Determine if same pointers are passed to a macro - c++

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.

Answer 2

So you want to check if same name is passed to both arguments, right? Then this simple trick works
#define ASSERT_IF_NULL_2(_ptr1, _ptr2)\
{\
int _check_##_ptr1;\
int _check_##_ptr2;\
}\
ASSERT(_ptr1);\
ASSERT(_ptr2);
int main ()
{
int* ptr1;
int* ptr2;
ASSERT_IF_NULL_2 (ptr1, ptr1); // error: redeclaration of `int _check_ptr1'
ASSERT_IF_NULL_2 (ptr1, ptr2); // OK
}
EDIT (By OP, Ajay):
On Visual C++ compiler, we can use MS specific keyword __if_exists to give error through static-assert :
#define ASSERT_IF_NULL_2(_ptr1, _ptr2)\
{\
int _check_##_ptr1;\
__if_exists(_check_##_ptr2)
STATIC_ASSERT(false, "Same pointer passed twice")
}\
Since static_assert keyword doesnt exist for pre-VS2010 compilers, one may use custom-written STATIC_ASSERT.

Answer 3

Are you trying to check if the pointers point to the same memory or if they point at the same value?
In the first case
xx = 1 / (ptr-ptr);
should work.
in the second case
xx = 1 / (*ptr-*ptr);
should work.

Answer 4

Try one of these maybe:
ASSERT_IF_NULL(ptr1 == ptr1);
ASSERT(ptr1 != ptr2)
As pointed outby Nawaz you want a compile error so try this:
1/(ptr1 != ptr2);
1/static_cast<int>(ptr1 != ptr2);
static_assert(ptr1 != ptr2, "Pointers are different") //if you dont use c++0x look here:http://stackoverflow.com/a/1664651/258418