Step by step expansion of variable parameters in class templates

1.myclasst<Args…> Inherit

template<typename...Args>
class myclasst
{
    
public:
	myclasst()
	{
    
		cout << "myclasst::myclasst() Yes , Variable parameters ：" << sizeof...(Args) << endl;
	}
};

template<typename...Args>
class myclasst2 :public myclasst<Args...>
{
    
public:
	myclasst2()
	{
    
		cout << "myclasst2::myclasst2() Yes , Variable parameters ：" << sizeof...(Args) << endl;
	}

};
int main()
{
    	
	
	myclasst2<double, float, int> tmpobj;
	system("pause");
	return 0;
}

result ：

From the running results, it can be seen that the myclasst2<double,float,int> class .

2.myclasst… Inherit

Now put it another way , as follows ：

template<typename...Args>
class myclasst2 :public myclasst<Args>...
{
    
public:
	myclasst2()
	{
    
		cout << "myclasst2::myclasst2() Yes , Variable parameters ：" << sizeof...(Args) << endl;
	}

};

result ：

It's modified here myclasst2 The inherited parent class , Turned out to be myclasst<Args...>, Now the … Put it outside the right angle bracket , Change into myclasst<Args>...
(1) without doubt , It will be instantiated myclasst2<double,float,int> class , because tmpobj Is the object belonging to this class ;
(2)Args It represents a package type , Here the package type is double,float,int common 3 A type of ; and myclasst<Args>... After instantiation, this expression represents 3 Classes , Namely myclasst<double>,myclasst<float>,myclasst<int>
(3) So in this case myclasst2<double,float,int> The parent class of class actually has 3 individual . So for statements ：

class myclasst2:public myclasst<Args>...

It's actually equivalent to ：

class myclasst2:public myclasst<double>,public myclasst<double>,public myclasst<int>