Friend declaration and template constructor in c++ class template

Friend declarations for class templates:

When granting access to all instances of a given template, a declaration of the class template or function template does not need to exist in the scope. When you want to restrict a friendship relationship to a specific instantiation, you must declare the class or function before it can be used in a friend declaration.

template <class T>
class test
{
    template <class U> friend ostream& operator<< (ostream &os, const test<U> &obj); //友元的所有实例均具有访问权
    ...
};

class test;
template <class Type> ostream& operator<< (ostream &os, const test<Type> &obj);
template <class T>
class test
{
    friend ostream& operator<< <T> (ostream &os, const test<T> &obj);//友元为的T类型实例才有访问权
    ...
};

Template constructor:

　In a template class, when the constructor and template constructor exist at the same time, the constructor is called first. A template constructor is called only if its interface is exactly met. The compiler will never treat a template constructor as a constructor, and even if the client does not define a copy constructor, the compiler will generate a default copy constructor.

template <class T>
class test
{
public:
    test() { cout << "in my test construct" << endl;}
    test(const test &) { cout << "in my test copy" << endl;}
    template <class V>
    test(const test<V> &) { cout << "in my template copy" << endl;}
};


int main()
{
    test<int> t1;
    test<int> t2(t1);
    test<double> t3(t1);
    return 0;
}

The template test(const test &) function here should be called the type conversion constructor, which can convert the type of a test into test, that is, a template class with different template parameters. This application is, for example, if I have an array of type int and want to pass it to a parameter of type double array, this constructor can complete the conversion.

The output structure of the program is:

in my test construct

in my test copy

in my template copy

It is applied in the pair and auto_ptr classes of stl