Template class friendship

Question

I recently came across a c++ piece of code where a class is made friend to itself. As I have read on different forums a class is already a friend to itself. So I was wondering if there is a specific reason why one would want to make a class friend to itself explicitly.

Another question would be, what's the reason in making a class a friend of itself?

Maybe someone with a lot of experience can clarify this topic.

Here is the code example, to illustrate my question:

template < typename T>
class Foo: public T
{
protected:
   template < typename U>
   friend class Foo;
}

Answer 1

There's no point indeed to make a class a friend to itself, except if it is a template class. For example the following code makes sense:

template <class T>
class A
{
    template<class U>
    friend class A;
}

An STL example is the std::_Ptr_base which is the base class for std::shared_ptr and std::weak_ptr.