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Overview

template< class Key, class Value, /* ... */ >
class unordered_map;

The std::unordered_multimap is a container that stores key-value pairs. It works almost as std::unordered_map, the only difference is that the keys can repeat themselves. If the keys of both elements are the same, their values are compared to determine if they should be added to the multimap.

The order of the elements stored is not specified, unlike in std::multimap.

Technical details

Technical definition of an unordered multimap

Unordered multimap is an unordered associative container that supports equivalent keys (an unordered_multimap may contain multiple copies of each key value) and that associates values of another type with the keys.

The unordered_multimap class supports forward iterators. Search, insertion, and removal 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. This allows fast access to individual elements, since once the hash is computed, it refers to the exact bucket the element is placed into.

The iteration order of this container is not required to be stable (so, for example, std::equal cannot be used to compare two std::unordered_multimaps), except that every group of elements whose keys compare equivalent (compare equal with key_eq() as the comparator) forms a contiguous subrange in the iteration order, also accessible with equal_range().

std::unordered_multimap meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

std::unordered_multimap

Defined inunordered_multimap

Template parameters

pubKeyThe type of the stored keys.
pubValueThe type of the stored values.
pubHashA function object that perfoms hashing on two objects of type Key.
pubKeyEqual

A function object taking two arguments of type Key and determining if the keys are equal. Must return bool.

pubAllocatorAn allocator type responsible for allocating and deallocating memory. Must satisfy Allocator.

Type names

pubkey_typeKey
pubmapped_typeValue
pubvalue_typestd::pair<const Key, Value>
pubsize_typeUnsigned integer type (usually
std::size_t
).
pubdifference_typeSigned integer type (usually
std::ptrdiff_t
).
pubhasherHash
pubkey_equalKeyEqual
puballocator_typeAllocator
pubreferencevalue_type&
pubconst_referenceconst value_type&
pubpointerAllocator::pointer(until C++11)
std::allocator_traits<Allocator>::pointer(since C++11)
pubconst_pointerAllocator::const_pointer(until C++11)
std::allocator_traits<Allocator>::const_pointer(since C++11)
pubiteratorLegacyBidirectionalIterator to value_type
pubconst_iteratorLegacyBidirectionalIterator to const value_type
publocal_iterator

Like iterator but only to iterate across a single bucket.

pubconst_local_iterator

Like const_iterator but only to iterate across a single bucket.

pubnode_type (since C++17) A specialization of node handle representing a container node.

Member functions

pub(constructors)

Constructs a new unordered_multimap.

pub(destructor)

Destructs an unordered_multimap.

puboperator=

Assigns one unordered_multimap to another.

pubget_allocator

Returns an associated allocator.

Iterators

pubbegin
cbegin

Returns an iterator to the beginning.

pubend
cend

Returns an iterator to the end.

Capacity

pubempty

Returns true if an unordered_multimap is empty, otherwise false.

pubsize

Returns the number of elements in an unordered_multimap.

pubmax_size

Returns the maximum possible number of elements.

Modifiers

pubclear

Clears the contents of an unordered_multimap.

pubinsert

Inserts elements or nodes (extracted with .extract()) (since C++17).

pubemplace

Constructs a new element in place.

pubemplace_hint

Constructs elements in-place using a hint (iterator).

puberase

Erases elements.

pubswap

Swaps two unordered_multimaps.

pubextract (since C++17)

Extracts nodes from an unordered_multimap (can be later inserted somewhere else).

pubmerge (since C++17)

Merges two unordered_multimaps together.

Lookup

pubcount

Returns the number of elements matching a specific key.

pubfind

Searches for an element and returns an iterator to it, or end iterator if not found.

pubcontains (since C++20)

Returns true if an element is inside an unordered_multimap, otherwise false.

pubequal_range

Returns a range of elements matching a specific key.

Bucket interface

pubbegin
cbegin

Returns an iterator to the beginning of a specified bucket.

pubend
cend

Returns an iterator to the end of a specified bucket.

pubbucket_count

Returns a number of buckets.

pubmax_bucket_count

Returns a maximum number of buckets.

pubbucket_size

Returns a number of elements in a specific bucket.

pubbucket

Returns a bucket for a specific key.

Hash policy

pubload_factor

Returns an average number of elements per bucket.

pubmax_load_factor

Manages a maximum average number of elements per bucket.

pubrehash

Reserves at least the specified number of buckets and regenerates the hash table.

pubreserve

Reserves space for at least a specified number of elements and regenerates the hash table.

