Question

So, I’m trying to check whether a particular object is already present in the set. For this, I use the count() method. Now, it doesn’t seem to return the right answer. Let me explain the problem a bit more clearly —

I have declared a class this way

class Node{
        public:
                Node(int _state=0, int _cost=0)
                {
                        state = _state;
                        cost = _cost;
                }

                bool operator<(const Node& rhs)
                {
                        return cost < rhs.cost;
                }

                bool operator==(const Node& rhs)
                {
                        cout << "== operator method used" << endl;
                        if (rhs.state == state)
                                return true;
                        return false;
                }

                int state;
                int cost;
};

in my code, I declare a set like this —

set<Node*> myset;

after a few insertions, myset is like this {{1, 5}, {2, 6}, {3, 9}}

now I check whether {1, 7} is part of the set. How would it do it? I have written a operator== method in Node class, which is never called. Then on what basis does count() check if the object is already in the set?… I would want the count to work in a fashion that if {1, 5} is already there in myset, it should view {1, 7} as a duplicate entry.

Answer 1

how does count in c++ stl set work?

It uses operator< by default.

In fact, in general, C++ Standard Library containers use !(a < b) && !(b < a) to determine the property of equivalence.

You can override the comparator used to perform this check by providing your own Compare template argument to the container type, though there is rarely a reason to — you should usually simply define operator< for your type instead, as you have done. (Make sure that it creates a Strict Weak Ordering, though.)

in my code, I declare a set like this —

set<Node*> myset;

after a few insertions, myset is like this {{1, 5}, {2, 6}, {3, 9}}

No, your set is never like this. Your set contains pointers, not Nodes. Make it a set<Node> instead.