检测 CMake 中生成器表达式的存在

Question

以下问题的用例是部署的宏包，因此输入完全是用户定义的。

考虑以下最小示例：

function(foo)
    cmake_parse_arguments(FOO "" "POSSIBLY_RELATIVE_POSSIBLY_GENEX_PATH" "" ${ARGN})
    # Chosen as an example for a necessary step which doesn't have a generator expression equivalent
    get_filename_component(
        ABSOLUTE_PATH
        "${FOO_POSSIBLY_RELATIVE_POSSIBLY_GENEX_PATH}"
        REALPATH
        BASE_DIR
            "${CMAKE_CURRENT_SOURCE_DIR}"
    )
    # Compatible with generator expressions by itself
    add_custom_command(
        OUTPUT
            ${MY_OUTPUT}
        COMMAND
            ${MY_TOOL}
            ${ABSOLUTE_PATH}
    )
endfunction()

add_custom_command itself supports generator expressions, but the need to go via get_filename_component to handle one class of input (relative paths, for brevity in calling code), and the use of that intermediate step being impossible for the other class of input (build type dependent generator expressions, which get scrambled by get_filename_component) clashes.

get_filename_component effectively can't be used whenever POSSIBLY_RELATIVE_POSSIBLY_GENEX_PATH had contained any genex components. get_filename_component has no generator expression equivalent either, so substitution of that logic isn't possible.

Is there a robust way to detect the presence of valid generator expressions in a CMake variable, so it can be used as a signal not to attempt any non-genex transformations on that input?

Answer 1

You could use command string(GENEX_STRIP) which strips generator expressions from the string, and compare its result with original string:

string(GENEX_STRIP "${FOO_POSSIBLY_RELATIVE_POSSIBLY_GENEX_PATH}" no_genex)
if(FOO_POSSIBLY_RELATIVE_POSSIBLY_GENEX_PATH STREQUAL no_genex)
  # The string doesn't contain generator expressions.
  # It is safe to use e.g. 'get_filename_component'
else()
  # The string contains generator expressions.
  # This case requires special handling.
endif()