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      std::unordered_map

      From cppreference.com
      <cpp‎ |container
       
       
       
      std::unordered_map
      Member types
      Member functions
      Non-member functions
      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 bucket containing the element.

      Two keys are consideredequivalent 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 ofContainer,AllocatorAwareContainer,UnorderedAssociativeContainer.

      All member functions ofstd::unordered_map areconstexpr: it is possible to create and usestd::unordered_map objects in the evaluation of a constant expression.

      However,std::unordered_map objects generally cannot beconstexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.

      		(since C++26)

      Contents

      [edit]Iterator invalidation

      OperationsInvalidated
      All read only operations,swap,std::swapNever
      clear,rehash,reserve,operator=Always
      insert,emplace,emplace_hint,operator[]Only if causes rehash
      eraseOnly to the element erased

      [edit]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.

      [edit]Template parameters

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

      [edit]Member types

      Type Definition
      key_typeKey[edit]
      mapped_typeT[edit]
      value_typestd::pair<const Key, T>[edit]
      size_type Unsigned integer type (usuallystd::size_t)[edit]
      difference_type Signed integer type (usuallystd::ptrdiff_t)[edit]
      hasherHash[edit]
      key_equalKeyEqual[edit]
      allocator_typeAllocator[edit]
      referencevalue_type&[edit]
      const_referenceconst value_type&[edit]
      pointerstd::allocator_traits<Allocator>::pointer[edit]
      const_pointerstd::allocator_traits<Allocator>::const_pointer[edit]
      iteratorLegacyForwardIterator andConstexprIterator(since C++26) tovalue_type[edit]
      const_iteratorLegacyForwardIterator andConstexprIterator(since C++26) toconst value_type[edit]
      local_iterator An iterator type whose category, value, difference, pointer and
      reference types are the same asiterator. This iterator
      can be used to iterate through a single bucket but not across buckets[edit]
      const_local_iterator An iterator type whose category, value, difference, pointer and
      reference types are the same asconst_iterator. This iterator
      can be used to iterate through a single bucket but not across buckets[edit]
      node_type(since C++17) a specialization ofnode handle representing a container node[edit]
      insert_return_type(since C++17) type describing the result of inserting anode_type, a specialization of

      template<class Iter,class NodeType>
      struct/*unspecified*/
      {
          Iter     position;
         bool     inserted;
          NodeType node;
      };
      instantiated with template argumentsiterator andnode_type.[edit]

      [edit]Member functions

      constructs theunordered_map
      (public member function)[edit]
      destructs theunordered_map
      (public member function)[edit]
      assigns values to the container
      (public member function)[edit]
      returns the associated allocator
      (public member function)[edit]
      Iterators
      returns an iterator to the beginning
      (public member function)[edit]
      returns an iterator to the end
      (public member function)[edit]
      Capacity
      checks whether the container is empty
      (public member function)[edit]
      returns the number of elements
      (public member function)[edit]
      returns the maximum possible number of elements
      (public member function)[edit]
      Modifiers
      clears the contents
      (public member function)[edit]
      inserts elementsor nodes(since C++17)
      (public member function)[edit]
      inserts a range of elements
      (public member function)[edit]
      inserts an element or assigns to the current element if the key already exists
      (public member function)[edit]
      constructs element in-place
      (public member function)[edit]
      constructs elements in-place using a hint
      (public member function)[edit]
      inserts in-place if the key does not exist, does nothing if the key exists
      (public member function)[edit]
      erases elements
      (public member function)[edit]
      swaps the contents
      (public member function)[edit]
      (C++17)
      		 extracts nodes from the container
      (public member function)[edit]
      (C++17)
      		 splices nodes from another container
      (public member function)[edit]
      Lookup
      access specified element with bounds checking
      (public member function)[edit]
      access or insert specified element
      (public member function)[edit]
      returns the number of elements matching specific key
      (public member function)[edit]
      finds element with specific key
      (public member function)[edit]
      (C++20)
      		 checks if the container contains element with specific key
      (public member function)[edit]
      returns range of elements matching a specific key
      (public member function)[edit]
      Bucket interface
      returns an iterator to the beginning of the specified bucket
      (public member function)[edit]
      returns an iterator to the end of the specified bucket
      (public member function)[edit]
      returns the number of buckets
      (public member function)[edit]
      returns the maximum number of buckets
      (public member function)[edit]
      returns the number of elements in specific bucket
      (public member function)[edit]
      returns the bucket for specific key
      (public member function)[edit]
      Hash policy
      returns average number of elements per bucket
      (public member function)[edit]
      manages maximum average number of elements per bucket
      (public member function)[edit]
      reserves at least the specified number of buckets and regenerates the hash table
      (public member function)[edit]
      reserves space for at least the specified number of elements and regenerates the hash table
      (public member function)[edit]
      Observers
      returns function used to hash the keys
      (public member function)[edit]
      returns the function used to compare keys for equality
      (public member function)[edit]

      [edit]Non-member functions

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

      Deduction guides

      		(since C++17)

      [edit]Notes

      Feature-test macroValueStdFeature
      __cpp_lib_containers_ranges202202L(C++23)Ranges construction and insertion for containers
      __cpp_lib_constexpr_unordered_map202502L(C++26)constexprstd::unordered_map

      [edit]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 pairsauto print_key_value=[](constauto& key,constauto& 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(conststd::pair<conststd::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(constauto&[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(constauto& n: u)        print_key_value(n.first, n.second);}

      Possible output:

      Iterate and print key-value pairs of unordered_map, beingexplicit 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]

      [edit]Defect reports

      The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

      DRApplied toBehavior as publishedCorrect behavior
      LWG 2050C++11the definitions ofreference,const_reference,pointer
      andconst_pointer were based onallocator_type      		based onvalue_type and
      std::allocator_traits

      [edit]See also

      collection of key-value pairs, hashed by keys
      (class template)[edit]
      collection of key-value pairs, sorted by keys, keys are unique
      (class template)[edit]
      (C++23)
      		 adapts two containers to provide a collection of key-value pairs, sorted by unique keys
      (class template)[edit]
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