friend question

This is a discussion on friend question within the C++ Programming forums, part of the General Programming Boards category; i want to make class A friend of class B, so when i declare an instance of A within B ...

  1. #1
    flashing vampire black's Avatar
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    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.
    Last edited by black; 08-02-2004 at 03:05 AM.
    Never end on learning~

  2. #2
    vae victus! skorman00's Avatar
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    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.

  3. #3
    flashing vampire black's Avatar
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    sorry i forgot to add template line above class A, even i add that line my compiler will still report an error, any suggestions ?
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  4. #4
    vae victus! skorman00's Avatar
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    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.

  5. #5
    flashing vampire black's Avatar
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    oh thanx let me try it. but it is strange that my book didnt mention anything about this, mmm...
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  6. #6
    flashing vampire black's Avatar
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    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 ???
    Never end on learning~

  7. #7
    vae victus! skorman00's Avatar
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    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.

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