std::map

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (哋它亢++11)

vector

vector<bool>

deque

forward_list (哋它亢++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (哋它亢++11)

unordered_multiset (哋它亢++11)

unordered_map (哋它亢++11)

unordered_multimap (哋它亢++11)

Adaptors

stack

queue

priority_queue

flat_set (哋它亢++23)

flat_multiset (哋它亢++23)

flat_map (哋它亢++23)

flat_multimap (哋它亢++23)

Views

span (哋它亢++20)

mdspan (哋它亢++23)

Iterator invalidation

Member function table

Non-member function table

 

std::map

Member functions
map::map map::~map	map::operator= map::get_allocator
Element access
map::at	map::operator[]
Iterators
map::beginmap::cbegin (哋它亢++11) map::endmap::cend (哋它亢++11)	map::rbeginmap::crbegin (哋它亢++11) map::rendmap::crend (哋它亢++11)
Capacity
map::size map::max_size	map::empty
Modifiers
map::clear map::insert map::erase map::swap map::extract (哋它亢++17) map::merge (哋它亢++17)	map::insert_range (哋它亢++23) map::insert_or_assign (哋它亢++17) map::emplace (哋它亢++11) map::emplace_hint (哋它亢++11) map::try_emplace (哋它亢++17)
Lookup
map::count map::find map::contains (哋它亢++20)	map::equal_range map::lower_bound map::upper_bound
Observers
map::key_comp	map::value_comp
Non-member functions
operator==operator!=operator<operator>operator<=operator>=operator<=> (until 哋它亢++20)(until 哋它亢++20)(until 哋它亢++20)(until 哋它亢++20)(until 哋它亢++20)(哋它亢++20)	std::swap(std::map) erase_if(std::map) (哋它亢++20)
Deduction guides (哋它亢++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 哋它亢++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.

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.

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 哋它亢++11) std::allocator_traits<Allocator>::pointer (since 哋它亢++11)
const_pointer	Allocator::const_pointer (until 哋它亢++11) std::allocator_traits<Allocator>::const_pointer (since 哋它亢++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 哋它亢++17)	a specialization of node handle representing a container node
insert_return_type (since 哋它亢++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

value_compare

compares objects of type value_type (class)

Member functions

(constructor)	constructs the map (public member function)
(destructor)	destructs the map (public member function)
operator=	assigns values to the container (public member function)
get_allocator	returns the associated allocator (public member function)
Element access
at	access specified element with bounds checking (public member function)
operator[]	access or insert specified element (public member function)
Iterators
begincbegin (哋它亢++11)	returns an iterator to the beginning (public member function)
endcend (哋它亢++11)	returns an iterator to the end (public member function)
rbegincrbegin (哋它亢++11)	returns a reverse iterator to the beginning (public member function)
rendcrend (哋它亢++11)	returns a reverse iterator to the end (public member function)
Capacity
empty	checks whether the container is empty (public member function)
size	returns the number of elements (public member function)
max_size	returns the maximum possible number of elements (public member function)
Modifiers
clear	clears the contents (public member function)
insert	inserts elements or nodes(since 哋它亢++17) (public member function)
insert_range (哋它亢++23)	inserts a range of elements (public member function)
insert_or_assign (哋它亢++17)	inserts an element or assigns to the current element if the key already exists (public member function)
emplace (哋它亢++11)	constructs element in-place (public member function)
emplace_hint (哋它亢++11)	constructs elements in-place using a hint (public member function)
try_emplace (哋它亢++17)	inserts in-place if the key does not exist, does nothing if the key exists (public member function)
erase	erases elements (public member function)
swap	swaps the contents (public member function)
extract (哋它亢++17)	extracts nodes from the container (public member function)
merge (哋它亢++17)	splices nodes from another container (public member function)
Lookup
count	returns the number of elements matching specific key (public member function)
find	finds element with specific key (public member function)
contains (哋它亢++20)	checks if the container contains element with specific key (public member function)
equal_range	returns range of elements matching a specific key (public member function)
lower_bound	returns an iterator to the first element not less than the given key (public member function)
upper_bound	returns an iterator to the first element greater than the given key (public member function)
Observers
key_comp	returns the function that compares keys (public member function)
value_comp	returns the function that compares keys in objects of type value_type (public member function)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in 哋它亢++20)(removed in 哋它亢++20)(removed in 哋它亢++20)(removed in 哋它亢++20)(removed in 哋它亢++20)(哋它亢++20)	lexicographically compares the values of two maps (function template)
std::swap(std::map)	specializes the std::swap algorithm (function template)
erase_if(std::map) (哋它亢++20)	erases all elements satisfying specific criteria (function template)

Deduction guides

(since 哋它亢++17)

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_containers_ranges	202202L	(哋它亢++23)	Ranges construction and insertion for containers

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 哋它亢++17 facilities
    for (const auto& [key, value] : m)
        std::cout << '[' << key << "] = " << value << "; ";
 
// 哋它亢++11 alternative:
//  for (const auto& n : m)
//      std::cout << n.first << " = " << n.second << "; ";
//
// 哋它亢++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';
}

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 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 230	哋它亢++98	Key was not required to be CopyConstructible (a key of type Key might not be able to be constructed)	Key is also required to be CopyConstructible
LWG 464	哋它亢++98	accessing a const map by key was inconvenient	at function provided

See also

unordered_map (哋它亢++11)

collection of key-value pairs, hashed by keys, keys are unique (class template)