is there an iterator across unique keys in a std::multimap?

c++, multimap

Solution

You can use `upper_bound` to increment the iterator position instead of `++`:

#include <map>
#include <string>
#include <iostream>

using namespace std;

int main()
{
  multimap<int,string> mm;
  mm.insert(make_pair(1, "a"));
  mm.insert(make_pair(1, "lemon"));
  mm.insert(make_pair(2, "peacock"));
  mm.insert(make_pair(3, "angel"));

  for( auto it = mm.begin(), end = mm.end();
       it != end;
       it = mm.upper_bound(it->first)
  )
    cout << it->first << ' ' << it->second << endl;
  return 0;
}

This results in:

1 a
2 peacock
3 angel

Problem

Is there a simple or standard way to have a multimap iterator which iterate across unique keys in a multimap? i.e. for a set that looks like: `{1, "a"}, {1, "lemon"}, {2, "peacock"}, {3, "angel"}` an iterator which would start at `{1, "a"}` then incrementing would point to `{2, "peacock"}` and then incrementing again would point to `{3, "angel"}`?

Original source