Can a derived class access a private method of a protected inner class of the parent class that is a friend of the inner class?

Question

Considering this class:

class Matchable
{
  protected:
    class Match {
      friend class Matchable;

      void append( const Match& match ) {}
    };

  public:
    virtual bool match( const Source& source ) = 0;
};

... where the outer class Matchable is a friend of the inner class Match, and considering this class:

class Literal : public Matchable {

  bool match( const Source& source ) override {
    Matchable::Match m;
    Matchable::Match cm;

    m.append( cm );

    return true;
  }

}

... where Literal is derived from Matchable, I seem to be able to instantiate Matchable::Match in Literal::match() without a problem, yet I am unable to call the private method Matchable::Match::append(), where I expected Literal to inherit the "friendliness" of Matchable.

Is this expected behavior and if so, is there a way to make Literal access private methods of its parent inner class Match?

Answer 1

Yes, this is expected behavior. See friend declaration

Friendship is not inherited (your friend's children are not your friends)

You might provide a delegate method in Matchable:

class Matchable
{
  protected:
    class Match {
      friend class Matchable;
      void append( const Match& match ) {}
    };
    void appendMatch( Match& match, const Match& matched ) { 
        match.append(matched);
    }

  public:
    virtual bool match( const Source& source ) = 0;
};

then

class Literal : public Matchable {

  bool match( const Source& source ) override {
    Matchable::Match m;
    Matchable::Match cm;

    appendMatch(m, cm);

    return true;
  }

}

Otherwise you might make Match::append public (which make friend declaration meaningless).