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

Question

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"}?

Answer 1

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