sfinae_3_ways.cpp

// After the Wiki example

#include <iostream>
#include <type_traits>
#include <experimental/type_traits>

template <typename T>
struct has_foobar1 {

    typedef char yes[1];
    typedef char no[2];

    template <typename C>
    static yes& test(typename C::foobar*);

    template <typename>
    static no& test(...);

    static const bool value = sizeof(test<T>(nullptr)) == sizeof(yes);

};

template <typename... Ts>
using void_t = void;

template <typename T, typename = void>
struct has_foobar2 : std::false_type {};

template <typename T>
struct has_foobar2 <T, void_t<typename T::foobar>> : std::true_type {};

struct foo {
    typedef int foobar;
};

template <typename T>
using has_foobar3 = typename T::foobar;

int main() {

    std::cout << std::boolalpha;

    std::cout << "\nPre C++11 SFINAE" << std::endl;
    std::cout << has_foobar1<foo>::value << std::endl;
    std::cout << has_foobar1<int>::value << std::endl;

    std::cout << "\nC++11 SFINAE" << std::endl;
    std::cout << has_foobar2<foo>::value << std::endl;
    std::cout << has_foobar2<int>::value << std::endl;

    std::cout << "\nC++11 SFINAE after Library Fundamental v2" << std::endl;
    std::cout << std::experimental::is_detected<has_foobar3, foo>::value << std::endl;
    std::cout << std::experimental::is_detected<has_foobar3, int>::value << std::endl;

    return 0;
}

Output:

Pre C++11 SFINAE
true
false

C++11 SFINAE
true
false

C++11 SFINAE after Library Fundamental v2
true
false