Backward compatible `add_compile_definitions`

Question

I am working on fixing some CMake files that have been made with a version newer than 3.10, but should have been coded to be compatible with 3.10.

There is a particular line:

add_compile_definitions(SOME_BOOL_VARIABLE)

that I can not figure out how to change to make it work with version 3.10. The add_definitions(SOME_BOOL_VARIABLE) command was the obvious thing that I tried, but produces errors with "no such file of directory".

Answer 1

From the CMake documentation, the functionality of add_definitions() and add_compile_definitions() is essentially the identical:

Adds preprocessor definitions to the compiler command line for targets in the current directory and below (whether added before or after this command is invoked).

The difference is in the syntax accepted for the command arguments. The older add_definitions() command requires the -D flag be prepended to each definition.

add_definitions(-DSOME_BOOL_VARIABLE) 

The newer add_compile_definitions() command (available in CMake 3.12 and above) is cleaner, and does not require the -D flag prefix.

---

If you are refactoring your code, modern CMake encourages a target-centric approach. Whenever possible, you should prefer the target_compile_definitions() command to add preprocessor definitions to only those targets that require them.

target_compile_definitions(MyLibraryTarget PRIVATE SOME_BOOL_VARIABLE)