相等可比较类型的C++11静态断言？

Question

How to static_assert a template type is EqualityComparable concept in C++11？

Answer 1

您可以使用以下类型特征：

#include <type_traits>

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

template<typename T>
struct is_equality_comparable<T,
    typename std::enable_if<
        true, 
        decltype(std::declval<T&>() == std::declval<T&>(), (void)0)
        >::type
    > : std::true_type
{
};

你可以这样测试它：

struct X { };
struct Y { };

bool operator == (X const&, X const&) { return true; }

int main()
{
    static_assert(is_equality_comparable<int>::value, "!"); // Does not fire
    static_assert(is_equality_comparable<X>::value, "!"); // Does not fire
    static_assert(is_equality_comparable<Y>::value, "!"); // Fires!
}

这是一个。