Difference between add_compile_options and SET(CMAKE_CXX_FLAGS…)

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  • CMAKE_CXX_FLAGS is used to add flags for all C++ targets. That’s handy to pass general arguments like warning levels or to selected required C++ standards. It has no effect on C or Fortran targets and the user might pass additional flags.

  • add_compile_options adds the options to all targets within the directory and its sub-directories. This is handy if you have a library in a directory and you want to add options to all the targets related to the library, but unrelated to all other targets. Additionally, add_compile_options can handle arguments with generator expressions. The documentation explicitly states, that

This command can be used to add any options, but alternative commands
exist to add preprocessor definitions (target_compile_definitions()
and add_definitions()) or include directories
(target_include_directories() and include_directories()).

  • add_definitions is intended to pass pre-processor values of the type -DFOO -DBAR=32 (/D on Windows) which defines and sets pre-processor variables. You could pass any flag, but the flags of the above form are detected and added to [COMPILE_DEFINITIONS][2] property, which you can later read and change. Here, you can use generator expressions, too. The documentation mentions scopes for directories, targets and source files.

For a given target, CMake will collect all flags from CMAKE_CXX_FLAGS, the target’s and directory’s COMPILE_DEFINITIONS and from all add_compile_options which affect the target.
CMAKE_CXX_FLAGS are not altered by the other commands or vice versa. This would violate the scope of these commands.

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