Question

#include <iostream>
#define true false
#define false true
int main() {
    std::cout << false << true;
}

Why does it output “01”?

Answer 1

As Jerry Coffin notes, you cannot define a macro with a name that is a keyword.

However, we could consider another, similar example, with well-defined behavior and the same result. Consider:

int TRUE = 1;
int FALSE = 0;

#define TRUE FALSE
#define FALSE TRUE

std::cout << FALSE << TRUE;

When you use FALSE, it is identified as the macro FALSE and is replaced by that macro’s replacement list, which is the single token, TRUE. That replacement is then rescanned for further macros to replace.

The TRUE in the replacement is then identified as a macro and is replaced by its replacement list, which is the single token FALSE. That replacement is again rescanned.

If we continued on rescanning and replacing, we’d end up in an infinite loop, so the C (and C++) preprocessing specifications state that macro replacement never recurses within a replacement list.

Since replacement of FALSE in this final replacement list would result in recursion, macro replacement stops and we are left with FALSE, which is the name of an int with a value of 0.