Question

I need to implement a set of sets in my application.
Using QSet with a custom class requires providing a qHash() function and an operator==.

The code is as follows:

    class Custom{
        int x;
        int y;
        //some other irrelevant here
    }
    inline uint qHash(Custom* c){
        return (qHash(c->x) ^ qHash(c->y));
    }
    bool operator==(Custom &c1, Custom &c2){
        return ((c1.x==c2.x) && (c1.y == c2.y));
    }

    //now I can use: QSet<Custom*>

How can I implement qHash(QSet<Custom*>), to be able to use QSet< QSet<SomeClass*> >?

Edit:

Additional question:
In my application the “set of sets” can contain up to 15000 sets. Each subset up to 25 Custom class pointers. How to guarantee that qHash(QSet<Custom*>) will be unique enough?

Answer 1

A common way to hash containers is to combine the hashes of all elements. Boost provides hash_combine and hash_range for this purpose. This should give you an idea how to implement this for the results of your qHash.

So, given your qHash for Custom:

uint qHash(const QSet<Custom*>& c) {
  uint seed = 0;

  for(auto x : c) {
    seed ^= qHash(x) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
  }

  return seed;
}