void_t usage test case

gistfile1.cpp

#include <type_traits>
#include <iostream>

using namespace std;

template<class... > using void_t = void;

template< class, class = void >
struct has_type_member : false_type { };

template< class T >
struct has_type_member<T, void_t<typename T::type>> : true_type { };

template< class, class = void >
struct has_type_data_member : false_type { };

template< class T >
struct has_type_data_member<T, void_t<decltype(T::type)>> : true_type { };

struct foo {
    int type;
};

struct bar {};

struct baz {
    typedef int type;
};

int main() {
    if (has_type_member<foo>::value) {
        std::cout << "foo has a type member\n";
    } else {
        std::cout << "foo does not have a type member\n";
    }

    if (has_type_member<bar>::value) {
        std::cout << "bar has a type member\n";
    } else {
        std::cout << "bar does not have a type member\n";
    }

    if (has_type_member<baz>::value) {
        std::cout << "baz has a type member\n";
    } else {
        std::cout << "baz does not have a type member\n";
    }

    if (has_type_data_member<foo>::value) {
        std::cout << "foo has a type data member\n";
    } else {
        std::cout << "foo does not have a type data member\n";
    }

    if (has_type_data_member<bar>::value) {
        std::cout << "bar has a type data member\n";
    } else {
        std::cout << "bar does not have a type data member\n";
    }

    if (has_type_data_member<baz>::value) {
        std::cout << "baz has a type data member\n";
    } else {
        std::cout << "baz does not have a type data member\n";
    }

}