How to add custom flag in CMake building

Question

My main goal is to use a flag when building a project in command line. So I can add own flags like DEBUG or MY_FLAG.

#ifdef DEBUG
#define debug_print(format, ...) printf(format, __VA_ARGS__)
#else
#define debug_print(...)
#endif

int main() {
    debug_print("%s\n", "Debugging mode");

    return 0;
}

I build the program with:

cmake -H. -Bbuild
make -C build -j

then run with

./build/main

I added add_definitions(-DDEBUG) to the CMakeList file which works fine. But I want to add the flag in the cmake -H. -Build process.
I tried with cmake -H. -Build -DCMAKE_BUILD_TYPE=DEBUG but it's not working.

Answer 1

I had a similar problem. The way I solved it was by adding this line to CMakeLists.txt:

add_compile_options("$<$<NOT:$<CONFIG:RELEASE,MINSIZEREL,RELWITHDEBINFO>>:-DDEBUG>")

This way, whenever -DMAKE_BUILD_TYPE is set to either RELEASE, MINSIZERE, or RELWITHDEBINFO, my custom flag DEBUG will not be defined.