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, \

Answer 1

Using upper_bound would result in an easy-to-read loop but each call will perform a binary tree search, resulting in an O(n log n) instead of O(n) traversal. If the difference in efficiency matters, you can structure your traversal like this:

typedef std::multimap MapType;
MapType container;
for (MapType::iterator it = container.begin(); it != container.end(); ) {
  std::string key = it->first;

  doSomething(key);

  // Advance to next non-duplicate entry.
  do {
    ++it;
  } while (it != container.end() && key == it->first);
}