std:: map

std::map is a sorted associative container that contains key-value pairs with unique keys. Keys are sorted by using the comparison function Compare . Search, removal, and insertion operations have logarithmic complexity. Maps are usually implemented as Red–black trees .

Iterators of std::map iterate in ascending order of keys, where ascending is defined by the comparison that was used for construction. That is, given

• m , a std::map
• it_l and it_r , dereferenceable iterators to m , with it_l < it_r .

m. value_comp ( ) ( * it_l, * it_r ) == true (least to greatest if using the default comparison).

Everywhere the standard library uses the Compare requirements, uniqueness is determined by using the equivalence relation. In imprecise terms, two objects a and b are considered equivalent (not unique) if neither compares less than the other: ! comp ( a, b ) && ! comp ( b, a ) .

std::map meets the requirements of Container , AllocatorAwareContainer , AssociativeContainer and ReversibleContainer .

Template parameters

Member types

Member classes

Member functions

Non-member functions

Notes

Example

#include <iostream>
#include <map>
#include <string>
#include <string_view>
 
void print_map(std::string_view comment, const std::map<std::string, int>& m)
{
    std::cout << comment;
    // Iterate using C++17 facilities
    for (const auto& [key, value] : m)
        std::cout << '[' << key << "] = " << value << "; ";
 
// C++11 alternative:
//  for (const auto& n : m)
//      std::cout << n.first << " = " << n.second << "; ";
//
// C++98 alternative:
//  for (std::map<std::string, int>::const_iterator it = m.begin(); it != m.end(); ++it)
//      std::cout << it->first << " = " << it->second << "; ";
 
    std::cout << '\n';
}
 
int main()
{
    // Create a map of three (string, int) pairs
    std::map<std::string, int> m{{"CPU", 10}, {"GPU", 15}, {"RAM", 20}};
 
    print_map("1) Initial map: ", m);
 
    m["CPU"] = 25; // update an existing value
    m["SSD"] = 30; // insert a new value
    print_map("2) Updated map: ", m);
 
    // Using operator[] with non-existent key always performs an insert
    std::cout << "3) m[UPS] = " << m["UPS"] << '\n';
    print_map("4) Updated map: ", m);
 
    m.erase("GPU");
    print_map("5) After erase: ", m);
 
    std::erase_if(m, [](const auto& pair){ return pair.second > 25; });
    print_map("6) After erase: ", m);
    std::cout << "7) m.size() = " << m.size() << '\n';
 
    m.clear();
    std::cout << std::boolalpha << "8) Map is empty: " << m.empty() << '\n';
}

1) Initial map: [CPU] = 10; [GPU] = 15; [RAM] = 20;
2) Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30;
3) m[UPS] = 0
4) Updated map: [CPU] = 25; [GPU] = 15; [RAM] = 20; [SSD] = 30; [UPS] = 0;
5) After erase: [CPU] = 25; [RAM] = 20; [SSD] = 30; [UPS] = 0;
6) After erase: [CPU] = 25; [RAM] = 20; [UPS] = 0;
7) m.size() = 3
8) Map is empty: true

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also