Why Do I Need a Separate Parameter for a Friend Template Class Inside a Class Template?

Class Template with Template Class Friend: Delving into the Mechanics

Imagine constructing a binary tree (BT) class with an element class (BE) describing each node, resembling the following structure:

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 setup encounters a curious quirk. Attempting to declare the friend as template friend class BT; fails, necessitating the usage of a separate parameter U (or any non-T parameter).

This distinction stems from the concept of template shadowing. Template parameters cannot duplicate each other within the scope of nested templates. Consequently, different parameter names are indispensable for nested templates.

Consider the following constructs:

template<typename T>
struct foo {
  template<typename U>
  friend class bar;
};

Here, bar is declared as a friend to foo regardless of its own template arguments. All variations of bar, whether bar, bar, bar, or others, become friends to any instantiation of foo.

In contrast, the following declaration:

template<typename T>
struct foo {
  friend class bar<T>;
};

Implies that bar is only a friend to foo when bar's template argument aligns with foo's. Only bar would be considered a friend to foo.

Therefore, in your particular scenario, adopting the form friend class bar; should effectively establish the desired friend relationship between BE and BT.

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