Function Macro definition problem

[Ericxx]

Hi all,

I have a macro function defined as follows in xx.h:

int swap_tmp_int;
#define INT_SWAP(X,Y)  swap_tmp_int=X; X=Y; Y=swap_tmp_int;

Then I used this function in another cpp file and got the following error when built:

"swap_tmp_int first defined here" Please kindly suggest.

Thank you,
Eric

[Lindley]

You can't define a global variable like swap_tmp_int in a header file.

However, in this case there's no need. Just #include <algorithm> and use std::swap().

Or, if you prefer, write it as an inline function rather than a macro so that it can have a local variable. You should prefer inline functions over macros most of the time anyway, since they're a lot easier to debug.

[laserlight]

If this is actually C rather than C++, then you might do something like this:

#define INT_SWAP(x, y) do { int t = x; x = y; y = t; } while (0)

But if I were to infer C++ from "cpp file", then std::swap as what Lindley mentioned is the way to go.

[ninja9578]

If this is actually C rather than C++, then you might do something like this:

#define INT_SWAP(x, y) do { int t = x; x = y; y = t; } while (0)

But if I were to infer C++ from "cpp file", then std::swap as what Lindley mentioned is the way to go.

Why do/while(0)?

#define INT_SWAP(x, y) { int t = x; x = y; y = t; }

[laserlight]

Why do/while(0)?

Try compiling this C program:

#include <stdio.h>

#define INT_SWAP(x, y) { int t = x; x = y; y = t; }

int main(void)
{
    int a = 1, b = 2;

    if (a < b)
        INT_SWAP(a, b);
    else
        puts("No swap");

    printf("&#37;d %d\n", a, b);
    return 0;
}

[potatoCode]

Why do/while(0)?

In general, do/while(0) completes the function block and enforces the scope

#define INT_SWAP(x, y) { int t = x; x = y; y = t; }

If this macro is preceeded by an expression statement that requires the terminator such as if statement, the macro will fail.
Another reason to put a pair of curly braces even for a single statement

    // no do/while(0) required
    if (a < b)
    {
        INT_SWAP(a, b);
    }
    else
    {
        puts("No swap");
    }

[ninja9578]

Try compiling this C program:

#include <stdio.h>

#define INT_SWAP(x, y) { int t = x; x = y; y = t; }

int main(void)
{
    int a = 1, b = 2;

    if (a < b)
        INT_SWAP(a, b);
    else
        puts("No swap");

    printf("%d %d\n", a, b);
    return 0;
}

Ah, got it. I guess I never ran into a problem like that because I use braces for even single line if statements if there is an else.

[Ericxx]

Thanks, I have managed to build the code by changing the function as an inline function. But I still cannot figure out what's wrong with the previous declaration...

regards,
Eric

[laserlight]

But I still cannot figure out what's wrong with the previous declaration...

You must have read Lindley's explanation in post #2. Is there anything about it that you would like to have clarified?

[Lindley]

Well, I didn't say why you can't define global variables in header files. The reason is, if multiple cpp files include the header, then they each get "different copies" of the global. So you end up with two global variables that have the same name, and the linker chokes on that.

There are ways around the problem, but none of them is the "right" solution in this case so I won't go into details.

[Zaccheus]

Try compiling this C program:

#include <stdio.h>

#define INT_SWAP(x, y) { int t = x; x = y; y = t; }

int main(void)
{
    int a = 1, b = 2;

    if (a < b)
        INT_SWAP(a, b);
    else
        puts("No swap");

    printf("%d %d\n", a, b);
    return 0;
}

error C2181: illegal else without matching if

Classic !