std::map

From cppreference.com
< cpp‎ | container
 
C++
 
Containers library
Sequence
(C++11)
(C++11)
Associative
map
Unordered associative
(C++11)
(C++11)
(C++11)
(C++11)
Adaptors
Views
(C++20)
 
std::map
Member functions
Element access
Iterators
(C++11)
(C++11)
(C++11)
(C++11)
Capacity
Modifiers
(C++17)
(C++11)
(C++11)
(C++17)
(C++17)
(C++17)
Lookup
(C++20)
Observers
Non-member functions
(C++20)
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)
Deduction guides (C++17)
 
Defined in header <map>
template<

    class Key,
    class T,
    class Compare = std::less<Key>,
    class Allocator = std::allocator<std::pair<const Key, T> >

> class map;
 (1)
namespace pmr {

    template <class Key, class T, class Compare = std::less<Key>>
    using map = std::map<Key, T, Compare,
                         std::pmr::polymorphic_allocator<std::pair<const Key,T>>>

}
 (2) (since C++17)

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.

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.

Member types

Member type Definition
key_type Key
mapped_type T
value_type std::pair<const Key, T>
size_type Unsigned integer type (usually std::size_t)
difference_type Signed integer type (usually std::ptrdiff_t)
key_compare Compare
allocator_type Allocator
reference value_type&
const_reference const value_type&
pointer
Allocator::pointer (until C++11)
std::allocator_traits<Allocator>::pointer (since C++11)
const_pointer
Allocator::const_pointer (until C++11)
std::allocator_traits<Allocator>::const_pointer (since C++11)
iterator LegacyBidirectionalIterator to value_type
const_iterator LegacyBidirectionalIterator to const value_type
reverse_iterator std::reverse_iterator<iterator>
const_reverse_iterator std::reverse_iterator<const_iterator>
node_type (since C++17) a specialization of node handle representing a container node
insert_return_type (since C++17) type describing the result of inserting a node_type, a specialization of

template <class Iter, class NodeType>
struct /*unspecified*/ {
    Iter     position;
    bool     inserted;
    NodeType node;
};
instantiated with template arguments iterator and node_type.

Member classes

 compares objects of type value_type
(class)

Member functions

 constructs the map
(public member function)
destructs the map
(public member function)
assigns values to the container
(public member function)
returns the associated allocator
(public member function)
Element access
access specified element with bounds checking
(public member function)
access or insert specified element
(public member function)
Iterators
(C++11)
 returns an iterator to the beginning
(public member function)
(C++11)
 returns an iterator to the end
(public member function)
(C++11)
 returns a reverse iterator to the beginning
(public member function)
(C++11)
 returns a reverse iterator to the end
(public member function)
Capacity
checks whether the container is empty
(public member function)
returns the number of elements
(public member function)
returns the maximum possible number of elements
(public member function)
Modifiers
clears the contents
(public member function)
inserts elements or nodes (since C++17)
(public member function)
(C++17)
 inserts an element or assigns to the current element if the key already exists
(public member function)
(C++11)
 constructs element in-place
(public member function)
(C++11)
 constructs elements in-place using a hint
(public member function)
(C++17)
 inserts in-place if the key does not exist, does nothing if the key exists
(public member function)
erases elements
(public member function)
swaps the contents
(public member function)
(C++17)
 extracts nodes from the container
(public member function)
(C++17)
 splices nodes from another container
(public member function)
Lookup
returns the number of elements matching specific key
(public member function)
finds element with specific key
(public member function)
(C++20)
 checks if the container contains element with specific key
(public member function)
returns range of elements matching a specific key
(public member function)
returns an iterator to the first element not less than the given key
(public member function)
returns an iterator to the first element greater than the given key
(public member function)
Observers
returns the function that compares keys
(public member function)
returns the function that compares keys in objects of type value_type
(public member function)

Non-member functions

(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)
 lexicographically compares the values in the map
(function template)
specializes the std::swap algorithm
(function template)
(C++20)
 Erases all elements satisfying specific criteria
(function template)

Deduction guides (since C++17)

Example

Run this code
#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 (strings, 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';
}

Output: 

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.

DR Applied to Behavior as published Correct behavior
LWG 464 C++98 accessing a const map by key was inconvenient at function provided

See also

(C++11)
 collection of key-value pairs, hashed by keys, keys are unique
(class template)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/map&oldid=137548"