol	bare
c_restrict	<PREFIX>_RESTRICT	restrict	yes
cxx_constexpr	<PREFIX>_CONSTEXPR	constexpr	yes
cxx_deleted_functions	<PREFIX>_DELETED_FUNCTION	= delete
cxx_extern_templates	<PREFIX>_EXTERN_TEMPLATE	extern
cxx_final	<PREFIX>_FINAL	final	yes
cxx_noexcept	<PREFIX>_NOEXCEPT	noexcept	yes
cxx_noexcept	<PREFIX>_NOEXCEPT_EXPR(X)	noexcept(X)
cxx_override	<PREFIX>_OVERRIDE	override	yes

Compatibility Implementation Macros

Some features are suitable for wrapping in a macro with a backward compatibility implementation if the compiler does not support the feature.

When the cxx_static_assert feature is not provided by the compiler, a compatibility implementation is available via the <PREFIX>_STATIC_ASSERT(COND) and <PREFIX>_STATIC_ASSERT_MSG(COND, MSG) function-like macros. The macros expand to static_assert where that compiler feature is available, and to a compatibility implementation otherwise. In the first form, the condition is stringified in the message field of static_assert. In the second form, the message MSG is passed to the message field of static_assert, or ignored if using the backward compatibility implementation.

The cxx_attribute_deprecated feature provides a macro definition <PREFIX>_DEPRECATED, which expands to either the standard [[deprecated]] attribute or a compiler-specific decorator such as __attribute__((__deprecated__)) used by GNU compilers.

The cxx_alignas feature provides a macro definition <PREFIX>_ALIGNAS which expands to either the standard alignas decorator or a compiler-specific decorator such as __attribute__ ((__aligned__)) used by GNU compilers.

The cxx_alignof feature provides a macro definition <PREFIX>_ALIGNOF which expands to either the standard alignof decorator or a compiler-specific decorator such as __alignof__ used by GNU compilers.

Feature	Define	Symbol	bare
cxx_alignas	<PREFIX>_ALIGNAS	alignas
cxx_alignof	<PREFIX>_ALIGNOF	alignof
cxx_nullptr	<PREFIX>_NULLPTR	nullptr	yes
cxx_static_assert	<PREFIX>_STATIC_ASSERT	static_assert
cxx_static_assert	<PREFIX>_STATIC_ASSERT_MSG	static_assert
cxx_attribute_deprecated	<PREFIX>_DEPRECATED	[[deprecated]]
cxx_attribute_deprecated	<PREFIX>_DEPRECATED_MSG	[[deprecated]]
cxx_thread_local	<PREFIX>_THREAD_LOCAL	thread_local

A use-case which arises with such deprecation macros is the deprecation of an entire library. In that case, all public API in the library may be decorated with the <PREFIX>_DEPRECATED macro. This results in very noisy build output when building the library itself, so the macro may be may be defined to empty in that case when building the deprecated library:

add_library(compat_support ${srcs})
target_compile_definitions(compat_support
  PRIVATE
    CompatSupport_DEPRECATED=
)