For Cmake, can you modify the release/debug compiler flags with `add_compiler_flags()` command?

Question

In the man page for add_compile_options() I don't see any mention of how to modify the Release/Debug compiler flags. Can you use add_compiler_options() to modify the Release/Debug compiler flags? If yes, how?

If no, is the recommended canonical method to modify the release/debug cmake variables[1] as described here ?

[1] i.e. set the cmake variables CMAKE_<LANG>_FLAGS_<TYPE> (for lang c/c++ it would be: CMAKE_CXX_FLAGS_RELEASE, CMAKE_CXX_FLAGS_DEBUG, CMAKE_C_FLAGS_RELEASE, CMAKE_C_FLAGS_DEBUG).

Answer 1

If you want to reuse your compiler settings through several of your projects or you need to differentiate the compiler options between C and C++, I would recommend the CMAKE_C_FLAGS/CMAKE_CXX_FLAGS variant with a toolchain file for each of your supported compilers (see e.g. here or here).

But if you just need some additional C++ compiler options in your project, taking add_compile_options(), target_compile_options() or target_compile_features() is the way to go.

And yes, you can differentiate between DEBUG and RELEASE there.

Examples

1. The add_compile_options() command does take generator expressions:

   add_compile_options("$<$<CONFIG:DEBUG>:/MDd>")
   

   or

   add_compile_options(
       "$<$<CONFIG:RELEASE>:-std=gnu99>"
       "$<$<CONFIG:DEBUG>:-std=gnu99 -g3>"
   )
   

2. Better to check also the compiler id:

   add_compile_options("$<$<AND:$<CXX_COMPILER_ID:MSVC>,$<CONFIG:DEBUG>>:/MDd>")
   

   or

   if (MSVC)
       add_compile_options("$<$<CONFIG:DEBUG>:/MDd>")
   endif()
   

3. Even better to let CMake decide the correct compiler options for you. So you can either set the CXX_STANDARD needed for your target:

   set_property(TARGET tgt PROPERTY CXX_STANDARD 11)
   

   or give the compiler feature your target needs with target_compile_features()

   add_library(mylib requires_constexpr.cpp)
    # cxx_constexpr is a usage-requirement
    target_compile_features(mylib PUBLIC cxx_constexpr)
   

References

• CMake CMAKE_CXX_FLAGS enabled optimization unexpectly
• Change default value of CMAKE_CXX_FLAGS_DEBUG and friends in CMake
• CMake generator expression, differentiate C / C++ code