friend class in TC++ book

[mickey]

hello,

I read TC++ vol2. (html ed 2004) at page 456 (chap design patterns) "the inner class idiom".
In the class "Outer" it writes:

class Outer {
 .....
  class Inner1;
  friend class Outer::Inner1;
  ...........
};

class Outer {
 .....
  class Inner1;
  friend class Outer::Inner1;
  ...........
};

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I tried to get out those two lines and it compiles; why? What sense have they, please?

thanks,