Concatenation of tokens in variadic macros

4
votes

In C, is it possible to concatenate each of the variable arguments in a a variadic macro?

Example: 

MY_MACRO(A, B, C) // will yield HDR_A, HDR_B, HDR_C
MY_MACRO(X, Y)    // will yield HDR_X, HDR_Y

The normal ## operator has special meaning for variadic macros (avoiding the comma for empty argument list). And concatenation when used with __VA_ARGS__ takes place with the first token only.

Example: 

#define MY_MACRO(...) HDR_ ## __VA_ARGS__

MY_MACRO(X, Y)    // yields HDR_X, Y

Suggestions?

1
cmacrosvariadic
Have a look at stackoverflow.com/a/1872506/832273 this will probably help – Ulrich Dangel

1 Answers

6
votes

First, the comma rule you are mentioning is a gcc extension, standard C doesn't have it and most probably will never have it since the feature can be achieved by different means.

What you are looking for is meta programming with macros, which is possible, but you'd need some tricks to achieve that. P99 provides you with tools for that:

#define MY_PREFIX(NAME, X, I) P99_PASTE2(NAME, X)
#define MY_MACRO(...) P99_FOR(HDR_, P99_NARG(__VA_ARGS__), P00_SEQ, MY_PREFIX, __VA_ARGS__)
  • Here MY_PREFIX describes what has to be done with the individual items.
  • P00_SEQ declares how the items should be separated
  • P99_NARGS just counts the number of arguments