Let’s say I have two files main.cpp and generate_header.cpp. generate_header.cpp generates a header containing data that main.cpp needs to run. based on this answer, I can use the following code to run generate_header.cpp before building main.cpp:

add_custom_target(run_generate_header 
    COMMAND generate_header
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
    COMMENT "generating header"
)

add_dependencies(main_target run_generate_header)

However, as generating the header is slow, I would like to generate it only if it does not contain data.

To achieve this, I thought about using a preprocessor macro HEADER_GENERATED, and check_cxx_symbol_exists to generate the header only if the macro is not defined.

check_cxx_symbol_exists(HEADER_GENERATED ${HEADER_PATH} HEADER_GENERATED)

if (NOT HEADER_GENERATED)
   add_dependencies(main_target run_generate_header)
endif()

The problem with this approach is that check_cxx_symbol_exists runs during the configuration stage, not the build stage, so if the header is erased after configuration, it will not be regenerated and the build will fail. Is there a way to check if the header contains data during the build stage? Thanks in advance!