Introduction

C++ has eight different associative containers. Four of them are associative containers with sorted keys: std::set, std::map, std::multiset and std::multimap. The other four are associative containers with unsorted keys: std::unordered_set, std::unordered_map, std::unordered_multiset and std::unordered_multimap. The associative containers are special containers. That means they support all of the operations described in the chapter Interface of all containers.

Overview

All eight ordered and unordered containers have in common that they associate a key with a value. You can use the key to get the value. To get a classification of the associative containers, three simple questions need to be answered:

• Are the keys sorted?
• Does the key have an associated value?
• Can a key appear more than once?

The following table with $2^3$ = 8 rows give the answers to the three questions. I answer a fourth question in the table. How fast is the access time of a key in the best case?

Associative container	Sorted	Associated value	More identical keys	Access time
std::set	yes	no	no	logarithmic
std::unordered_set	no	no	no	constant
std::map	yes	yes	no	logarithmic
std::unordered_map	no	yes	no	constant
std::multiset	yes	no	yes	logarithmic
std::unordered_multiset	no	no	yes	constant
std::multimap	yes	yes	yes	logarithmic
std::unordered_multimap	no	yes	yes	constant

Characteristics for associative containers

Since C++98, C++ has ordered associative containers; with C++11, C++ has in addition unordered associative containers. Both classes have a very similar interface. That’s the reason that the following code sample is identical for std::map and std::unordered_map. To be more precise, the interface of std::unordered_map is a superset of the interface of std::map. The same holds for the remaining three unordered associative containers. So the porting of your code from the ordered to unordered containers is quite easy.

You can initialize the containers with an initialiser list and add new elements with the index operator. To access the first element of the key/value pair p, you have p.first, and for the second element, you have p.second. p.first is the key and p.second is the associated value of the pair.