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

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 type information for IPC(inter-process) communication with the description structure per type:

struct my_type_info
{
    bool   is_pointer;
    size_t size;         //for double* will be 4 on i386. that is sizeof(double*)
    size_t base_size;    //for double* will be 8. that is sizeof(double)
};

The problem appears when into my system goes something like class MyOnlyDeclaredClass; I got compilation error, obviously by reason I can't take size of it.

boost type_traits http://www.boost.org/doc/libs/1_48_0/libs/type_traits/doc/html/index.html suggests many compile-time classes, but there is no 'is_incomplete'

Interesting compilers are VS2008, VS2010, clang 3, gcc-4.6, gcc-4.7

Answer 1

Do not try to do that.

It is fundamentally unsound. Templates are parametrized by types, not instantiation point. A class type is not complete or not in itself, it is complete at some point during translation.

A template instantiated on some types must have the exact same semantic in every instantiation.

If not, the behaviour is not defined.

Answer 2

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

struct char256 { char x[256]; };

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

template <typename>
char is_complete_helper(...);

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

Example:

#include <cstdio>

struct F;
struct G {};

int main()
{
    printf("%d %d\n", is_complete<F>::value, is_complete<G>::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)