11.4. The Unordered Containers

The new standard defines four unordered associative containers. Rather than using a comparison operation to organize their elements, these containers use a hash function and the key type’s == operator. An unordered container is most useful when we have a key type for which there is no obvious ordering relationship among the elements. These containers are also useful for applications in which the cost of maintaining the elements in order is prohibitive.

Although hashing gives better average case performance in principle, achieving good results in practice often requires a fair bit of performance testing and tweaking. As a result, it is usually easier (and often yields better performance) to use an ordered container.