template<class T>
class Array
{
private:
   //...

public:
    //...
    friend ostream& operator<<(ostream &os, const Array<T> &obj);
};

template<class T>
ostream& operator<<(ostream &os, const Array<T> &obj)
{
    //..

    return os;
}

cout << "arr: " << arr << endl;

template<typename U>
friend ostream& operator<<(ostream &os, const Array<U> &obj);