Example
Regular Map
A map is an associative container, containing key-value pairs.
#include <string>
#include <map>
std::map<std::string, size_t> fruits_count;
In the above example, std::string is the key type, and size_t is a value.
The key acts as an index in the map. Each key must be unique, and must be ordered.
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If you need mutliple elements with the same key, consider using
multimap(explained below) -
If your value type does not specify any ordering, or you want to override the default ordering, you may provide one:
#include <string> #include <map> #include <cstring> struct StrLess { bool operator()(const std::string& a, const std::string& b) { return strncmp(a.c_str(), b.c_str(), 8)<0; //compare only up to 8 first characters } } std::map<std::string, size_t, StrLess> fruits_count2;If
StrLesscomparator returnsfalsefor two keys, they are considered the same even if their actual contents differ.
Multi-Map
Multimap allows multiple key-value pairs with the same key to be stored in the map. Otherwise, its interface and creation is very similar to the regular map.
#include <string>
#include <map>
std::multimap<std::string, size_t> fruits_count;
std::multimap<std::string, size_t, StrLess> fruits_count2;
Hash-Map (Unordered Map)
A hash map stores key-value pairs similar to a regular map. It does not order the elements with respect to the key though. Instead, a hash value for the key is used to quickly access the needed key-value pairs.
#include <string>
#include <unordered_map>
std::unordered_map<std::string, size_t> fruits_count;
Unordered maps are usually faster, but the elements are not stored in any predictable order. For example, iterating over all elements in an unordered_map gives the elements in a seemingly random order.
