An STL-style container’s performance can be dramatically affected by
minor changes to the underlying data structure’s invariants, which
in turn can be dramatically constrained by the container’s chosen API.
Since the Boost.Unordered performance project
has put std::unordered_foo back in the spotlight, I thought this
would be a good week to talk about my favorite little-known STL trivia
tidbit: std::unordered_multiset’s decision to support .equal_range
dramatically affects its performance!