NAME¶

std::map - std::map

Synopsis¶

Defined in header <map>
template<

class Key,
class T, (1)
class Compare = std::less<Key>,
class Allocator = std::allocator<std::pair<const Key, T> >

> class map;
namespace pmr {

template <class Key, class T, class Compare = std::less<Key>>
using map = std::map<Key, T, Compare, (2) (since C++17)
std::pmr::polymorphic_allocator<std::pair<const Key,T>>>

}

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)
Allocator::const_pointer (until
const_pointer 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
type describing the result of inserting a node_type, a
specialization of

template <class Iter, class NodeType>
insert_return_type (since struct /*unspecified*/ {
C++17) 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¶

begin returns an iterator to the beginning
cbegin (public member function)
(C++11)
end returns an iterator to the end
cend (public member function)
(C++11)
rbegin returns a reverse iterator to the beginning
crbegin (public member function)
(C++11)
rend returns a reverse iterator to the end
crend (public member function)
(C++11)

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)
inserts elements
insert or nodes
(since C++17)
(public member function)
insert_or_assign inserts an element or assigns to the current element if the key
(C++17) already exists
(public member function)
emplace constructs element in-place
(C++11) (public member function)
emplace_hint constructs elements in-place using a hint
(C++11) (public member function)
try_emplace inserts in-place if the key does not exist, does nothing if the key
(C++17) exists
(public member function)
erase erases elements
(public member function)
swap swaps the contents
(public member function)
extract extracts nodes from the container
(C++17) (public member function)
merge splices nodes from another container
(C++17) (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 checks if the container contains element with specific key
(C++20) (public member function)
equal_range returns range of elements matching a specific key
(public member function)
returns an iterator to the first element not less than the given
lower_bound 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)
returns the function that compares keys in objects of type
value_comp value_type
(public member function)

Non-member functions¶

operator==
operator!=
operator<
operator<=
operator>
operator>= lexicographically compares the values in the map
operator<=> (function template)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(C++20)
std::swap(std::map) specializes the std::swap algorithm
(function template)
erase_if(std::map) Erases all elements satisfying specific criteria
(C++20) (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 at function provided
inconvenient

See also¶

unordered_map collection of key-value pairs, hashed by keys, keys are unique
(C++11) (class template)