Define preprocessor macro through CMake?

Question

How do I define a preprocessor variable through CMake?

The equivalent code would be #define foo.

Answer 1

For a long time, CMake had the add_definitions command for this purpose. However, recently the command has been superseded by a more fine grained approach (separate commands for compile definitions, include directories, and compiler options).

An example using the new add_compile_definitions:

add_compile_definitions(OPENCV_VERSION=${OpenCV_VERSION}) add_compile_definitions(WITH_OPENCV2) 

Or:

add_compile_definitions(OPENCV_VERSION=${OpenCV_VERSION} WITH_OPENCV2) 

The good part about this is that it circumvents the shabby trickery CMake has in place for add_definitions. CMake is such a shabby system, but they are finally finding some sanity.

Find more explanation on which commands to use for compiler flags here: https://cmake.org/cmake/help/latest/command/add_definitions.html

Likewise, you can do this per-target as explained in Jim Hunziker's answer.

Answer 2

To do this for a specific target, you can do the following:

target_compile_definitions(my_target PRIVATE FOO=1 BAR=1) 

You should do this if you have more than one target that you're building and you don't want them all to use the same flags. Also see the official documentation on target_compile_definitions.