std::unordered_map(3)

NAME
std::unordered_map - std::unordered_map

Synopsis
   Defined in header <unordered_map>
   template<
   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. 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
                        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>
   insert_return_type (since struct /*unspecified*/ {
   C++17) Iter position;
                        bool inserted;
                        NodeType node;
                        };
                        instantiated with template arguments iterator and
                        node_type.

Member functions
   constructor constructs the unordered_map
   (C++11) (public member function)
   destructor destructs the unordered_map
   (C++11) (public member function)
   operator= assigns values to the container
   (C++11) (public member function)
   get_allocator returns the associated allocator
   (C++11) (public member function)

Iterators
   begin returns an iterator to the beginning
   cbegin (public member function)
   (C++11)
   end returns an iterator to the end
   cend (public member function)
   (C++11)

Capacity
   empty checks whether the container is empty
   (C++11) (public member function)
   size returns the number of elements
   (C++11) (public member function)
   max_size returns the maximum possible number of elements
   (C++11) (public member function)

Modifiers
   clear clears the contents
   (C++11) (public member function)
                     inserts elements
   insert or nodes
   (C++11) (since C++17)
                     (public member function)
   insert_or_assign inserts an element or assigns to the current element if the key
   (C++17) already exists
                     (public member function)
   emplace constructs element in-place
   (C++11) (public member function)
   emplace_hint constructs elements in-place using a hint
   (C++11) (public member function)
   try_emplace inserts in-place if the key does not exist, does nothing if the
   (C++17) key exists
                     (public member function)
   erase erases elements
   (C++11) (public member function)
   swap swaps the contents
   (C++11) (public member function)
   extract extracts nodes from the container
   (C++17) (public member function)
   merge splices nodes from another container
   (C++17) (public member function)

Lookup
   at access specified element with bounds checking
   (C++11) (public member function)
   operator[] access or insert specified element
   (C++11) (public member function)
   count returns the number of elements matching specific key
   (C++11) (public member function)
   find finds element with specific key
   (C++11) (public member function)
   contains checks if the container contains element with specific key
   (C++20) (public member function)
   equal_range returns range of elements matching a specific key
   (C++11) (public member function)

Bucket interface
   begin(size_type) returns an iterator to the beginning of the specified bucket
   cbegin(size_type) (public member function)
   (C++11)
   end(size_type) returns an iterator to the end of the specified bucket
   cend(size_type) (public member function)
   (C++11)
   bucket_count returns the number of buckets
   (C++11) (public member function)
   max_bucket_count returns the maximum number of buckets
   (C++11) (public member function)
   bucket_size returns the number of elements in specific bucket
   (C++11) (public member function)
   bucket returns the bucket for specific key
   (C++11) (public member function)

Hash policy
   load_factor returns average number of elements per bucket
   (C++11) (public member function)
   max_load_factor manages maximum average number of elements per bucket
   (C++11) (public member function)
   rehash reserves at least the specified number of buckets and regenerates
   (C++11) the hash table
                     (public member function)
   reserve reserves space for at least the specified number of elements and
   (C++11) regenerates the hash table
                     (public member function)

Observers
   hash_function returns function used to hash the keys
   (C++11) (public member function)
   key_eq returns the function used to compare keys for equality
   (C++11) (public member function)

Non-member functions
   operator== compares the values in the unordered_map
   operator!= (function template)
   (removed in C++20)
   std::swap(std::unordered_map) specializes the std::swap algorithm
   (C++11) (function template)
   erase_if(std::unordered_map) Erases all elements satisfying specific criteria
   (C++20) (function template)
  Deduction guides(since C++17)

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