The library guarantees that iterators access elements in order by key.The begin operation yields the element with the lowest key.Incrementing that iterator yields elements in nondescending order.

• 除此之外，关联容器还定义一些新操作，并对某些顺序容器同样提供的操作重新定义了其含义或返回类型，这些操作的差别体现了关联容器中键的使用。