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

From cppreference.com
< cpp‎ | container
(Using KeyEqual everywhere)
m (→‎Synopsis: fmt: unified the view)
Line 3: Line 3:
 
{{dcl begin}}
 
{{dcl begin}}
 
{{dcl header|unordered_map}}
 
{{dcl header|unordered_map}}
−
{{dcl|num=1|| since=c++11 |1=
+
{{dcl|num=1||since=c++11|1=
 
template<
 
template<
 
     class Key,
 
     class Key,
 
     class T,
 
     class T,
−
     class Hash {{=}} std::hash<Key>,
+
     class Hash = std::hash<Key>,
−
     class KeyEqual {{=}} std::equal_to<Key>,
+
     class KeyEqual = std::equal_to<Key>,
−
     class Allocator {{=}} std::allocator< std::pair<const Key, T> >
+
     class Allocator = std::allocator< std::pair<const Key, T> >
 
> class unordered_map;
 
> class unordered_map;
 
}}
 
}}
 
{{dcl|num=2|since=c++17|1=
 
{{dcl|num=2|since=c++17|1=
 
namespace pmr {
 
namespace pmr {
−
    template <class Key,  
+
template <
−
              class T,
+
    class Key,
−
              class Hash = std::hash<Key>,
+
    class T,
−
              class KeyEqual = std::equal_to<Key>>
+
    class Hash = std::hash<Key>,
−
    using unordered_map = std::unordered_map<Key, T, Hash, KeyEqual,
+
    class KeyEqual = std::equal_to<Key>
−
                              std::pmr::polymorphic_allocator<std::pair<const Key,T>>>;
+
> using unordered_map = std::unordered_map<Key, T, Hash, KeyEqual,
 +
                            std::pmr::polymorphic_allocator<std::pair<const Key,T>>>;
 
}
 
}
 
}}
 
}}
Line 32: Line 33:
 
===Iterator invalidation===
 
===Iterator invalidation===
  
−
{| class="dsctable" style="font-size:0.8em"
+
{|class="dsctable" style="font-size:0.8em"
 
! Operations
 
! Operations
 
! Invalidated
 
! Invalidated
 
|-
 
|-
−
| All read only operations, {{lc|swap}}, {{lc|std::swap}}
+
|All read only operations, {{lc|swap}}, {{lc|std::swap}}
−
| Never
+
|Never
 
|-
 
|-
−
| {{lc|clear}}, {{lc|rehash}}, {{lc|reserve}}, {{lc|1=operator=}}
+
|{{lc|clear}}, {{lc|rehash}}, {{lc|reserve}}, {{lc|1=operator=}}
−
| Always
+
|Always
 
|-
 
|-
−
| {{lc|insert}}, {{lc|emplace}}, {{lc|emplace_hint}}, {{lc|operator[]}}
+
|{{lc|insert}}, {{lc|emplace}}, {{lc|emplace_hint}}, {{lc|operator[]}}
−
| Only if causes rehash
+
|Only if causes rehash
 
|-
 
|-
−
| {{lc|erase}}
+
|{{lc|erase}}
−
| Only to the element erased
+
|Only to the element erased
 
|}
 
|}
  
Line 56: Line 57:
 
