CODEWITHSUNDEEP
c++cc-preprocessor
jmlopez
jmlopez

Reputation: 4953

Swapping values of macros

This may seem silly but I seem to have forgotten the order of replacement of macros. Can someone help me figure out how to correctly swap the values of two macros? Consider the following:

#include <stdlib.h>
#include <stdio.h>

#define var1 5
#define var2 10

#define _VAL(a) a
#define VAL(a) _VAL(a)


int main(){

    printf("var1 = %d, var2 = %d\n", var1, var2);

#define TEMP VAL(var1)
#undef var1
#define var1 VAL(var2)
#undef var2
#define var2 VAL(TEMP)

    printf("var1 = %d, var2 = %d\n", var1, var2);
}

All I want is to have var1 to be replaced by 10 and var2 to be replaced by 5. Any ideas of how to fix this mess?

I'm trying to use this to try to figure something out for this other question:

C Macro to protect definitions

Upvotes: 0

Views: 175

Answers (1)

ugoren
ugoren

Reputation: 16441

Can't be done.
As @jeffamaphone explains in his comment, macro definitions are not assignments.
#define A B doesn't care about the value of B. It just remembers that A should be replaced to B. Later, when A is seen in the source, it's replaced to B, which may then be replaced again, with whatever B is defined to be at that time.

Upvotes: 2

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