C++ Primer, Fifth Edition

11.2.2. Requirements on Key Type

The associative containers place constraints on the type that is used as a key. We’ll cover the requirements for keys in the unordered containers in § 11.4 (p. 445). For the ordered containers—map, multimap, set, and multiset—the key type must define a way to compare the elements. By default, the library uses the < operator for the key type to compare the keys. In the set types, the key is the element type; in the map types, the key is the first type. Thus, the key type for word_count in § 11.1 (p. 421) is string. Similarly, the key type for exclude is string.

Note

Callable objects passed to a sort algorithm ...