Implement is by yourself_ default_ constructible

Let's first introduce c++ In the standard library std::is_default_constructible, The main function of this class template is to determine whether the object of a class can be constructed by default , As shown in the following example

class Ax
{
    

};

class Bx
{
    
public:
	Bx(int x)
	{
    

	}
};

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

result ：

How can we achieve it by ourselves ？ The code is as follows ：

template<typename T>
struct IsConstructibile
{
    
private:
	template<typename = decltype(T())>
	static std::true_type test(void*);
	template<typename = int>
	static std::false_type test(...);
public:
	static constexpr bool value = std::is_same<decltype(test(nullptr)),std::true_type>::value;
};

The above implementation , It is implemented by overloading member functions , Well written , Worth learning . Let's analyze ：
（1） There are two static member function templates with the same name test(). first test() return std::true_type, the second test() return std::false_type; first test(）, The formal parameter is void *, The second is …, This is a C Ellipsis formal parameter in language , Acceptable 0 To any number of arguments . Pay special attention to decltype(T()), This is the key to implementation .
（2） For these two test(), When called , The compiler will prefer the version with specific formal parameters , Only when the version does not match, the ellipsis version will be selected .
（3） about decltype(test(nullptr), If passed to IsConstructibile The type of T Support default construction , Then obviously the compiler will choose the first test, Then the return value is std::true_type, So that value by true;
If passed to IsConstructibile The type of T Default construction is not supported , that decltype(T()) The writing method of is not tenable at all , according to SFINAE characteristic , The compiler will choose the second test, The return value is false_type, So the overall value is false. Let's look at the test results ：