C++ Primer, Fifth Edition

Requirements on Key Type for Unordered Containers

By default, the unordered containers use the == operator on the key type to compare elements. They also use an object of type hash<key_type> to generate the hash code for each element. The library supplies versions of the hash template for the built-in types, including pointers. It also defines hash for some of the library types, including strings and the smart pointer types that we will describe in Chapter 12. Thus, we can directly define unordered containers whose key is one of the built-in types (including pointer types), or a string, or a smart pointer.

However, we cannot directly define an unordered container that uses a our own class types for its key type. Unlike the containers, we cannot ...