[boost] [type_traits] is_complete<T> possible realization

Subject: [boost] [type_traits] is_complete<T> possible realization
From: Antony Polukhin (antoshkka_at_[hidden])
Date: 2012-11-05 03:34:47

Hi,

Yesterday I had a need to detect, is class forward declared or is it complete.

Here are some thoughts:

template <class T, int Misc>
struct is_complete {
    typedef char false_type;
    typedef int true_type;

    template <class T1>
    static typename boost::enable_if_c<sizeof(T1), true_type>::type test(int);

    template <class T1>
    static false_type test(...);

    enum ENU { value = (
               sizeof(test<T>(0)) == sizeof(true_type)
    )};
};

'Misc' parameter is required to work around compilers one time instantiation:

struct incomplete;
template <int N> struct incomplete_template;
struct complete{};
struct complete_abstract{
    virtual void foo() =0;
};

void test();

void foo1() {
    cout
            << is_complete<incomplete, __LINE__>::value
            << is_complete<incomplete_template<0> , __LINE__>::value
            << is_complete<incomplete_template<1> , __LINE__>::value
            << is_complete<complete, __LINE__>::value
            << is_complete<complete_abstract, __LINE__>::value
            << endl;
}

struct incomplete{};
template <> struct incomplete_template<0>{};

void foo2() {
    cout
            << is_complete<incomplete, __LINE__>::value // Without
second template parameter it would be still returning 0
            << is_complete<incomplete_template<0> , __LINE__>::value
            << is_complete<incomplete_template<1> , __LINE__>::value
            << is_complete<complete, __LINE__>::value
            << is_complete<complete_abstract, __LINE__>::value
            << endl;

}

int main()
{
    foo1(); // Outputs 00011
    foo2(); // Outputs 11011
    return 0;
}

I was thinking of changing 'Misc' parameter definition to something like this:

template <class T, class... Misc>
struct is_complete ;

Than it can be used in template functions/classes just like this:

template <class T, class... SomeClasses>
void test_function() {
    is_complete<T, SomeClasses..., boost::mpl::int_<__LINE__> >::value;
}

So,

Is there interest in such metafunction?
Can somebody give advices how to improve it? (I`m not sure that
implementation is portable, I`ve tested it only on GCC-4.6 and
CLANG-3.0, MSVC compiler will be available only tomorrow)

--
Best regards,
Antony Polukhin