Modify a local instance of own class

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;
    }
}

"Incomplete type not allowed" means you can't create an instance of a class that hasn't been fully-defined yet. A class isn't "complete" until the compiler reads its closing brace and semicolon. A class can't contain an instance of itself; that'd be a circular definition. — Wyzard, Nov 07 '12 at 01:35

score 4 · Accepted Answer · edited May 23 '17 at 11:48

Turn Pi into a reference or a pointer to Node.

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.
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++?

score 1 · Answer 2 · answered Nov 07 '12 at 01:14

Try this: (Send pointer to objects instead of class objects directly)

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
}

Java 'auto-magically' converts those objects, with C++, you have to do it yourself.

bames53 · Answer 3 · 2014-01-21T21:50:37.063

1

Java's syntax hides the fact that it uses pointers everywhere. In C++ you have to use pointer syntax explicitly in order to mimic Java:

// Java
class Node {
    private Node Pi; // This is actually a pointer
}

// C++
class Node {
    private:
        Node *Pi; // So you use a pointer in C++ too
};

edited Jan 21 '14 at 21:50

answered Nov 07 '12 at 01:20

bames53

86,085
15
179
244

score 0 · Answer 4 · answered Nov 07 '12 at 01:15

0

Change the following code :

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

To:

class Node {
    private:
        Node* Pi; 
};

answered Nov 07 '12 at 01:15

user1804740

41
2

Modify a local instance of own class

4 Answers4