11.3. Operations on Associative Containers

In addition to the types listed in Table 9.2 (p. 330), the associative containers define the types listed in Table 11.3. These types represent the container’s key and value types.

Table 11.3. Associative Container Additional Type Aliases

For the set types, the key_type and the value_type are the same; the values held in a set are the keys. In a map, the elements are key–value pairs. That is, each element is a pair object containing a key and a associated value. Because we cannot change an element’s key, the key part of these pairs is const:

set<string>::value_type v1;      // v1 is a string
set<string>::key_type v2;        // v2 is a string
map<string, int>::value_type v3; // v3 is a pair<const string, int>
map<string, int>::key_type v4;   // v4 is a string
map<string, int>::mapped_type v5; // v5 is an int

As with the sequential containers (§ 9.2.2, p. 332), we use the scope operator to fetch a type member—for example, map<string, int>::key_type.

Only the map types (unordered_map, unordered_multimap, multimap, and map) define mapped_type .