How to access private members of other template class instances? [duplicate]

问题

This question already has an answer here:

How to share protected members between C++ template classes? (1 answer)

Closed 5 years ago.

This extremely minimal example will fail to compile because A<int> cannot access the private member i in A<double>

template <class T>
class A {
    int i;
  public:
    template <class U>
    void copy_i_from( const A<U> & a ){
        i = a.i;
    }
};

int main(void) {
    A<int> ai;
    A<double> ad;
    ai.copy_i_from(ad);
    return 0;
}

I cannot find the correct way to make those template instances friends.

回答1:

template <class T>
class A {
    template<class U>
    friend class A;
    int i;
  public:
    template <class U>
    void copy_i_from( const A<U> & a ){
        i = a.i;
    }
};

int main(void) {
    A<int> ai;
    A<double> ad;
    ai.copy_i_from(ad);
    return 0;
}

来源：https://stackoverflow.com/questions/26422160/how-to-access-private-members-of-other-template-class-instances