Check if object type is callable

is_callable.hpp

namespace detail {

template <typename T>
struct has_call_operator {
private:
    struct _fallback { void operator()(); };
    struct _derived : T, _fallback {};

    template<typename U, U> struct _check;

    template<typename>
    static std::true_type test(...);

    template<typename C>
    static std::false_type test(
    		_check<void (_fallback::*)(), &C::operator()>*);

public:
    static const bool value =
    		std::is_same< decltype(test<_derived>(0)), std::true_type >::value;
};

}  // namespace detail

template < typename T >
struct is_callable : std::conditional<
	std::is_class< T >::value,
	detail::has_call_operator< T >,
	std::is_function<T> >::type {
};