CMake - remove a compile flag for a single translation unit

Question

I would like to remove a set compile flag for a single translation unit. Is there a way to do this? (e.g. using set_property?)

Note: the compile-flag ha

Answer 1

If there is no compile flag negation option (which would be the standard way of CMake handling this for non-IDE configurators), you have to remove this compile flag from CMAKE_CXX_FLAGS.

Possible Approaches

1. Make COMPILE_FLAGS a source file property which is INHERITED from a directory property with the same name. Now you can modify COMPILE_FLAGS property of each directory and source file individually:

   # Remove '-fname' from global flags
   string(REPLACE "-fname" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   
   define_property(
       SOURCE
       PROPERTY COMPILE_FLAGS 
       INHERITED 
       BRIEF_DOCS "brief-doc"
       FULL_DOCS  "full-doc"
   )
   
   # Add '-fname' for directory
   set_directory_properties(PROPERTIES COMPILE_FLAGS "-fname")
   
   add_executable(${PROJECT_NAME} "main.cpp")
   
   # Remove '-fname' from source file
   set_source_files_properties("main.cpp" PROPERTIES COMPILE_FLAGS "")
   

2. If some property - like COMPILE_OPTIONS - is only available on target level, you have to move the files you want to have different settings for into a separate new target itself.

   This can be done by using OBJECT libraries:

   # Remove '-fname' from global flags
   string(REPLACE "-fname" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
   
   # Set '-fname' for directory, but if you also use add_compile_options() be aware that
   # this will overwrite your previous setting. Append/remove properties instead.
   set_directory_properties(
       PROPERTIES 
           COMPILE_OPTIONS "-fname"
   )
   
   # Remove '-fname' from source file(s) in separate object library target
   add_library(${PROJECT_NAME}_no_name OBJECT "main.cpp")
   set_target_properties(
       ${PROJECT_NAME}_no_name
       PROPERTIES 
           COMPILE_OPTIONS ""
   )
   
   # Use object library target in e.g. executable target
   add_executable(${PROJECT_NAME} $)
   

References

• CMake per file optimizations
• Override compile flags for single files
• clang-tidy cmake exclude file from check