cpp metaprogramming, is_container (sfinae for templates)

is_container.cpp

// taken from http://stackoverflow.com/a/31207079
// assuming that only containers have value_type
#include <cassert>
#include <vector>

namespace detail {

// the whole point of this structure is to get template-based sfinae working
template <class... _> struct resolved {};

}   // namespace detail

// template partial specialization mechanism is necessary hence 2nd arg
template <class T, class _ = void> 
struct is_container : public std::false_type {};

// partial template specialization
// at first sight std::conditional would always result in 3rd argument (void) but
// it needs to deduce types first which may fail on 2nd argument; thanks to
// sfinae such an failure is not an error and the compiler tries another struct
template <class T>
struct is_container<T, std::conditional_t<
        false,
        detail::resolved<typename T::value_type>,
        void
    >> : public std::true_type {};

template <class T> constexpr bool is_container_v = is_container<T>::value;

struct Foo {};
struct Bar { using value_type = void; };

int main(int, char**) {
    assert(!is_container_v<Foo>);
    assert(is_container_v<Bar>);
    assert(is_container_v<std::vector<int>>);
    assert(!is_container_v<int>);
    return 0;
}