C preprocessor concatenation outside of #define

Question

I was wondering why we can't use token concatenation outside of defines.

This comes up when I want these at the same time:

conflict-free naming in a library (or for "generics")
debugability; when using a define for this then the whole code gets merged into a line and the debugger will only show the line where the define was used

Some people might want an example (actual question is below that):

lib.inc:

#ifndef NAME
    #error includer should first define NAME
#endif
void NAME() { // works
}
// void NAME##Init() { // doesn't work
// }

main.c:

#define NAME conflictfree
#include "lib.inc"
int main(void) {
    conflictfree();
    // conflictfreeInit();
    return 0;
}

Error:

In file included from main.c:2:0:
lib.h:6:10: error: stray '##' in program
 void NAME##Init();
          ^

The rule of thumb is "concat only in define". And if I remember correctly: The reason is because of the preprocessor-phases. Question: Why does it not work. The phases-argument sounds like it was once an implementation-limitation (instead of a logical reason) and then found its way into the standard. What could be so difficult about accepting NAME##Init() if NAME() works fine?

C preprocessor concatenation outside of #define

3 Answers