C++ template operator overload for template class

An example code to perform template operator overload for a template class in C++ is provided.

Run and consider the output of the example below.

#include <iostream>
#include <vector>

using std::ostream;
using std::vector;
using std::cout;

template<class T>
class List {
    private:
        std::vector<T> vec;
    public:
        void push_back(T t){
        vec.push_back(t);
        };

        template<class U>
        friend ostream& operator<<(ostream& os, const List<U>& L );
};

template<class T>
ostream& operator<<(ostream& os, const List<T>& L ){  
    for (T it : L.vec){
        os << it << " .. ";
    }
    return os;
}

int main(void){  
    List<int> L;
    L.push_back(1);
    L.push_back(2);
    L.push_back(3);
    std::cout << L;

    return 0;
}  

The expected output is:

1 .. 2 .. 3 ..