Question

I have defined a custom assert macro. This works fine for all other comparisons. However, I get the compiler error:

ISO C++ forbids comparison between pointer and integer

when using the macro shown below (DWASSERT) to compare pointers as in the code below.

#define DWASSERT(condition,printstatement)  if(!condition){ printf(printstatement); assert(condition); }

#include <stdio.h>

int main()
{
    int target = 0;
    int* ptr1 = &target;
    int* ptr2 = &target;

    //Normal comparison works fine
    if(ptr1 == ptr2)
        printf("Equal");

    //Comparison using Macro generates compiler
    //error on the next line
    DWASSERT(ptr1 == ptr2, "Pointers not equal!\n");
    return 0;
}

While I can simply avoid using DWASSERT for this case, I am curious as to why this compiler error is generated.

Answer 1

The problem is that DWASSERT(ptr1 == ptr2, ... gets expanded to
if(!ptr1 == ptr2){ printf(...

Do you see what’s going on? !ptr1 == ptr2 is equivalent to (!ptr1) == (ptr2), and since !ptr1 is an integer type and ptr2 is a pointer type, you get your error.

What you need to do to fix this is to change your macro definition to:

#define DWASSERT(condition,printstatement)  if(!(condition)){ printf...

Also, keep in mind that it is a bad idea to use printf the way you have, with an arbitrary string as a format. At some point somebody will give you a string with a % in it and things will break. You should use something like puts(x) or printf("%s", x).