Question

I want to check a header in my source file and if its already defined, I want to undefine it.

Is it possible?

Answer 1

You can’t do what you seem to want to do. If you don’t want the header included your choices are

don’t include it in the first place

conditionally include it or not via the prepocessor e.g.

#ifdef INC_MY_HEADER
#include "myheader.h"
#endif

or assuming the header has header guards just define the header guard before the header is included e.g. from the commend line -DMY_HEADER or similar

If the real problem is that you have different headers defining things with the same name then you probably need to look into namespaces so you don’t get clashes