std::map

 

C++

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Feature test macros (C++20)

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Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

vector<bool>

inplace_vector (C++26)

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

Views

span (C++20)

mdspan (C++23)

Tables

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 (C++11) map::endmap::cend (C++11) map::rbeginmap::crbegin (C++11)      map::rendmap::crend (C++11) Capacity map::size map::max_size map::empty Observers map::key_comp map::value_comp Modifiers map::clear map::insert map::erase map::swap map::extract (C++17) map::merge (C++17) map::insert_range (C++23) map::insert_or_assign (C++17) map::emplace (C++11) map::emplace_hint (C++11) map::try_emplace (C++17) Lookup map::count map::find map::contains (C++20) map::equal_range map::lower_bound map::upper_bound

Non-member functions

operator==operator<=> (C++20) std::swap(std::map) erase_if(std::map) (C++20)   operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until 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

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 (C++11)	returns an iterator to the beginning (public member function)
endcend (C++11)	returns an iterator to the end (public member function)
rbegincrbegin (C++11)	returns a reverse iterator to the beginning (public member function)
rendcrend (C++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 C++17) (public member function)
insert_or_assign (C++17)	inserts an element or assigns to the current element if the key already exists (public member function)
emplace (C++11)	constructs element in-place (public member function)
emplace_hint (C++11)	constructs elements in-place using a hint (public member function)
try_emplace (C++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 (C++17)	extracts nodes from the container (public member function)
merge (C++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 (C++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 C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++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) (C++20)	erases all elements satisfying specific criteria (function template)

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 (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 C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 230	C++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	C++98	accessing a const map by key was inconvenient	at function provided

See also

unordered_map (C++11)

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