Observers

pubhash_function

Returns an internal function object that hashes keys.

pubkey_eq

Returns an internal function object that compares keys.

Non-member functions

puboperator==
operator!= (removed in C++20)
Compares values in an unordered_multimap.
pubstd::swap (std::unordered_multimap)An overload for a std::swap algorithm.
pubstd::erase_if (std::unordered_multimap) (since C++20)Overload for a std::erase_if algorithm.

Deduction guides (since C++17)

Click to expand

Deduction guides

// (1)
template< class InputIt, class Alloc >
unordered_map( InputIt, InputIt, Alloc )
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>,
std::hash<iter_key_t<InputIt>>,
std::equal_to<iter_key_t<InputIt>>, Alloc>;
// (2)
template< class InputIt, class Hash, class Alloc >
unordered_map( InputIt, InputIt, typename /*see below*/::size_type, Hash, Alloc )
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash,
std::equal_to<iter_key_t<InputIt>>, Alloc>;
// (3)
template< class InputIt, class Alloc >
unordered_map( InputIt, InputIt, typename /*see below*/::size_type, Alloc )
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>,
std::hash<iter_key_t<InputIt>>,
std::equal_to<iter_key_t<InputIt>>, Alloc>;
// (4)
template< class InputIt,
class Hash = std::hash<iter_key_t<InputIt>>,
class Pred = std::equal_to<iter_key_t<InputIt>>,
class Alloc = std::allocator<iter_to_alloc_t<InputIt>> >
unordered_map( InputIt, InputIt,
typename /*see below*/::size_type = /*see below*/,
Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash, Pred, Alloc>;

(1 - 4) allow deduction from an iterator range

// (5)
template< class Key, class T, typename Alloc >
unordered_map( std::initializer_list<std::pair<Key, T>>, Alloc )
-> unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;
// (6)
template< class Key, class T, class Hash, class Alloc >
unordered_map( std::initializer_list<std::pair<Key, T>>,
typename /*see below*/::size_type, Hash, Alloc )
-> unordered_map<Key, T, Hash, std::equal_to<Key>, Alloc>;
// (7)
template< class Key, class T, typename Alloc >
unordered_map( std::initializer_list<std::pair<Key, T>>,
typename /*see below*/::size_type, Alloc )
-> unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;
// (8)
template< class Key, class T, class Hash = std::hash<Key>,
class Pred = std::equal_to<Key>,
class Alloc = std::allocator<std::pair<const Key, T>> >
unordered_map( std::initializer_list<std::pair<Key, T>>,
typename /*see below*/::size_type = /*see below*/,
Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
-> unordered_map<Key, T, Hash, Pred, Alloc>;

(5 - 8) allow deduction from a std::initializer_list

Aliases iter_key_t, iter_val_t and iter_to_alloc_t are defined as if follows:

template< class InputIt >
using iter_key_t = std::remove_const_t<
typename std::iterator_traits<InputIt>::value_type::first_type>;
template< class InputIt >
using iter_val_t = typename std::iterator_traits<InputIt>::value_type::second_type;
template< class InputIt >
using iter_to_alloc_t = std::pair<
std::add_const_t<typename std::iterator_traits<InputIt>::value_type::first_type>,
typename std::iterator_traits<InputIt>::value_type::second_type
>
zanotuj

Note that these aliases aren't guaranteed to be found anywhere in the standard library. They are defined solely for presentation purposes of these deduction guides and weren't present in the standard library at the point of writing of this document.

Overload resolution

In order for any of the deduction guides to participate in overload resolution, the folllowing requirements must be met: A deduction guide for an unordered associative container shall not participate in overload resolution if any of the following are true:

zanotuj

The extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum:

  • Integral types do not qualify as input iterators.

Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum:

More examples

Basic manipulation

Create, insert and print
#include <unordered_map>
#include <iostream>
#include <string>

int main() {
std::unordered_map<std::string, std::string> player_locations {
{"player2", "Capital City"},
{"player3", "The Harbor"}
};

player_locations["player1"] = "Mine";

for (const auto& [key, value] : player_locations)
std::cout << key << " is in " << value << '\n';

return 0;
}
Result
player3 has got Shield
player2 has got Health potion
player2 has got Sword
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.
uwaga

Note, this article is not finished! You can help by editing this doc page.

