friend question

i want to make class A friend of class B, so when i declare an instance of A within B its private members will also in handy. but things go wrongly when i compile it, my code is right below:

Code:

---

template <class T>
class A
{
        friend B<T>;
        public:
                //
        private:
                T data;
};
template <class T>
class B
{
        public :
                //
        private:
                A<T>* a;
}

---

i just can not figure out the problem, any ideas ?

btw, i am with Dev-C++ 5, thanx.

Code:

---

friend B<T>;
T data;

---

A doesn't know what a T is.

Code:

---

private: A<T>* a;

---

A isn't templated

You need to either tell what kind of template B needs to be by giving it a known type and create that member data so that it is of that chosen type, or template A itself. Also, by the looks of what you're trying to do, you might want to try out private inheritance, although it may not be necessary.

sorry i forgot to add template line above class A, even i add that line my compiler will still report an error, any suggestions ?

well, A need's to know what class B looks like since it has the friend declaration, so B needs to be declared before A....but...B needs to know what A looks like since it has A as a private member, so A needs to go first. Curse you circular dependance!

if you put a forward declarations of B infront of A, all should be fine. It just tells the compiler that B is indeed a class, but it's defined somewhere else.

Code:

---

template <class T> class B;

---

By the way, it's become standard practice to use template <typename> instead of <class>, but to my knowledge there is no difference.

oh thanx let me try it. but it is strange that my book didnt mention anything about this, mmm...

my god i tried that but failed either. these are what my code looks like:

Code:

---

template <class T>
class Stack;
template <class T>
class StackNode
{
        friend Stack<T>;
        public :
                //
        private :
                T __data;
                StackNode* __next;
};
template <class T>
class Stack
{
    //
};

---

what's wrong again ???

Nothing seems to be wrong, and that snippet compiled fine on my machine. If it's not the same error, I ask that you post what the error messages are. Other than that, I can only suggest that you make StackNode a struct instead of a class with private fields, making the friend declaration unecessary. It wouldn't solve the problem, merely get around it.