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std::unordered_map(3) C++ Standard Libary 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>, class KeyEqual = std::equal_to<Key> (2) (since C++17) > using unordered_map = std::unordered_map<Key, T, Hash, KeyEqual, std::pmr::polymorphic_allocator<std::pair<const Key, T>>>; } 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 predi- cate returns true when passed those keys. If two keys are equivalent, the hash func- tion must return the same value for both keys. std::unordered_map meets the requirements of Container, Alloca- torAwareContainer, UnorderedAssociativeContainer. 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 con- tainer 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<Alloca- tor>::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_iter- ator. 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 represent- ing a container node type describing the result of inserting a node_type, a specialization of template<class Iter, class NodeType> struct /*unspecified*/ insert_return_type { (since C++17) Iter position; bool inserted; NodeType node; }; instantiated with template arguments itera- tor 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 begin returns an iterator to the beginning cbegin (public member function) end returns an iterator to the end cend (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) inserts elements insert or nodes (since C++17) (public member function) insert_range inserts a range of elements (C++23) (public member function) insert_or_assign inserts an element or assigns to the current ele- ment if the key (C++17) 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 inserts in-place if the key does not exist, does nothing if the (C++17) key exists (public member function) erase erases elements (public member function) swap swaps the contents (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 (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 checks if the container contains element with spe- cific key (C++20) (public member function) equal_range returns range of elements matching a specific key (public member function) Bucket interface begin(size_type) returns an iterator to the beginning of the speci- fied bucket cbegin(size_type) (public member function) end(size_type) returns an iterator to the end of the specified bucket cend(size_type) (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) reserves at least the specified number of buckets and regenerates rehash the hash table (public member function) reserves space for at least the specified number of elements and reserve 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!= compares the values in the un- ordered_map (C++11) (function template) (C++11)(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 spe- cific criteria (C++20) (function template) Deduction guides (since C++17) Notes Feature-test macro Value Std Feature __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] Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior the definitions of reference, LWG 2050 C++11 const_reference, pointer based on value_type and and const_pointer were based on std::allocator_traits allocator_type See also map collection of key-value pairs, sorted by keys, keys are unique (class template) Category: * Todo with reason http://cppreference.com 2024.06.10 std::unordered_map(3)
NAME | Synopsis | Notes | Template parameters | Member types | Member functions | Iterators | Capacity | Modifiers | Lookup | Bucket interface | Hash policy | Observers | Non-member functions | Notes | Example | Possible output: | See also | Category:
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