How to create a std::set with custom comparator in C++?

Question

How do I create a set of pairs, the elements of which (the pairs) are sorted with a custom bool function? I write

set ,compare> my         


        

Answer 1

You should use a functional object. Here is an example

#include <iostream>
#include <set>
#include <utility>

struct Compare
{
    bool operator ()( const std::pair<int, int> &p1, 
                      const std::pair<int, int> &p2 ) const
    {
        return ( p1.second - p1.first  > p2.second - p2.first );
    }
};

int main() 
{
    std::set<std::pair<int, int>, Compare> s;

    return 0;
}

Answer 2

Method 1: use functor

Write a class that overloads the operator()so it can be called like a function:

struct compare {
    bool operator() (const pair<int,int> &lhs, const pair<int,int> &rhs) const{
         return (lhs.second-lhs.first > rhs.second-rhs.first);
    }
};

Then, you can use the class name as the type parameter

set<pair<int,int>, compare> myset;

Method 2: use function pointer

Assuming compare is the function you want to use:

set<pair<int,int>, bool(*)(const pair<int,int> &lhs, 
                           const pair<int,int> &rhs)
   > myset(&compare);