Incomplete Type Is Not Allowed

Question

Attempting to create as simple parent/child node class but running into Incomplete Type Is Not Allowed error. Why?

class Node {
public:
    Node ParentNode;  //Error on this line
    string NodeName;


    Node(Node *node) : ParentNode(*node) {

    }
};

If it was allowed then you got an infinite compilation. The compiler will try recursively to insert the definition of Node for each its instance.:) — Vlad from Moscow, Dec 12 '17 at 20:39
What's the size of a `Node`? Ignoring padding, it's the size of a `Node` + the size of a `string`. See how that doesn't really make sense? — Kevin, Dec 12 '17 at 20:41
Possible duplicate of [Can a c++ class include itself as an attribute?](https://stackoverflow.com/questions/2706129/can-a-c-class-include-itself-as-an-attribute) — Kevin, Dec 12 '17 at 20:43

score 6 · Answer 1 · edited Feb 28 '20 at 17:48

6

You are trying:

Node ParentNode;  //Error on this line

but Node is not a complete type at this point (you are in fact defining it at this point), a structure cannot contain an instance of itself, it can contain a pointer or reference to an instance of itself but not an actual instance. After all, if such recursive containment were allowed, where would it end?

edited Feb 28 '20 at 17:48

Błażej Michalik

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answered Dec 12 '17 at 20:43

SoronelHaetir

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score 3 · Answer 2 · answered Dec 12 '17 at 20:57

The error hints at the fact that the definition of a class is incomplete while the class is being defined.

An object can't contain an instance of its own type – it would be infinitely large.

You want

Node* ParentNode;

and

Node(Node *node) : ParentNode(node)

Incomplete Type Is Not Allowed

2 Answers2