Question

I’ve got a class where one of the private properties is an instance of the class. In Java (because I’m used to Java) this looks like:

class Node {
    private Node Pi;
}

And now I’m writing this in C++, and when I try to do the same thing, I get a red underline (I’m using Visual C++ 2010 Express), “Incomplete type is not allowed”.

class Node {
    private:
        Node Pi; //Incomplete type is not allowed
};

I’ve tried fixing this by changing it to Node Pi(void); but then why I try to use another function that modifies this property I get “Expression must be a modifiable lvalue”.

class Node {
    private:
        Node Pi(void);
    public:
        void setPi(Node n);
};
void Node::setPi(Node n) {
    Pi = n; //Node Node::Pi() expression must be a modifiable lvalue
}

Normally in Java there’s no problem with this, so my question is, how do I implement this? I’m not a beginner to C++, but I haven’t written with it for a while.

class Node {
    private Node Pi;
    void setPi(Node n) {
        this.Pi = n;
    }
}

Answer 1

Turn Pi into a reference or a pointer to Node.

1. A reference (Node& Pi) will result in nicer syntax, but can’t be null, will have to be initialized at construction and cannot be changed to refer to a different object.
2. A pointer (Node* Pi) will require less pretty syntax, but can be null and can be made to point to different objects at different times.

For more details, see What are the differences between a pointer variable and a reference variable in C++?