How Can You Declare Class Templates as Friends in C ?

Declaring Class Templates as Friends: A Deeper Dive

In C , it is possible to define classes as friends to other classes, granting access to private members regardless of encapsulation. However, when dealing with class templates, understanding the underlying friend relationships becomes crucial.

Consider the example code:

template<class T> class BE {
    T *data;
    BE *l, *r;
public:
    template<class U> friend class BT;
};

template<class T> class BT {
    BE<T> *root;
public:
...
private:
...
};

This code defines a class BE representing tree elements and a class BT representing a binary tree. Initially, the attempt was made to declare the friend class as:

template<class T> friend class BT;

However, this approach failed. To understand why, it is essential to recognize that template parameters cannot shadow each other. In nested templates, each level must have unique template parameter names.

In contrast, consider this declaration:

template<class T> class BE {
    template<class U> friend class BT<U>;
};

This signifies that BT is a friend of BE regardless of BT's template arguments. Every instance of BT, regardless of the type it stores, will be a friend of BE.

Alternatively, if the following declaration is used:

template<class T> class BE {
    friend class BT<T>;
};

This means that BT is only a friend of BE when BT's template argument matches BE's. In this case, for example, BT<char> would be a friend of BE<char>, but not of BE<int> or BE<float>.

In the case of the binary tree example, declaring the friend class as friend class BT<T>; would be sufficient to provide the desired relationship between BT and BE.

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