===Member types===
 
===Member types===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc hitem | Member type | Definition}}
+
{{dsc hitem|Member type|Definition}}
−
{{dsc inc | cpp/container/dsc key_type | unordered_map}}
+
{{dsc inc|cpp/container/dsc key_type|unordered_map}}
−
{{dsc inc | cpp/container/dsc mapped_type | unordered_map}}
+
{{dsc inc|cpp/container/dsc mapped_type|unordered_map}}
−
{{dsc inc | cpp/container/dsc value_type | unordered_map}}
+
{{dsc inc|cpp/container/dsc value_type|unordered_map}}
−
{{dsc inc | cpp/container/dsc size_type | unordered_map}}
+
{{dsc inc|cpp/container/dsc size_type|unordered_map}}
−
{{dsc inc | cpp/container/dsc difference_type | unordered_map}}
+
{{dsc inc|cpp/container/dsc difference_type|unordered_map}}
−
{{dsc inc | cpp/container/dsc hasher | unordered_map}}
+
{{dsc inc|cpp/container/dsc hasher|unordered_map}}
−
{{dsc inc | cpp/container/dsc key_equal | unordered_map}}
+
{{dsc inc|cpp/container/dsc key_equal|unordered_map}}
−
{{dsc inc | cpp/container/dsc allocator_type | unordered_map}}
+
{{dsc inc|cpp/container/dsc allocator_type|unordered_map}}
−
{{dsc inc | cpp/container/dsc reference | unordered_map}}
+
{{dsc inc|cpp/container/dsc reference|unordered_map}}
−
{{dsc inc | cpp/container/dsc const_reference | unordered_map}}
+
{{dsc inc|cpp/container/dsc const_reference|unordered_map}}
−
{{dsc inc | cpp/container/dsc pointer | unordered_map}}
+
{{dsc inc|cpp/container/dsc pointer|unordered_map}}
−
{{dsc inc | cpp/container/dsc const_pointer | unordered_map}}
+
{{dsc inc|cpp/container/dsc const_pointer|unordered_map}}
−
{{dsc inc | cpp/container/dsc iterator | unordered_map}}
+
{{dsc inc|cpp/container/dsc iterator|unordered_map}}
−
{{dsc inc | cpp/container/dsc const_iterator | unordered_map}}
+
{{dsc inc|cpp/container/dsc const_iterator|unordered_map}}
−
{{dsc inc | cpp/container/dsc local_iterator | unordered_map}}
+
{{dsc inc|cpp/container/dsc local_iterator|unordered_map}}
−
{{dsc inc | cpp/container/dsc const_local_iterator | unordered_map}}
+
{{dsc inc|cpp/container/dsc const_local_iterator|unordered_map}}
−
{{dsc inc | cpp/container/dsc node_type | unordered_map}}
+
{{dsc inc|cpp/container/dsc node_type|unordered_map}}
−
{{dsc inc | cpp/container/dsc insert_return_type | unordered_map}}
+
{{dsc inc|cpp/container/dsc insert_return_type|unordered_map}}
 
{{dsc end}}
 
{{dsc end}}
  
 
===Member functions===
 
===Member functions===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/container/dsc constructor | unordered_map}}
+
{{dsc inc|cpp/container/dsc constructor|unordered_map}}
−
{{dsc inc | cpp/container/dsc destructor | unordered_map}}
+
{{dsc inc|cpp/container/dsc destructor|unordered_map}}
−
{{dsc inc | cpp/container/dsc operator{{=}} | unordered_map}}
+
{{dsc inc|cpp/container/dsc operator{{=}}|unordered_map}}
−
{{dsc inc | cpp/container/dsc get_allocator | unordered_map}}
+
{{dsc inc|cpp/container/dsc get_allocator|unordered_map}}
  
−
{{dsc h2 | Iterators}}
+
{{dsc h2|Iterators}}
−
{{dsc inc | cpp/container/dsc begin | unordered_map}}
+
{{dsc inc|cpp/container/dsc begin|unordered_map}}
−
{{dsc inc | cpp/container/dsc end | unordered_map}}
+
{{dsc inc|cpp/container/dsc end|unordered_map}}
  
−
{{dsc h2 | Capacity}}
+
{{dsc h2|Capacity}}
−
{{dsc inc | cpp/container/dsc empty | unordered_map}}
+
{{dsc inc|cpp/container/dsc empty|unordered_map}}
−
{{dsc inc | cpp/container/dsc size | unordered_map}}
+
{{dsc inc|cpp/container/dsc size|unordered_map}}
−
{{dsc inc | cpp/container/dsc max_size | unordered_map}}
+
{{dsc inc|cpp/container/dsc max_size|unordered_map}}
  
