Trait

cppreference std::is_member_function_pointer

#include<type_traits>
template<class T>
struct is_member_function_pointer_helper: std::false_type
{
};

template<class T, class U>
struct is_member_function_pointer_helper<T U::*> : std::is_function<T>
{
};

template<class T>
struct is_member_function_pointer: is_member_function_pointer_helper<typename std::remove_cv<T>::type>
{
};

NOTE:

1、上述是典型的specialization+inheritance+trait

2、struct is_member_function_pointer_helper<T U::*> : std::is_function<T> 中的 T 如何理解？

根据 std::is_function<T> 推测: 它是一个pointer？

#include <type_traits>

class A {
public:
    void member() { }
};

int main()
{
    // fails at compile time if A::member is a data member and not a function
    static_assert(std::is_member_function_pointer<decltype(&A::member)>::value,
                  "A::member is not a member function."); 
}

cppreference std::is_member_pointer

template< class T >
struct is_member_pointer_helper         : std::false_type {};

template< class T, class U >
struct is_member_pointer_helper<T U::*> : std::true_type {};

template< class T >
struct is_member_pointer : 
    is_member_pointer_helper<typename std::remove_cv<T>::type> {};