std::unordered_map

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

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::unordered_map

Member types
Member functions
unordered_map::unordered_map unordered_map::~unordered_map unordered_map::operator= unordered_map::get_allocator Iterators unordered_map::beginunordered_map::cbegin unordered_map::endunordered_map::cend Capacity unordered_map::size unordered_map::max_size unordered_map::empty Modifiers unordered_map::clear unordered_map::erase unordered_map::swap unordered_map::extract (C++17) unordered_map::merge (C++17) unordered_map::insert unordered_map::insert_range (C++23) unordered_map::insert_or_assign (C++17)   unordered_map::emplace unordered_map::emplace_hint unordered_map::try_emplace (C++17)	Lookup unordered_map::at unordered_map::operator[] unordered_map::count unordered_map::find unordered_map::contains (C++20) unordered_map::equal_range Bucket interface unordered_map::begin(size_type)unordered_map::cbegin(size_type) unordered_map::end(size_type)unordered_map::cend(size_type) unordered_map::bucket_count unordered_map::max_bucket_count unordered_map::bucket_size unordered_map::bucket Hash policy unordered_map::load_factor unordered_map::max_load_factor unordered_map::rehash unordered_map::reserve Observers unordered_map::hash_function unordered_map::key_eq
Non-member functions
operator==operator!= (until C++20)	std::swap(std::unordered_map) erase_if(std::unordered_map) (C++20)
Deduction guides(C++17)

 

Defined in header <unordered_map>
template<     class Key,     class T,     class Hash = std::hash<Key>,     class KeyEqual = std::equal_to<Key>,     class Allocator = std::allocator< std::pair<const Key, T> > > class unordered_map;	(1)	(since C++11)
namespace pmr {     template <class Key,               class T,               class Hash = std::hash<Key>,               class Pred = std::equal_to<Key>>     using unordered_map = std::unordered_map<Key, T, Hash, Pred,                               std::pmr::polymorphic_allocator<std::pair<const Key,T>>>; }	(2)	(since C++17)

Unordered map is an associative container that contains key-value pairs with unique keys. Search, insertion, and removal of elements have average constant-time complexity.

Internally, the elements are not sorted in any particular order, but organized into buckets. Which bucket an element is placed into depends entirely on the hash of its key. Keys with the same hash code appear in the same bucket. This allows fast access to individual elements, since once the hash is computed, it refers to the exact bucket the element is placed into.

std::unordered_map meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

Iterator invalidation

Operations	Invalidated
All read only operations, swap, std::swap	Never
clear, rehash, reserve, operator=	Always
insert, emplace, emplace_hint, operator[]	Only if causes rehash
erase	Only to the element erased

Notes

• The swap functions do not invalidate any of the iterators inside the container, but they do invalidate the iterator marking the end of the swap region.

• References and pointers to either key or data stored in the container are only invalidated by erasing that element, even when the corresponding iterator is invalidated.

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)
hasher	Hash
key_equal	KeyEqual
allocator_type	Allocator
reference	value_type&
const_reference	const value_type&
pointer	std::allocator_traits<Allocator>::pointer
const_pointer	std::allocator_traits<Allocator>::const_pointer
iterator	LegacyForwardIterator to value_type
const_iterator	LegacyForwardIterator to const value_type
local_iterator	An iterator type whose category, value, difference, pointer and reference types are the same as iterator. This iterator can be used to iterate through a single bucket but not across buckets
const_local_iterator	An iterator type whose category, value, difference, pointer and reference types are the same as const_iterator. This iterator can be used to iterate through a single bucket but not across buckets
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 functions