−
{{dsc h2 | Modifiers}}
+
{{dsc h2|Modifiers}}
−
{{dsc inc | cpp/container/dsc clear | unordered_map}}
+
{{dsc inc|cpp/container/dsc clear|unordered_map}}
−
{{dsc inc | cpp/container/dsc insert | unordered_map}}
+
{{dsc inc|cpp/container/dsc insert|unordered_map}}
−
{{dsc inc | cpp/container/dsc insert_or_assign | unordered_map}}
+
{{dsc inc|cpp/container/dsc insert_or_assign|unordered_map}}
−
{{dsc inc | cpp/container/dsc emplace | unordered_map}}
+
{{dsc inc|cpp/container/dsc emplace|unordered_map}}
−
{{dsc inc | cpp/container/dsc emplace_hint | unordered_map}}
+
{{dsc inc|cpp/container/dsc emplace_hint|unordered_map}}
−
{{dsc inc | cpp/container/dsc try_emplace | unordered_map}}
+
{{dsc inc|cpp/container/dsc try_emplace|unordered_map}}
−
{{dsc inc | cpp/container/dsc erase | unordered_map}}
+
{{dsc inc|cpp/container/dsc erase|unordered_map}}
−
{{dsc inc | cpp/container/dsc swap | unordered_map}}
+
{{dsc inc|cpp/container/dsc swap|unordered_map}}
−
{{dsc inc | cpp/container/dsc extract | unordered_map}}
+
{{dsc inc|cpp/container/dsc extract|unordered_map}}
−
{{dsc inc | cpp/container/dsc merge | unordered_map}}
+
{{dsc inc|cpp/container/dsc merge|unordered_map}}
  
−
{{dsc h2 | Lookup}}
+
{{dsc h2|Lookup}}
−
{{dsc inc | cpp/container/dsc at | unordered_map}}
+
{{dsc inc|cpp/container/dsc at|unordered_map}}
−
{{dsc inc | cpp/container/dsc operator_at | unordered_map}}
+
{{dsc inc|cpp/container/dsc operator_at|unordered_map}}
−
{{dsc inc | cpp/container/dsc count | unordered_map}}
+
{{dsc inc|cpp/container/dsc count|unordered_map}}
−
{{dsc inc | cpp/container/dsc find | unordered_map}}
+
{{dsc inc|cpp/container/dsc find|unordered_map}}
−
{{dsc inc | cpp/container/dsc contains | unordered_map}}
+
{{dsc inc|cpp/container/dsc contains|unordered_map}}
−
{{dsc inc | cpp/container/dsc equal_range | unordered_map}}
+
{{dsc inc|cpp/container/dsc equal_range|unordered_map}}
  
−
{{dsc h2 | Bucket interface}}
+
{{dsc h2|Bucket interface}}
−
{{dsc inc | cpp/container/dsc begin(int) | unordered_map}}
+
{{dsc inc|cpp/container/dsc begin(int)|unordered_map}}
−
{{dsc inc | cpp/container/dsc end(int) | unordered_map}}
+
{{dsc inc|cpp/container/dsc end(int)|unordered_map}}
−
{{dsc inc | cpp/container/dsc bucket_count | unordered_map}}
+
{{dsc inc|cpp/container/dsc bucket_count|unordered_map}}
−
{{dsc inc | cpp/container/dsc max_bucket_count | unordered_map}}
+
{{dsc inc|cpp/container/dsc max_bucket_count|unordered_map}}
−
{{dsc inc | cpp/container/dsc bucket_size | unordered_map}}
+
{{dsc inc|cpp/container/dsc bucket_size|unordered_map}}
−
{{dsc inc | cpp/container/dsc bucket | unordered_map}}
+
{{dsc inc|cpp/container/dsc bucket|unordered_map}}
  
−
{{dsc h2 | Hash policy}}
+
{{dsc h2|Hash policy}}
−
{{dsc inc | cpp/container/dsc load_factor | unordered_map}}
+
{{dsc inc|cpp/container/dsc load_factor|unordered_map}}
−
{{dsc inc | cpp/container/dsc max_load_factor | unordered_map}}
+
{{dsc inc|cpp/container/dsc max_load_factor|unordered_map}}
−
{{dsc inc | cpp/container/dsc rehash | unordered_map}}
+
{{dsc inc|cpp/container/dsc rehash|unordered_map}}
−
{{dsc inc | cpp/container/dsc reserve | unordered_map}}
+
{{dsc inc|cpp/container/dsc reserve|unordered_map}}
  
