Unordered Containers

Unordered associative containers differ from associative containers in that the elements have no defined order. Visually, unordered containers are often imagined as bags of elements. Because the elements are not sorted, unordered containers do not accept a comparator object to provide an order to the elements. On the other hand, all the unordered containers depend on a hash function.

he user can provide a Functor (function object) as a parameter to specify how the keys should be hashed:

Figure 5.13: Table presenting unordered containers and their descriptions

Typically, unordered containers are implemented as hash tables. The position in the array is determined using the hash function, which given a value returns the position at which it should be stored. Ideally, most of the elements will be mapped into different positions, but the hash function can potentially return the same position for different elements. This is called a collision. This problem is solved by using...