std::unordered_map

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C++
 
Containers library
 
std::unordered_map
Member types
Member functions
Iterators
Capacity
Modifiers
(C++23)
(C++17)
(C++17)
(C++17)
(C++17)
Lookup
(C++20)
Bucket interface
Hash policy
Observers
Non-member functions
(until C++20)
(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 KeyEqual = std::equal_to<Key>
> using unordered_map = std::unordered_map<Key, T, Hash, KeyEqual,
                            std::pmr::polymorphic_allocator<std::pair<const Key,T>>>;

}
 (2) (since C++17)

std: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.

Two keys are considered equivalent if the map's key equality predicate returns true when passed those keys. If two keys are equivalent, the hash function must return the same value for both keys.

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.

Template parameters

This section is incomplete
Reason: Add descriptions of the template parameters.

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

(C++11)
 constructs the unordered_map
(public member function)
(C++11)
 destructs the unordered_map
(public member function)
(C++11)
 assigns values to the container
(public member function)
(C++11)
 returns the associated allocator
(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)
Capacity
(C++11)
 checks whether the container is empty
(public member function)
(C++11)
 returns the number of elements
(public member function)
(C++11)
 returns the maximum possible number of elements
(public member function)
Modifiers
(C++11)
 clears the contents
(public member function)
(C++11)
 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)
(C++11)
 erases elements
(public member function)
(C++11)
 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
(C++11)
 access specified element with bounds checking
(public member function)
(C++11)
 access or insert specified element
(public member function)
(C++11)
 returns the number of elements matching specific key
(public member function)
(C++11)
 finds element with specific key
(public member function)
(C++20)
 checks if the container contains element with specific key
(public member function)
(C++11)
 returns range of elements matching a specific key
(public member function)
Bucket interface
(C++11)
 returns an iterator to the beginning of the specified bucket
(public member function)
(C++11)
 returns an iterator to the end of the specified bucket
(public member function)
(C++11)
 returns the number of buckets
(public member function)
(C++11)
 returns the maximum number of buckets
(public member function)
(C++11)
 returns the number of elements in specific bucket
(public member function)
(C++11)
 returns the bucket for specific key
(public member function)
Hash policy
(C++11)
 returns average number of elements per bucket
(public member function)
(C++11)
 manages maximum average number of elements per bucket
(public member function)
(C++11)
 reserves at least the specified number of buckets and regenerates the hash table
(public member function)
(C++11)
 reserves space for at least the specified number of elements and regenerates the hash table
(public member function)
Observers
(C++11)
 returns function used to hash the keys
(public member function)
(C++11)
 returns the function used to compare keys for equality
(public member function)

Non-member functions

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

Deduction guides

 (since C++17)

Notes

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

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 = [](const auto& key, const auto& value)
    {
        std::cout << "Key:[" << key << "] Value:[" << value << "]\n";
    };
 
    std::cout << "Iterate and print key-value pairs of unordered_map, being\n"
                 "explicit with their types:\n";
    for (const std::pair<const std::string, std::string>& n : u)
        print_key_value(n.first, n.second);
 
    std::cout << "\nIterate and print key-value pairs using C++17 structured binding:\n";
    for (const auto& [key, value] : u)
        print_key_value(key, value);
 
    // Add two new entries to the unordered_map
    u["BLACK"] = "#000000";
    u["WHITE"] = "#FFFFFF";
 
    std::cout << "\nOutput values by key:\n"
                 "The HEX of color RED is:[" << u["RED"] << "]\n"
                 "The HEX of color BLACK is:[" << u["BLACK"] << "]\n\n";
 
    std::cout << "Use operator[] with non-existent key to insert a new key-value pair:\n";
    print_key_value("new_key", u["new_key"]);
 
    std::cout << "\nIterate and print key-value pairs, using `auto`;\n"
                 "new_key is now one of the keys in the map:\n";
    for (const auto& n : u)
        print_key_value(n.first, n.second);
}

Possible output: 

Iterate and print key-value pairs of unordered_map, being
explicit with their types:
Key:[BLUE] Value:[#0000FF]
Key:[GREEN] Value:[#00FF00]
Key:[RED] Value:[#FF0000]
 
Iterate and print key-value pairs using C++17 structured binding:
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]
 
Use operator[] with non-existent key to insert a new key-value pair:
Key:[new_key] Value:[]
 
Iterate and print key-value pairs, using `auto`;
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

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