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;
}

• syb0rg: I see that you have made a recent edit that invalidates some advice in the answers. It is often not recommended to make changes to your code once it has been reviewed (or even posted). Therefore, I have rolled back your edit so that all the answers can still be considered valid.

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)

• Simon Forsberg: But that doesn't support other arguments than ints (or other ordinal types), does it?
• syb0rg: @SimonAndréForsberg You are correct. I have added in another solution that works with more types than just ints.
• Morwenn: Note that is you are using GCC 4.9, a somewhat better solution would be to use the __auto_type extension whose behaviour is similar to the C++11 keyword auto.

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;
}