std::unordered_map (3) - Linux Manuals

std::unordered_map: std::unordered_map


std::unordered_map - std::unordered_map


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

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. 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


* 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 (until C++20)
                                Hash::transparent_key_equal if defined and names a type, otherwise KeyEqual (since C++20)

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
const_iterator Constant LegacyForwardIterator
                                An iterator type whose category, value, difference, pointer and
local_iterator reference types are the same as iterator. This iterator
                                can be used to iterate through a single bucket but not across buckets
                                An iterator type whose category, value, difference, pointer and
const_local_iterator 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
                                type describing the result of inserting a node_type, a specialization of

                                  template <class Iter, class NodeType> struct /*unspecified*/ {
                                      Iter position;
insert_return_type(since C++17) bool inserted;
                                      NodeType node;

                                instantiated with template arguments iterator and node_type.

Member functions

                  constructs the unordered_map
constructor (public member function)
                  destructs the unordered_map
destructor (public member function)
                  assigns values to the container
operator= (public member function)
                  returns the associated allocator
get_allocator (public member function)


begin returns an iterator to the beginning
cbegin (public member function)

end_ returns an iterator to the end
cend (public member function)


                  checks whether the container is empty
empty (public member function)
                  returns the number of elements
size (public member function)
                  returns the maximum possible number of elements
max_size (public member function)


                  clears the contents
clear (public member function)
                  inserts elements
                  or nodes
insert (since C++17)
                  (public member function)

insert_or_assign inserts an element or assigns to the current element if the key already exists
                  (public member function)
                  constructs element in-place
emplace (public member function)
                  constructs elements in-place using a hint
emplace_hint (public member function)

try_emplace inserts in-place if the key does not exist, does nothing if the key exists
                  (public member function)
                  erases elements
erase (public member function)
                  swaps the contents
swap (public member function)

extract extracts nodes from the container
                  (public member function)

merge splices nodes from another container
                  (public member function)


                  access specified element with bounds checking
at (public member function)
                  access or insert specified element
operator[] (public member function)
                  returns the number of elements matching specific key
count (public member function)
                  finds element with specific key
find (public member function)

contains checks if the container contains element with specific key
                  (public member function)
                  returns range of elements matching a specific key
equal_range (public member function)

Bucket interface

                  returns an iterator to the beginning of the specified bucket
begin(size_type) (public member function)
                  returns an iterator to the end of the specified bucket
end(size_type) (public member function)
                  returns the number of buckets
bucket_count (public member function)
                  returns the maximum number of buckets
max_bucket_count (public member function)
                  returns the number of elements in specific bucket
bucket_size (public member function)
                  returns the bucket for specific key
bucket (public member function)

Hash policy

                  returns average number of elements per bucket
load_factor (public member function)
                  manages maximum average number of elements per bucket
max_load_factor (public member function)
                  reserves at least the specified number of buckets.
rehash This regenerates the hash table.
                  (public member function)
                  reserves space for at least the specified number of elements.
reserve This regenerates the hash table.
                  (public member function)


                  returns function used to hash the keys
hash_function (public member function)
                  returns the function used to compare keys for equality
key_eq (public member function)

Non-member functions

                              compares the values in the unordered_map
operator== (function template)

std::swap(std::unordered_map) specializes the std::swap algorithm
                              (function template)

erase_if(std::unordered_map) Erases all elements satisfying specific criteria
                              (function template)

Deduction_guides(since C++17)


// 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 = {

      // Iterate and print keys and values of unordered_map
      for( const auto& n : u ) {
          std::cout << "Key:[" << n.first << "] Value:[" << n.second << "]\n";

      // Add two new entries to the unordered_map
      u["BLACK"] = "#000000";
      u["WHITE"] = "#FFFFFF";

      // Output values by key
      std::cout << "The HEX of color RED is:[" << u["RED"] << "]\n";
      std::cout << "The HEX of color BLACK is:[" << u["BLACK"] << "]\n";

      return 0;


  Key:[RED] Value:[#FF0000]
  Key:[BLUE] Value:[#0000FF]
  Key:[GREEN] Value:[#00FF00]
  The HEX of color RED is:[#FF0000]
  The HEX of color BLACK is:[#000000]