std::map difference between index and insert calls

In addition to the fact that map::operator[] will replace an existing value is that operator[] map::will create and add to the map a default existing value to replace before the replacement occurs (the map::operator[]() call has to return a reference to something). For items that are expensive to create this could be a performance issue.

See "Item 24: Choose carefully between map::operator[] and map::insert when efficiency is important" in Scott Meyers' Effective STL.