Return a reference providing modifiable access to the mapped value associated with the specified key in this map. If this map does not already contain an element having key, insert an element with the key and a default-constructed VALUE, and return a reference to the newly mapped value. If key is movable, key is left in a (valid) unspecified state.
Declared in <bdlc_flathashmap.h>
VALUE&
operator[](KEY const& key);