Allowing access to a base class member from a particular derived class

Question

In a project, I have a class hierarchy implementing a graph with levels of hierarchy (that is, a graph node can be a graph itself — think of it as an electronic circuit where a gate can actually be an integrated chip, for instance). Thus, I have some class Base derived into Group and Leaf, and the class Base has an attribute ancestor, which I would like to be able to set from a given method of Group (when adding a child), but not from anywhere else:

class Group;

class Base {
    private: // or protected?
        Group* ancestor;
};

class Group : public Base {
    private:
        void setAncestor(Base* child) {
            // Something like
            child->ancestor = this;
        }
};

class Leaf : public Base {
    // ...
};

What would be the "correct" way to achieve that? Usually, I would use friend methods, but it seems to me that this is impossible in this context, since I would have to declare friend Group::setAncestor in Base, which must be declared before. (Also note that those declarations are in different files in the real code.)

This doesn't work unless `ancestor` is public. `Group` can access protected members of *its own* base class. It can not access protected members through `Base` pointers, as these might be bases of some unrelated class, like `Leaf`. — Bo Persson, Mar 27 '17 at 15:25
You might make the whole class friend, i.e. `friend class Group;`. — songyuanyao, Mar 27 '17 at 15:25

score 0 · Answer 1 · answered Mar 27 '17 at 15:40

0

data member Base::ancestor should have protected access. That way it will be available to derived classes.

answered Mar 27 '17 at 15:40

shargors

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Rama · Answer 2 · 2017-03-27T17:46:43.753

0

You can declare it as protected in Base, and in the Leaf class and from anywhere else use private inheritance, so protected members will be private.

class Group;

class Base {
    protected:
        Group* ancestor;
};

class Group : public Base {
    private:
        void setAncestor(Base* newAncestor) {
            // Something like
            ancestor = newAncestor->ancestor;
        }
};

class Leaf : private Base {
    // ...
};

Note that I change the name from child to newAncestor.

So when you create a new child you can do something like this:

Base ancestor;
Group child;
child->setAncestor(&ancestor)

edited Mar 27 '17 at 17:46

answered Mar 27 '17 at 15:50

Rama

3,222
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It won't have the same effect. – O'Neil Mar 27 '17 at 16:12
@O'Neil Off course `setAncestor()` has a different signature in my example. Just a suggestion. But I'm not sure if that's what you're referring to. – Rama Mar 27 '17 at 16:15
Yes this is it. The point is to have `ancestor` to point to **another** instance. Not `this`. – O'Neil Mar 27 '17 at 16:54

Allowing access to a base class member from a particular derived class

2 Answers2