C ++集<struct>

Question

I am solving a problem and need a custom set with special functions. The problem is I don't know how to implement them.

 struct S
    {
        int x;
        int freq;

        S(int X, int F)
        {
            x = X, freq = F;
        }
    };
    bool compare(const S &a, const S &b) 
    {
        if (a.freq != b.freq) return a.freq > b.freq;
        return a.x < b.x;
    }
    int main()
    {
        std::set<S> my_set;       //custom compare

        my_set.find(S(10, 11));   //find first element with x = 10
        my_set.insert(S(20, 30)); //normal insertion
        my_set.erase(S(20, 30));  //erase element with x = 20 && freq = 30

        cout << my_set.size() << endl;

    }

Answer 1

The second template argument to std::set is a custom comparator functor:

std::set<S, compare> my_set;       //custom compare

To make this work, your comparator needs to be a type that implements operator() (the function call operator), which can be a simple struct like this:

struct compare {
    bool operator()(const S &a, const S &b) 
    {
        if (a.freq != b.freq) return a.freq > b.freq;
        return a.x < b.x;
    }
};

Answer 2

std::set<S> by default uses std::less<S> , and you are allowed to specialize that:

namespace std {
  template <> struct less {
     bool operator()(const S &a, const S &b) { return compare(a,b); }
  };
}

(Or use a custom comparator, see Thomas' answer)