Making an undefined class as friend, and defining it later

Question

Making an unknown friend

template
class List
{
protected:

    class a {
        int x;
        int y;
    private:
        friend class b;         


        

Answer 1

//Run this- it now will compile for you

template  class b; //<----- forward declaration

template
class List
{
protected:


        class a {
        int x;
        int y;
        private:
          friend class b;  // <------------ Add 
        };
        template  class b { 
          int z;
          U y;
        };

        public:
        List() {
          a* ptr = (a *)new unsigned char[sizeof(a)];
        }
};

int main() {
    List  mylist;

}