Overview

template< class Key, class Value, /* ... */ >
class unordered_map;

The std::unordered_multimap is a container that stores key-value pairs. It works almost as std::unordered_map, the only difference is that the keys can repeat themselves. If the keys of both elements are the same, their values are compared to determine if they should be added to the multimap.

The order of the elements stored is not specified, unlike in std::multimap.

Technical details

Technical definition of an unordered multimap

Unordered multimap is an unordered associative container that supports equivalent keys (an unordered_multimap may contain multiple copies of each key value) and that associates values of another type with the keys.

The unordered_multimap class supports forward iterators. Search, insertion, and removal 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. This allows fast access to individual elements, since once the hash is computed, it refers to the exact bucket the element is placed into.

The iteration order of this container is not required to be stable (so, for example, std::equal cannot be used to compare two std::unordered_multimaps), except that every group of elements whose keys compare equivalent (compare equal with key_eq() as the comparator) forms a contiguous subrange in the iteration order, also accessible with equal_range().

std::unordered_multimap meets the requirements of Container, AllocatorAwareContainer, UnorderedAssociativeContainer.

std::unordered_multimap

Defined inunordered_multimap

Template parameters

pubKeyThe type of the stored keys.
pubValueThe type of the stored values.
pubHashA function object that perfoms hashing on two objects of type Key.
pubKeyEqual

A function object taking two arguments of type Key and determining if the keys are equal. Must return bool.

pubAllocatorAn allocator type responsible for allocating and deallocating memory. Must satisfy Allocator.

Type names

pubkey_typeKey
pubmapped_typeValue
pubvalue_typestd::pair<const Key, Value>
pubsize_typeUnsigned integer type (usually
std::size_t
).
pubdifference_typeSigned integer type (usually
std::ptrdiff_t
).
pubhasherHash
pubkey_equalKeyEqual
puballocator_typeAllocator
pubreferencevalue_type&
pubconst_referenceconst value_type&
pubpointerAllocator::pointer(until C++11)
std::allocator_traits<Allocator>::pointer(since C++11)
pubconst_pointerAllocator::const_pointer(until C++11)
std::allocator_traits<Allocator>::const_pointer(since C++11)
pubiteratorLegacyBidirectionalIterator to value_type
pubconst_iteratorLegacyBidirectionalIterator to const value_type
publocal_iterator

Like iterator but only to iterate across a single bucket.

pubconst_local_iterator

Like const_iterator but only to iterate across a single bucket.

pubnode_type (since C++17) A specialization of node handle representing a container node.

Member functions

pub(constructors)

Constructs a new unordered_multimap.

pub(destructor)

Destructs an unordered_multimap.

puboperator=

Assigns one unordered_multimap to another.

pubget_allocator

Returns an associated allocator.

Iterators

pubbegin
cbegin

Returns an iterator to the beginning.

pubend
cend

Returns an iterator to the end.

Capacity

pubempty

Returns true if an unordered_multimap is empty, otherwise false.

pubsize

Returns the number of elements in an unordered_multimap.

pubmax_size

Returns the maximum possible number of elements.

Modifiers

pubclear

Clears the contents of an unordered_multimap.

pubinsert

Inserts elements or nodes (extracted with .extract()) (since C++17).

pubemplace

Constructs a new element in place.

pubemplace_hint

Constructs elements in-place using a hint (iterator).

puberase

Erases elements.

pubswap

Swaps two unordered_multimaps.

pubextract (since C++17)

Extracts nodes from an unordered_multimap (can be later inserted somewhere else).

pubmerge (since C++17)

Merges two unordered_multimaps together.

Lookup

pubcount

Returns the number of elements matching a specific key.

pubfind

Searches for an element and returns an iterator to it, or end iterator if not found.

pubcontains (since C++20)

Returns true if an element is inside an unordered_multimap, otherwise false.

pubequal_range

Returns a range of elements matching a specific key.

Bucket interface

pubbegin
cbegin

Returns an iterator to the beginning of a specified bucket.

pubend
cend

Returns an iterator to the end of a specified bucket.

pubbucket_count

Returns a number of buckets.

pubmax_bucket_count

Returns a maximum number of buckets.

pubbucket_size

Returns a number of elements in a specific bucket.

pubbucket

Returns a bucket for a specific key.

Hash policy

pubload_factor

Returns an average number of elements per bucket.

pubmax_load_factor

Manages a maximum average number of elements per bucket.

pubrehash

Reserves at least the specified number of buckets and regenerates the hash table.

pubreserve

Reserves space for at least a specified number of elements and regenerates the hash table.

