# CheckCXXCompilerFlag

Check once whether the CXX compiler supports a given flag.

check_cxx_compiler_flag

      check_cxx_compiler_flag(<flag> <var>)

Check once that the `<flag>` is accepted by the compiler without a diagnostic. The result is stored in the internal cache variable specified by `<resultVar>`, with boolean `true` for success and boolean `false` for failure.

`true` indicates only that the compiler did not issue a diagnostic message when given the flag. Whether the flag has any effect is beyond the scope of this module.

Internally, `try_compile()` is used to perform the check. If `CMAKE_TRY_COMPILE_TARGET_TYPE` is set to `EXECUTABLE` (default), the check compiles and links an executable program. If set to `STATIC_LIBRARY`, the check is compiled but not linked.

See also `check_compiler_flag()` for a more general command syntax.

The compile and link commands can be influenced by setting any of the following variables prior to calling `check_cxx_compiler_flag()`. Unknown flags in these variables can case a false negative result.

> `CMAKE_REQUIRED_FLAGS`

>   String of additional flags to pass to the compiler. The string must be space-delimited--a ;-list will not work. The contents of `CMAKE_<LANG>_FLAGS` and its associated configuration-specific variable are automatically added to the compiler command before the contents of `CMAKE_REQUIRED_FLAGS`.

> `CMAKE_REQUIRED_DEFINITIONS`

>   A ;-list of compiler definitions of the form `-DFOO` or `-DFOO=bar`. A definition for the name specified by `<resultVar>` will also be added automatically.

> `CMAKE_REQUIRED_INCLUDES`

>   A ;-list of header search paths to pass to the compiler. These will be the only header search paths used--the contents of the `INCLUDE_DIRECTORIES` directory property will be ignored.

> `CMAKE_REQUIRED_LINK_OPTIONS`

>   Added in version 3.14.

>   A ;-list of options to add to the link command (see `try_compile()` for further details).

> `CMAKE_REQUIRED_LIBRARIES`

>   A ;-list of libraries to add to the link command. These can be the name of system libraries or they can be Imported Targets (see `try_compile()` for further details).

> `CMAKE_REQUIRED_LINK_DIRECTORIES`

>   Added in version 3.31.

>   A ;-list of libraries search paths to pass to the linker (see `try_compile()` for further details).

> `CMAKE_REQUIRED_QUIET`

>   Added in version 3.1.

>   If this variable evaluates to a boolean true value, all status messages associated with the check will be suppressed.