Override macro from the command line

Question

I would like to override a macro from the command line. Somewhere in my source, there is a definition like this:

#define MY_FOO 1

What I wo

Answer 1

You can't.

The command line is handled before any source and header files.
If a source file defines a macro, then it must not be defined before, and will get the new value from now on.
The only way to change it, is to #undef and #define it again, at a later point. If you have access to a header that is included after the definition, then you have a chance.