Observers

pubhash_function

Returns an internal function object that hashes keys.

pubkey_eq

Returns an internal function object that compares keys.

Non-member functions

puboperator==
operator!= (removed in C++20)
Compares values in an unordered_multimap.
pubstd::swap (std::unordered_multimap)An overload for a std::swap algorithm.
pubstd::erase_if (std::unordered_multimap) (since C++20)Overload for a std::erase_if algorithm.

Deduction guides (since C++17)

Click to expand

Deduction guides

// (1)
template< class InputIt, class Alloc >
unordered_map( InputIt, InputIt, Alloc )
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>,
std::hash<iter_key_t<InputIt>>,
std::equal_to<iter_key_t<InputIt>>, Alloc>;
// (2)
template< class InputIt, class Hash, class Alloc >
unordered_map( InputIt, InputIt, typename /*see below*/::size_type, Hash, Alloc )
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash,
std::equal_to<iter_key_t<InputIt>>, Alloc>;
// (3)
template< class InputIt, class Alloc >
unordered_map( InputIt, InputIt, typename /*see below*/::size_type, Alloc )
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>,
std::hash<iter_key_t<InputIt>>,
std::equal_to<iter_key_t<InputIt>>, Alloc>;
// (4)
template< class InputIt,
class Hash = std::hash<iter_key_t<InputIt>>,
class Pred = std::equal_to<iter_key_t<InputIt>>,
class Alloc = std::allocator<iter_to_alloc_t<InputIt>> >
unordered_map( InputIt, InputIt,
typename /*see below*/::size_type = /*see below*/,
Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
-> unordered_map<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash, Pred, Alloc>;

(1 - 4) allow deduction from an iterator range

// (5)
template< class Key, class T, typename Alloc >
unordered_map( std::initializer_list<std::pair<Key, T>>, Alloc )
-> unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;
// (6)
template< class Key, class T, class Hash, class Alloc >
unordered_map( std::initializer_list<std::pair<Key, T>>,
typename /*see below*/::size_type, Hash, Alloc )
-> unordered_map<Key, T, Hash, std::equal_to<Key>, Alloc>;
// (7)
template< class Key, class T, typename Alloc >
unordered_map( std::initializer_list<std::pair<Key, T>>,
typename /*see below*/::size_type, Alloc )
-> unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>;
// (8)
template< class Key, class T, class Hash = std::hash<Key>,
class Pred = std::equal_to<Key>,
class Alloc = std::allocator<std::pair<const Key, T>> >
unordered_map( std::initializer_list<std::pair<Key, T>>,
typename /*see below*/::size_type = /*see below*/,
Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
-> unordered_map<Key, T, Hash, Pred, Alloc>;

(5 - 8) allow deduction from a std::initializer_list

Aliases iter_key_t, iter_val_t and iter_to_alloc_t are defined as if follows:

template< class InputIt >
using iter_key_t = std::remove_const_t<
typename std::iterator_traits<InputIt>::value_type::first_type>;
template< class InputIt >
using iter_val_t = typename std::iterator_traits<InputIt>::value_type::second_type;
template< class InputIt >
using iter_to_alloc_t = std::pair<
std::add_const_t<typename std::iterator_traits<InputIt>::value_type::first_type>,
typename std::iterator_traits<InputIt>::value_type::second_type
>
zanotuj

Note that these aliases aren't guaranteed to be found anywhere in the standard library. They are defined solely for presentation purposes of these deduction guides and weren't present in the standard library at the point of writing of this document.

Overload resolution

In order for any of the deduction guides to participate in overload resolution, the folllowing requirements must be met: A deduction guide for an unordered associative container shall not participate in overload resolution if any of the following are true:

zanotuj

The extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum:

  • Integral types do not qualify as input iterators.

Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum:

More examples

Basic manipulation

Create, insert and print
#include <unordered_map>
#include <iostream>
#include <string>

int main() {
std::unordered_map<std::string, std::string> player_locations {
{"player2", "Capital City"},
{"player3", "The Harbor"}
};

player_locations["player1"] = "Mine";

for (const auto& [key, value] : player_locations)
std::cout << key << " is in " << value << '\n';

return 0;
}
Result
player3 has got Shield
player2 has got Health potion
player2 has got Sword
This article originates from this CppReference page. It was likely altered for improvements or editors' preference. Click "Edit this page" to see all changes made to this document.
Hover to see the original license.