Implement is by yourself_ class

std::is_class yes c++11 In the standard, it is used to judge whether a type is a class type （ But not a union type ） Class template for . The use of it was explained in my previous article . link ：is_class Use
Let's take a look at its implementation ：

template<typename T>
struct ISCLASS
{
    
private:
	template<typename U>
	static std::integral_constant<bool,!std::is_union<U>::value> test(int U::*p);

	template<typename>
	static std::integral_constant<bool, false> test(...);
public:
	static constexpr bool value = decltype(test<T>(nullptr))::value;
};

int main()
{
    
	cout << ISCLASS<Bx>::value << endl;
	cout << ISCLASS<Ax>::value << endl;
	cout << ISCLASS<int>::value << endl;
	cout << ISCLASS<float>::value << endl;
	system("pause");
	return 0;
}

result ：

As the above code , Implementation method and is_default_constructible similar , They all use member function overloading . For these two test Member functions , The compiler will still prefer the version with specific parameters , Special attention int U::*p, This expression is actually a member variable pointer . It can be understood as the following code ：

int A::*p = nullptr;

If U It's a class type , Then grammatically speaking ,int U::*p This kind of writing conforms to the grammatical rules , otherwise , If U Not a class type , This kind of writing does not conform to the grammatical rules , according to SFINAE characteristic , It matches the second one test() edition , And once you match the second test() The version will match successfully .

summary ： If passed to ISCLASS Type template parameters for T Is a class type but not a union type , that test() Back to you std::integral_constant<bool,ture> type , Otherwise, the return is std::integral_constant<bool,false> type .