Determine if same pointers are passed to a macro

Question

There are set of macros, for debugging, logging, stack-trace displaying etc. One of them is like:

#define ASSERT_IF_NULL_2(_ptr1, _ptr2) \ 
    ASSERT(_ptr1);  \
    ASSERT(_ptr2);

This is over simplified version of macro(s) I have written. I have custom-assertion dialog box if assertion (runtime) fails, logs such null-check failure into log file. Macros are also written such that non-pointers are checked at compile time (static-assert).

Now, I am looking for some static-assert to check if two pointers are actually same. Example:

int* ptr;
ASSERT_IF_NULL_2(ptr, ptr);

Should raise a compiler error since both arguments to macro are same. I don't care if pointers point to same memory (since that thing is runtime).

I have tried expression like:

int xx;
xx = 1 / (ptr-ptr);
xx = 1 / (&ptr - &ptr);

None of them gives divide-by-zero compiler error. Also, I have tried with template that takes void* is template argument:

template<void* T>
class Sample{};

But it doesn't allow local-variable pointers to be passed to template non-type argument.

I am using VC9, which doesn't support constexpr keyword (even VS2012 doesn't). I tried using 'const' instead, which doesnt throw error. I also used the expression as array-size, which always results in error.

int array[(&ptr - &ptr)]; // Even with ptrdiff_t

Answer 1

Alright, the solution turns out to be simple. Use static_assert and Stringizing Operator (#):

#define ASSERT_TWO(a,b) static_assert((void*)#a != (void*) #b, "'" #a  "' and '" #b "' are same variables passed to ASSERT_TWO.") 

Demo:

int *global;    

int main()
{
    int *local;

    ASSERT_TWO(global, local);  //should pass
    ASSERT_TWO(local, global);  //should pass

    ASSERT_TWO(global, global); //should fail
    ASSERT_TWO(local, local);   //should fail

    (void)local; //suppress unsused warning!
}

which results in very helpful error message:

main.cpp: In function 'int main()':
main.cpp:18:5: error: static assertion failed: 'global' and 'global' are same variables passed to ASSERT_TWO.
main.cpp:19:5: error: static assertion failed: 'local' and 'local' are same variables passed to ASSERT_TWO.

Online demo.

Hope that helps.

_{I recall that this will not help you, as you're using VC9, but I keep this answer as it might help others. As of now, you could use other solution which generates redeclaration error which is less helpful error message compared to the static_assert message.}