Question

Is it possible to do a macro selection using something similar to this (wrong) syntax?

#define FLAG MULT
#ifdef FLAG ADD
   int op(int a, int b) {return a + b;}
#endif
#ifdef FLAG MULT
   int op(int a, int b) {return a * b;}
#endif

Answer 1

You could do this:

#define ADD  0
#define MULT 1

#define FLAG MULT
#if (FLAG == ADD)
   int op(int a, int b) {return a + b;}
#elif (FLAG == MULT)
   int op(int a, int b) {return a * b;}
#endif

However, conditional compilation often leads to sadness (typically, the “unused” branch will end up unmaintained, and things will break when you switch back to using it).

C++ offers better mechanisms (run-time polymorphism, templates, etc.) to solve most (but not all) problems of this nature.