All elements with the same key are adjacent to each other. That is, if p and q are iterators that point to elements that have the same key, and if p precedes q , then every element in the range [p, q) has the same key as every other element.
Immutability of keys
Every element of an Associative Container has an immutable key. Objects may be inserted and erased, but an element in an Associative Container may not be modified in such a way as to change its key.