The functor may change non-key fields of the \p item.
While the functor \p f is calling the item \p item is locked.
- Returns <tt> std::pair<bool, bool> </tt> where \p first is \p true if operation is successfull,
+ Returns <tt> std::pair<bool, bool> </tt> where \p first is \p true if operation is successful,
\p second is \p true if new item has been added or \p false if the item with \p key
already is in the list.
*/