How to Define Friend Relationships in Template Classes with Different Template Arguments?

Delving into Class Templates with Template Class Friends

When defining a binary tree class (BT) and its element class (BE), it's necessary to establish a friend relationship for BT to access BE's private members. However, it's crucial to understand the underlying mechanics to define the relationship correctly.

Originally, you attempted to declare the friend relationship as template friend class BT. But this syntax introduces a naming conflict with the template parameter of BE. Template parameters within nested templates must have distinct names.

Instead, you should use different template parameter names, such as:

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

This declaration indicates that any BT class, regardless of its template arguments, is a friend of all BE classes with matching template arguments.

Consider the following examples to further clarify the different types of friend relationships:

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

In this case, bar is a friend of foo regardless of bar's template arguments. Any specialization of bar would be a friend of any specialization of foo.

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

Here, bar is only a friend of foo if its template argument matches foo's. So, only bar would be a friend of foo.

In your specific scenario, friend class bar; should suffice as it allows any specialization of bar to access BE's private members as long as its template argument matches the corresponding BE class.

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