Multiset in C++

What is multiset<int, greater<int>> ms;

From [1]

multiset<int, greater<int> > ms1;

This says that ms1 is a std::multiset, storing int, and using a comparison functor of the type std::greater<int>. You've left out constructor arguments, so the instance of the functor is a default constructed one, exactly as if you had passed std::greater<int>{}, like this:

multiset<int, greater<int> > ms1(greater<int>{});
#include <iostream>
#include <set>
using namespace std;
int
main ()
{
  multiset < int, greater<int>>ms1;
  ms1.insert(10);
  ms1.insert(20);
  for(auto& m: ms1)cout<<m<<endl;
  multiset < int, greater<int>>ms2 (greater <int> {});
  ms2.insert(10);
  ms2.insert(20);
  for(auto& m: ms2)cout<<m<<endl;
  return 0;
}

Reference

[1]https://stackoverflow.com/questions/46555103/what-is-the-difference-between-multisetint-greaterint-ms1-and-multiset