macro problem

[eklavya8]

if I have a macro defined as

#define swap(a,b,c) c t; t=a,a=b,b=t;

The above would work if a,b were numbers and 'c' was a data type like int.but it wont work for float * or any other pointer type.it will give a compile error ,why?


cheers!

[Salem]

You would need to show an example use of "it doesn't work".

[MK27]

You would need to show an example use of "it doesn't work".

I'd be more interested in where it does work...

Have you seen this one:

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

That only works with ints, tho.

[tabstop]

I'd be more interested in where it does work...

Have you seen this one:

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

That only works with ints, tho.

So far as I can tell it works everywhere for anything, provided you are in a context where swap can safely expand to two statements (so if it's inside an if, you're going to need braces), at least assuming C99 so we can have variables declared anywhere.

[laserlight]

I'd be more interested in where it does work...

I think that it should work in quite a few situations, but to enable it to work in more situations:

#define swap(a,b,c) do { c t; t=a,a=b,b=t; } while (0)

[MK27]

I think that it should work in quite a few situations, but

How would I call that then? I almost never use macros, so I am interested in the syntax. But if I try:

swap(x,y,z);

Where x y z are ints, I get:

error: ‘t’ undeclared (first use in this function)

for both versions of this. Of course, this will work:

#define swap2(a,b,t) {t=a,a=b,b=t;}

[Ronix]

swap generally involves 2 variables. In the macro, the third parameter specifies the type, so if x and y are ints, you would call it swap(x, y, int)

[laserlight]

How would I call that then?

Eh, eklavya8 described an example: "The above would work if a,b were numbers and 'c' was a data type like int."

Thus the example would be:

swap(x, y, int);

[MK27]

Got it, thanks (I was thinking, "t" for temp, and that the OP meant "submit a tmp variable of the correct type")!

So today is my day to learn about writing macros...

[MK27]

So far as I can tell it works everywhere for anything, provided you are in a context where swap can safely expand to two statements (so if it's inside an if, you're going to need braces), at least assuming C99 so we can have variables declared anywhere.

error: invalid operands to binary ^ (have ‘float’ and ‘float’)

#include <stdio.h>

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

int main() {
	float i=0.7f, j=22.11f;
	swap(i,j);
	printf("swap: %f %f\n",i,j);
	return 0;
}

[laserlight]

MK27, tabstop's statement was in response to your comment about wanting to know where eklavya8's swap macro will work. It was not in response to your comment about XOR swap.

[itCbitC]

error: invalid operands to binary ^ (have ‘float’ and ‘float’)

#include <stdio.h>

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

int main() {
	float i=0.7f, j=22.11f;
	swap(i,j);
	printf("swap: %f %f\n",i,j);
	return 0;
}

Because the bitwise operators work only on integral operands (singed or unsigned).