Difference between revisions of "cpp/container/unordered map"

Revision as of 01:41, 30 October 2023

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.

Member type	Definition
`key_type`	`Key`[edit]
`mapped_type`	`T`[edit]
`value_type`	std::pair<const Key, T>[edit]
`size_type`	Unsigned integer type (usually std::size_t)[edit]
`difference_type`	Signed integer type (usually std::ptrdiff_t)[edit]
`hasher`	`Hash`[edit]
`key_equal`	`KeyEqual`[edit]
`allocator_type`	`Allocator`[edit]
`reference`	value_type&[edit]
`const_reference`	const value_type&[edit]
`pointer`	std::allocator_traits<Allocator>::pointer[edit]
`const_pointer`	std::allocator_traits<Allocator>::const_pointer[edit]
`iterator`	LegacyForwardIterator to `value_type`[edit]
`const_iterator`	LegacyForwardIterator to const value_type[edit]
`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[edit]
`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[edit]
`node_type` (since C++17)	a specialization of node handle representing a container node[edit]
`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`.[edit]

Member functions

(constructor)	constructs the `unordered_map` (public member function) [edit]
(destructor)	destructs the `unordered_map` (public member function) [edit]
operator=	assigns values to the container (public member function) [edit]
get_allocator	returns the associated allocator (public member function) [edit]
Iterators
begincbegin	returns an iterator to the beginning (public member function) [edit]
endcend	returns an iterator to the end (public member function) [edit]
Capacity
empty	checks whether the container is empty (public member function) [edit]
size	returns the number of elements (public member function) [edit]
max_size	returns the maximum possible number of elements (public member function) [edit]
Modifiers
clear	clears the contents (public member function) [edit]
insert	inserts elements or nodes(since C++17) (public member function) [edit]
insert_range (C++23)	inserts a range of elements (public member function) [edit]
insert_or_assign (C++17)	inserts an element or assigns to the current element if the key already exists (public member function) [edit]
emplace	constructs element in-place (public member function) [edit]
emplace_hint	constructs elements in-place using a hint (public member function) [edit]
try_emplace (C++17)	inserts in-place if the key does not exist, does nothing if the key exists (public member function) [edit]
erase	erases elements (public member function) [edit]
swap	swaps the contents (public member function) [edit]
extract (C++17)	extracts nodes from the container (public member function) [edit]
merge (C++17)	splices nodes from another container (public member function) [edit]
Lookup
at	access specified element with bounds checking (public member function) [edit]
operator[]	access or insert specified element (public member function) [edit]
count	returns the number of elements matching specific key (public member function) [edit]
find	finds element with specific key (public member function) [edit]
contains (C++20)	checks if the container contains element with specific key (public member function) [edit]
equal_range	returns range of elements matching a specific key (public member function) [edit]
Bucket interface
begin(size_type)cbegin(size_type)	returns an iterator to the beginning of the specified bucket (public member function) [edit]
end(size_type)cend(size_type)	returns an iterator to the end of the specified bucket (public member function) [edit]
bucket_count	returns the number of buckets (public member function) [edit]
max_bucket_count	returns the maximum number of buckets (public member function) [edit]
bucket_size	returns the number of elements in specific bucket (public member function) [edit]
bucket	returns the bucket for specific key (public member function) [edit]
Hash policy
load_factor	returns average number of elements per bucket (public member function) [edit]
max_load_factor	manages maximum average number of elements per bucket (public member function) [edit]
rehash	reserves at least the specified number of buckets and regenerates the hash table (public member function) [edit]
reserve	reserves space for at least the specified number of elements and regenerates the hash table (public member function) [edit]
Observers
hash_function	returns function used to hash the keys (public member function) [edit]
key_eq	returns the function used to compare keys for equality (public member function) [edit]

Non-member functions

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

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 retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2050	C++11	the definitions of `reference`, `const_reference`, `pointer` and `const_pointer` were based on `allocator_type`	based on `value_type` and std::allocator_traits

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Sequence
array (C++11)
vector
vector<bool>
inplace_vector (C++26)
deque
forward_list (C++11)
list
Associative
set
multiset
map
multimap
Unordered associative
unordered_set (C++11)
unordered_multiset (C++11)
unordered_map (C++11)
unordered_multimap (C++11)
Adaptors
stack
queue
priority_queue
flat_set (C++23)
flat_multiset (C++23)
flat_map (C++23)
flat_multimap (C++23)
Views
span (C++20)
mdspan (C++23)
Tables
Iterator invalidation
Member function table
Non-member function table

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

cppreference.com

Namespaces

Variants

Views

Actions

Difference between revisions of "cpp/container/unordered map"

Revision as of 01:41, 30 October 2023

Contents

Iterator invalidation

Notes

Template parameters

Member types

Member functions

Iterators

Capacity

Modifiers

Lookup

Bucket interface

Hash policy

Observers

Non-member functions

Deduction guides

Notes

Example

Defect reports

See also

Navigation

Toolbox

@@ Line 10: / Line 10: @@
      class KeyEqual = std::equal_to<Key>,
      class Allocator = std::allocator<std::pair<const Key, T>>
->
+> class unordered_map;
-class unordered_map;
 }}
 {{dcl|num=2|since=c++17|1=
@@ Line 20: / Line 19: @@
          class Hash = std::hash<Key>,
          class KeyEqual = std::equal_to<Key>
-     >
+     > using unordered_map =
-    using unordered_map =
+          std::unordered_map<Key, T, Hash, KeyEqual,
-        std::unordered_map<Key, T, Hash, KeyEqual,
+              std::pmr::polymorphic_allocator<std::pair<const Key, T>>>;
-            std::pmr::polymorphic_allocator<std::pair<const Key, T>>>;
 }
 }}