(constructor)	constructs the unordered_map (public member function)
(destructor)	destructs the unordered_map (public member function)
operator=	assigns values to the container (public member function)
get_allocator	returns the associated allocator (public member function)
Iterators
begincbegin	returns an iterator to the beginning (public member function)
endcend	returns an 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	constructs element in-place (public member function)
emplace_hint	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
at	access specified element with bounds checking (public member function)
operator[]	access or insert specified element (public member function)
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)
Bucket interface
begin(size_type)cbegin(size_type)	returns an iterator to the beginning of the specified bucket (public member function)
end(size_type)cend(size_type)	returns an iterator to the end of the specified bucket (public member function)
bucket_count	returns the number of buckets (public member function)
max_bucket_count	returns the maximum number of buckets (public member function)
bucket_size	returns the number of elements in specific bucket (public member function)
bucket	returns the bucket for specific key (public member function)
Hash policy
load_factor	returns average number of elements per bucket (public member function)
max_load_factor	manages maximum average number of elements per bucket (public member function)
rehash	reserves at least the specified number of buckets and regenerates the hash table (public member function)
reserve	reserves space for at least the specified number of elements and regenerates the hash table (public member function)
Observers
hash_function	returns function used to hash the keys (public member function)
key_eq	returns the function used to compare keys for equality (public member function)

Non-member functions

operator==operator!= (C++11)(C++11)(removed in C++20)	compares the values in the unordered_map (function template)
std::swap(std::unordered_map) (C++11)	specializes the std::swap algorithm (function template)
erase_if(std::unordered_map) (C++20)	erases all elements satisfying specific criteria (function template)

Example

Run this code

#include <iostream>
#include <string>
#include <unordered_map>
 
int main()
{
    // Create an unordered_map of three strings (that map to strings)
    std::unordered_map<std::string, std::string> u = {
        {"RED","#FF0000"},
        {"GREEN","#00FF00"},
        {"BLUE","#0000FF"}
    };
 
    // Helper lambda function to print key:value pairs
    auto print_key_value = [](std::string const& key, std::string const& value) {
        std::cout << "Key:[" << key << "] Value:[" << value << "]\n";
    };
 
    std::cout << "Iterate and print keys and values of unordered_map, being\n"
                 "explicit with the type of the iterator, n:\n";
    for( const std::pair<const std::string, std::string>& n : u ) {
        print_key_value(n.first, n.second);
    }
    std::cout << "\n";
 
    std::cout << "Iterate and print keys and values of unordered_map,\n"
                 "using auto:\n";
    for( const auto& n : u ) {
        print_key_value(n.first, n.second);
    }
    std::cout << "\n";
 
    std::cout << "Iterate and print keys and values using structured binding\n"
                 "(since C++17):\n";
    for( const auto& [key, value] : u ) {
        print_key_value(key, value);
    }
    std::cout << "\n";
 
    // Add two new entries to the unordered_map
    u["BLACK"] = "#000000";
    u["WHITE"] = "#FFFFFF";
 
    std::cout << "Output values by key:\n";
    std::cout << "The HEX of color RED is:[" << u["RED"] << "]\n";
    std::cout << "The HEX of color BLACK is:[" << u["BLACK"] << "]\n\n";
 
    // Use of operator[] with non-existent key inserts a new value
    print_key_value("new_key", u["new_key"]);
    std::cout << "Iterating over the unordered_map shows `new_key` is now one\n"
                 "of the keys in the map:\n";
    for( const std::pair<const std::string, std::string>& n : u ) {
        print_key_value(n.first, n.second);
    }
}

Possible output:

Iterate and print keys and values of unordered_map, being
explicit with the type of the iterator, n:
Key:[BLUE] Value:[#0000FF]
Key:[GREEN] Value:[#00FF00]
Key:[RED] Value:[#FF0000]
 
Iterate and print keys and values of unordered_map,
using auto:
Key:[BLUE] Value:[#0000FF]
Key:[GREEN] Value:[#00FF00]
Key:[RED] Value:[#FF0000]
 
Iterate and print keys and values using structured binding
(since C++17):
Key:[BLUE] Value:[#0000FF]
Key:[GREEN] Value:[#00FF00]
Key:[RED] Value:[#FF0000]
 
Output values by key:
The HEX of color RED is:[#FF0000]
The HEX of color BLACK is:[#000000]
 
Key:[new_key] Value:[]
Iterating over the unordered_map shows `new_key` is now one
of the keys in the map:
Key:[new_key] Value:[]
Key:[WHITE] Value:[#FFFFFF]
Key:[BLACK] Value:[#000000]
Key:[BLUE] Value:[#0000FF]
Key:[GREEN] Value:[#00FF00]
Key:[RED] Value:[#FF0000]

See also

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