How to instantiate an comparison function (functor) when defining a map/set?

Question

I'm using a funciton object to specify a comparison function for map/set:

struct CompareObject {
    bool operator()(Node* const n1, Node* const n2) const;
}; 

As far as I understand defining a set like this will not create any instance of CompareObject and will pretend it is static:

std::multiset<Node*, CompareObject> set;

But in my problem I need to pass an instance of Tree to it for I'm using it in the actual comparision function:

bool
CompareObject::operator()(Node* const n1, Node* const n2) const {
  if (tree->getNoOfGood(n1) > tree->getNoOfGood(n2)) return false;
  if (tree->getNoOfGood(n2) > tree->getNoOfGood(n1)) return true;
  return false;
}

So, I'm adding some fields to CompareObject definition:

struct CompareObject {

  Tree& tree;              // added
  CompareObject(Tree& t);  // added

  bool operator()(Node* const n1, Node* const n2) const;
}; 

The issue I'm having is that I don't know how to instatiate this object with definition of the set.

The first thing that comes to my mind is:

std::multiset<Node*, CompareObjects(*this)> shapesMap; // not valid code

but not suprisingly it gives me an error: ‘this’ cannot appear in a constant-expression

Do you have any ideas how to go around this problem?

Answer 1

You can pass in an instance of the functor as a parameter to the set constructor. So something like multiset<Node*, CompareObject> shapesSet(CompareObject(myTree));