lower_bound upper_bound

《C++ Primer》11.3. Operations on Associative Containers

Finding Elements in a multimap or multiset

Finding an element in an associate container that requires unique keys is a simple matter—the element is or not in the container. For the containers that allow multiple keys, the process is more complicated: There may be many elements with the given key. When a multimap or multiset has multiple elements of a given key, those elements will be adjacent within the container.

A Different, Iterator-Oriented Solution

Alternatively, we can solve our problem using lower_bound and upper_bound. Each of those operations take a key and returns a iterator.

If the key is in the container,

the iterator returned from lower_bound will refer to the first instance of that key and

the iterator returned by upper_bound will refer just after the last instance of the key.

If the element in not int the multimap, then

lower_bound and upper_bound will return equal iterators; both will refer to the point at which the key can be inserted without disrupting the order.

Thus, calling lower_bound and upper_bound on the same key yields an iterator range that denotes all the elements with that key.