Preprocessor replacement

Question

These macros are including or excluding text:

#include <stdio.h>

#define SKIP_TEXT(text)
#define JOIN_TEXT(text) text

int main(void)
{
    #define S(TEXT) "a" TEXT("b") "c" TEXT("d")

    printf("%s\n", S(SKIP_TEXT));
    printf("%s\n", S(JOIN_TEXT));
    return 0;
}

Output:

ac
abcd

Now I'm trying to do the same without defining S for each string to evaluate, but I don't know how to replace TEXT with SKIP_TEXT or JOIN_TEXT

#include <stdio.h>

#define SKIP_TEXT(text)
#define JOIN_TEXT(text) text

#define S(s) S_EXEC(s)
#define S_EXEC_SKIP(s) s /* Here I want to skip text */
#define S_EXEC_JOIN(s) s /* Here I want to join text */
#define S_EXEC(s) S_EXEC_##s

int main(void)
{
    printf("%s\n", S(SKIP("a" TEXT("b") "c" TEXT("d")));
    printf("%s\n", S(JOIN("a" TEXT("b") "c" TEXT("d")));
    return 0;
}

Is there any way to evaluate the arguments from S() ?

Answer 1

What you are asking is probably impossible, as it would require symbol redefinion inside a macro, instead of a simple macro expansion.

Simplest alternative would probably be:

#define TEXT SKIP_TEXT
printf("%s\n", "a" TEXT("b") "c" TEXT("d"));
#define TEXT JOIN_TEXT
printf("%s\n", "a" TEXT("b") "c" TEXT("d"));

or without SKIP_TEXT / JOIN_TEXT -macros:

#define TEXT(x) 
printf("%s\n", "a" TEXT("b") "c" TEXT("d"));
#define TEXT(x) x
printf("%s\n", "a" TEXT("b") "c" TEXT("d"));

#undef TEXT is probably needed after each printf .