From cppreference.com
Defined in header
template<classKey,classT,classCompare=std::less<Key>,classAllocator=std::allocator<std::pair<constKey,T>>>classmap; (1) namespacepmr{template<classKey,classT,classCompare=std::less<Key>>usingmap=std::map<Key,T,Compare,std::pmr::polymorphic_allocator<std::pair<constKey,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
.
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
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
,
,
and
.
All member functions of std::map are constexpr: it is possible to create and use std::map objects in the evaluation of a constant expression.
However, defining a constexprstd::map variable is generally an error, because constant evaluation requires any dynamically allocated storage to be released in the same evaluation, which is usually not the case with std::map's initializer.
(since C++26)Template parameters
Member types
Type Definition key_typeKey
mapped_typeT
value_typestd::pair<constKey,T>
size_typeunsigned integer type (usually
)
difference_typesigned integer type (usually
)
key_compareCompare
allocator_typeAllocator
referencevalue_type&
const_referenceconstvalue_type&
pointerAllocator::pointer
(until C++11)std::allocator_traits<Allocator>::pointer
(since C++11)
const_pointerAllocator::const_pointer
(until C++11)std::allocator_traits<Allocator>::const_pointer
(since C++11)
iterator
and
(since C++26) to value_type
const_iterator
and
(since C++26) to constvalue_type
reverse_iteratorstd::reverse_iterator<iterator>
const_reverse_iteratorstd::reverse_iterator<const_iterator>
node_type(since C++17)a specialization of
representing a container node
insert_return_type(since C++17)type describing the result of inserting a node_type, a specialization of template<classIter,classNodeType>struct/* unspecified */{Iterposition;boolinserted;NodeTypenode;};instantiated with template arguments iterator and node_type.
Member classes
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++23)
inserts a range of elements
(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
Notes
macro ValueStdFeature
(C++23)Ranges construction and insertion for containers
(C++26)Constexpr std::mapExample
Run this code
#include<iostream>#include<map>#include<string>#include<string_view>voidprint_map(std::string_viewcomment,conststd::map<std::string,int>&m){std::cout<<comment;// Iterate using C++17 facilitiesfor(constauto&[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';}intmain(){// Create a map of three (string, int) pairsstd::map<std::string,int>m{{"CPU",10},{"GPU",15},{"RAM",20}};print_map("1) Initial map: ",m);m["CPU"]=25;// update an existing valuem["SSD"]=30;// insert a new valueprint_map("2) Updated map: ",m);// Using operator[] with non-existent key always performs an insertstd::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,[](constauto&pair){returnpair.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
C++98 Key was not required to be
(a key of type Key might not be able to be constructed) Key is also required to
be
C++98 accessing a const map by key was inconvenient at function provided See also
collection of key-value pairs, sorted by keys
(class template)
(C++11)
collection of key-value pairs, hashed by keys, keys are unique
(class template)
(C++23)
adapts two containers to provide a collection of key-value pairs, sorted by unique keys
(class template)