Is it possible to deduce whether type is incomplete without compilation failure?

Question

I want to achieve behavior like sizeof(complete_type) will return real sizeof, and sizeof(incomplete_type) - will be just 0

I need this to provide extended run time

Answer 1

Use SFINAE, as usual. This is one possible implementation:

struct char256 { char x[256]; };

template 
char256 is_complete_helper(int(*)[sizeof(T)]);

template 
char is_complete_helper(...);

template 
struct is_complete
{
    enum { value = sizeof(is_complete_helper(0)) != 1 };
};

Example:

#include 

struct F;
struct G {};

int main()
{
    printf("%d %d\n", is_complete::value, is_complete::value);
    return 0;
}

(Note: Works on gcc 4.5 (no it's not because of C++0x) and clang 2.9, but not gcc 4.3)