Macro that interchanges 2 arguments

Question

Define a macro swap(t, x, y) that interchanges two arguments of type t.(Block structure will help.)

The ideea is that a variable defined in a block structure exists only inside the block structure. So, I can create a temporary variable without affecting the code.

Here is my solution:

#include <stdio.h>

#define swap(t, x, y) {t tmp = x; x = y; y = tmp;}

int main() {
    int x = 10, y = 2;
    swap(int, x, y);

    printf("%d %d", x, y);
    return 0;
}

Answer 1

If you are using GCC, we can use the typeof()^(C99) keyword to get rid of one of the arguments. Also, add a do-while so the macro to be used in contexts where it would otherwise be problematic.

#define SWAP(a, b) do { typeof(a) t; t = a; a = b; b = t; } while(0)

---

You could also use an exclusive-or (^=) to get rid of that temporary variable, but that only works for integers.

#define SWAP(a, b) do { a ^= b; b ^= a; a ^= b; } while(0)

Answer 2

First, macros should have ALL_CAPS names to distinguish them from functions. Users need to be able to distinguish macros from functions because they behave slightly differently. For example, with your definition of swap() as a macro, this code (for illustration purposes — not that it's good code) fails to compile:

if (swap(int, x, y), x) {
    /* Do something */
}

Also, I'd put a defensive do { ... } while (0) around your definition. Otherwise, this would cause an unexpected compilation error:

int main() {
    int x = 10, y = 0;
    if (x != 0)
        SWAP(int, x, y);
    else
        x = 0;
    printf("%d %d\n", x, y);
    return 0;
}