−
{{dsc h2 | Observers}}
+
{{dsc h2|Observers}}
−
{{dsc inc | cpp/container/dsc hash_function | unordered_map}}
+
{{dsc inc|cpp/container/dsc hash_function|unordered_map}}
−
{{dsc inc | cpp/container/dsc key_eq | unordered_map}}
+
{{dsc inc|cpp/container/dsc key_eq|unordered_map}}
 
{{dsc end}}
 
{{dsc end}}
  
 
===Non-member functions===
 
===Non-member functions===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/container/dsc operator_cmp_unord | unordered_map}}
+
{{dsc inc|cpp/container/dsc operator_cmp_unord|unordered_map}}
−
{{dsc inc | cpp/container/dsc swap2 | unordered_map}}
+
{{dsc inc|cpp/container/dsc swap2|unordered_map}}
−
{{dsc inc | cpp/container/dsc erase_if | unordered_map}}
+
{{dsc inc|cpp/container/dsc erase_if|unordered_map}}
 
{{dsc end}}
 
{{dsc end}}
  
Line 143: Line 144:
 
===Example===
 
===Example===
 
{{example
 
{{example
−
|
+
|
−
| code=
+
|code=
 
#include <iostream>
 
#include <iostream>
 
#include <string>
 
#include <string>
Line 159: Line 160:
  
 
     // Helper lambda function to print key-value pairs
 
     // Helper lambda function to print key-value pairs
−
     auto print_key_value = [](const auto& key, const auto& value) {
+
     auto print_key_value = [](const auto& key, const auto& value)
 +
    {
 
         std::cout << "Key:[" << key << "] Value:[" << value << "]\n";
 
         std::cout << "Key:[" << key << "] Value:[" << value << "]\n";
 
     };
 
     };
Line 165: Line 167:
 
     std::cout << "Iterate and print key-value pairs of unordered_map, being\n"
 
     std::cout << "Iterate and print key-value pairs of unordered_map, being\n"
 
                 "explicit with their types:\n";
 
                 "explicit with their types:\n";
−
     for( const std::pair<const std::string, std::string>& n : u ) {
+
     for( const std::pair<const std::string, std::string>& n : u )
 
         print_key_value(n.first, n.second);
 
         print_key_value(n.first, n.second);
−
    }
 
  
 
     std::cout << "\nIterate and print key-value pairs using C++17 structured binding:\n";
 
     std::cout << "\nIterate and print key-value pairs using C++17 structured binding:\n";
−
     for( const auto& [key, value] : u ) {
+
     for( const auto& [key, value] : u )
 
         print_key_value(key, value);
 
         print_key_value(key, value);
−
    }
 
  
 
     // Add two new entries to the unordered_map
 
     // Add two new entries to the unordered_map
Line 187: Line 187:
 
     std::cout << "\nIterate and print key-value pairs, using `auto`;\n"
 
     std::cout << "\nIterate and print key-value pairs, using `auto`;\n"
 
                 "new_key is now one of the keys in the map:\n";
 
                 "new_key is now one of the keys in the map:\n";
−
     for( const auto& n : u ) {
+
     for( const auto& n : u )
 
         print_key_value(n.first, n.second);
 
         print_key_value(n.first, n.second);
−
    }
 
 
}
 
}
−
| p=true <!--the order of elements is not specified-->
+
|p=true <!--the order of elements is not specified-->
−
| output=
+
|output=
 
Iterate and print key-value pairs of unordered_map, being
 
Iterate and print key-value pairs of unordered_map, being
 
explicit with their types:
 
explicit with their types:
Line 223: Line 222:
 
===See also===
 
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/container/dsc map}}
+
{{dsc inc|cpp/container/dsc map}}
 
{{dsc end}}
 
{{dsc end}}
  
 
{{langlinks|de|es|fr|it|ja|ko|pt|ru|zh}}
 
{{langlinks|de|es|fr|it|ja|ko|pt|ru|zh}}

Revision as of 03:10, 9 November 2022

 
C++
 
Containers library
 
std::unordered_map
Member types
Member functions
Non-member functions
(until C++20)
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)

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.

Contents

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[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

 constructs the unordered_map
(public member function) [edit]
destructs the unordered_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 elements or nodes(since C++17)
(public member function) [edit]
(C++17)
 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]
(C++17)
 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]

Non-member functions

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

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

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