custom hash func? See short example on P 364 [[c++standard library]]. [[optimized c++]] has many examples too.
initialize? There’s a simple constructor taking a long initializer, but the insert() methods support the same and are more versatile.
insert? single pair; range (anotherMap.being(), end());
** insert single value — won’t overwrite pre-existing
** map1[key2] = val3 // overwrites pre-existing
** insert list of values — http://en.cppreference.com/w/cpp/container/unordered_map/insert
(returning the value) lookup? at() better than operator
a pointer type as key? useful technique.
erase? by a specific key. No need to call another function to really erase the node.
Q: create only if absent; no update please
Q2: create or uppate
Q2b: look up or create
A: operator 
Q1: update only; no create please
Q1b: look up only. No create please
A: find() method
Q: check for existance
A: c.find() is slightly better than c.count() esp. for multi_* containers