c - Define a preprocessor macro swap(t, x, y)

Question

I need to define a preprocessor macro swap(t, x, y) that will swap two arguments x and y of a given type t in C/C++.Can anyone have any opinion on how can i do it?

Answer 1

If you want to swap basic types like int or char (which implement the XOR operator) you can use the tripple XOR trick to swap the values without the need of an additional variable:

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

If you're swapping complex types (float, structs, ...) you need a helper variable:

#define SWAP_TYPE(type, a, b) 
    { 
        type __swap_temp; 
        __swap_temp = (b); 
        (b) = (a); 
        (a) = __swap_temp; 
    }

Usage of those two macros is like this:

int a = 6;
int b = 123;
float fa = 3.1415;
float fb = 2.7182;

SWAP(a, b);
SWAP_TYPE(float, fa, fb);