Chapter 10. Associative Containers

CONTENTS

Section 10.1 Preliminaries: the pair Type	356
Section 10.2 Associative Containers	358
Section 10.3 The map Type	360
Section 10.4 The set Type	372
Section 10.5 The multimap and multiset Types	375
Section 10.6 Using Containers: Text-Query Program	379
Chapter Summary	388
Defined Terms	388

This chapter completes our review of the standard library container types by looking at the associative containers. Associative containers differ in a fundamental respect from the sequential containers: Elements in an associative container are stored and retrieved by a key, in contrast to elements in a sequential container, which are stored and accessed sequentially by their position within the container.

Although the associative containers share much of the behavior of the sequential containers, they differ from the sequential containers in ways that support the use of keys. This chapter covers the associative containers and closes with an extended example that uses both associative and sequential containers.

Associative containers support efficient lookup and retrieval by a key. The two primary associative-container types are map and set. The elements in a map are keyvalue pairs: The key serves as an index into the map, and the value represents the data that are stored and retrieved. A set contains only a key and supports efficient queries to whether a given key is present.

In general, a set is most useful when we want to store a collection of distinct values efficiently, and a map is most useful when we wish to store (and possibly modify) a value associated with each key. We might use a set to hold words that we want to ignore when doing some kind of text processing. A dictionary would be a good use for a map: The word would be the key, and its definition would be the value.

An object of the map or set type may contain only a single element with a given key. There is no way to add a second element with the same key. If we need to have multiple instances with a single key, then we can use multimap or multi set, which do allow multiple elements with a given key.

The associative containers support many of the same operations as do the sequential containers. They also provide specialized operations that manage or use the key. In the sections that follow, we look at the associative container types and their operations in detail. We'll conclude the chapter by using the containers to implement a small text-query program.

Table 10.1. Associative Container Types

map	Associative array; elements stored and retrieved by key
set	Variable-sized collection with fast retrieval by key
multimap	map in which a key can appear multiple times
multiset	set in which a key can appear multiple times