How to redefine a C++ #define macro using information from the macro itself?

Question

Is it possible to redefine a C++ #define macro using information from the macro itself? I tried the code below, but because of the way the macros are evaluated the output was not what I expected.

#include <iostream>

#define FINAL_DEFINE "ABC"
#define NEW_DEFINE FINAL_DEFINE "DEF" // Want ABCDEF

#undef FINAL_DEFINE
#define FINAL_DEFINE NEW_DEFINE // Want ABCDEF, but get empty?


int main ()
{
    std::cout << FINAL_DEFINE << std::endl; // Want ABCDEF, but doesn't compile.
}

Answer 1

Macros in macro bodies are never expanded when the macro is defined -- only when the macro is used. That means that the definition of NEW_DEFINE is not "ABC" "DEF", it is exactly what appears on the #define line: FINAL_DEFINE "DEF".

So when you use FINAL_DEFINE, that gets expanded to NEW_DEFINE which then gets expanded to FINAL_DEFINE "DEF". At this point it will not recursively expand FINAL_DEFINE (as that would lead to an infinite loop) so no more expansion occurs.