Qt C++ operator == overload Qlist not called

Question

My operator == is not beeing called when i use QList or QHashMap

here is my code :

class Node
{
    QString     _state;
    Node*       _parent;
    // for ID generation purpose
    static int  _seqNumber;
    int         _id;
public:
    Node();
    inline bool operator== (const Node &node) const
    {
         return ( _id == node._id  );
    }
 }

Now if i use QHash for example :

    QHash<Node*, double> hashMap* = new QHash<Node*, double>();
    Node* node = new Node();

    hashMap->insert(node, 500);

    // value is never found, because operator== is not being called
    double value = hashMap->value(node); 

I can't get value or compare is node exists in the map because operator== is not called !!

If you can help i would apreciate that.

Answer 1

This is expected behaviour . You are using Node* as your key-type, but there is no special operator==(Node*,Node*) defined.

What you seem to intend is Node .

Answer 2

The Hash key is a pointer( Node* ) not a Node object. so the map or hash is comparing pointers in there. Thus, there's really no need to your operator== .

And you cannot ask compiler to use your function to compare two pointers. because a pointer is a primitive type just like an int . you cannot overload operator== to compare two int s.

So, I think the solution, could be to use your object as hash key like this:

QHash<Node, double> //[1]

or just use pointer and leave the comparison to pointer comparisons.

[1] But then, you'd have to provide a hash function for the QHash to work as well.