[SOLVED] Using preprocessor constant with cmake

Issue

I have a preprocessing constant in my code :

const unsigned int nbins= _NBINS;

And I would like to be able to change it when compiling. For example, I would like to be able to write:

cmake build_path [string to change _NBINS]

How can I do it using cmake?

Edit: I usually use makefiles so I was a bit confused For those who ask why I want to do that, it is because I want to autotest my program with various values for _NBINS. I want to recompile my program each time with the new value for _NBINS automatically. _NBINS needs to be a constant because its value is critical for optimisation purposes

Solution

How can I do it using cmake?

You can pass the value of your constant into a cmake variable then inside a cmake script get the value of that constant and pass it to compiler.

Call cmake with:

cmake build_path -D NBINS=something

And inside CMakeLists.txt:

add_executable(your_target ...)
target_compile_definitions(your_target PUBLIC NBINS=${NBINS})

or before the target:

add_compile_definitions(NBINS=${NBINS})

Note that in C++ identifiers with leading _ followed by uppercase letters are reserved. Do not use them in your code. Because of that, note I have removed the leading _ in the code snippets above.

Answered By - KamilCuk