Macro that interchanges 2 arguments

Question 1

Define a macroswap(t, x, y) that interchanges two arguments of typet.(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;}

Question 2

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.

Question 3

If you are using GCC, we can use thetypeof()^(C99) keyword to get rid of one of the arguments. Also,add ado-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)

Question 4

But that doesn't support other arguments than ints (or other ordinal types), does it?

Question 5

@SimonAndréForsberg You are correct. I have added in another solution that works with more types than justints.

Question 6

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 keywordauto.

Question 7

First, macros should haveALL_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 ofswap() 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 adefensivedo { ... } 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;}

syb0rg 21.9k10 gold badges113 silver badges193 bronze badges · Accepted Answer · 2017-05-23 12:40:53Z

If you are using GCC, we can use thetypeof()^(C99) keyword to get rid of one of the arguments. Also,add ado-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)

But that doesn't support other arguments than ints (or other ordinal types), does it?
@SimonAndréForsberg You are correct. I have added in another solution that works with more types than justints.
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 keywordauto.

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Macro that interchanges 2 arguments

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