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Overview

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

The std::multimap is a container that stores key-value pairs in a specified order. It works almost as std::map, the only difference is that the keys can repeat themselves. If the keys of both elements are the same, they are ordered by their values.

Technical details

Technical definition of a multimap

Multimap is an associative container that contains a sorted list of key-value pairs, while permitting multiple entries with the same key.

Sorting is done according to the comparison function Compare, applied to the keys. Search, insertion, and removal operations have logarithmic complexity.

The order of the key-value pairs whose keys compare equivalent is the order of insertion and does not change. (since C++11)

Everywhere the standard library uses the Compare requirements, equivalence is determined by using the equivalence relation as described on Compare.

In imprecise terms, two objects a and b are considered equivalent if neither compares less than the other: !comp(a, b) && !comp(b, a).

std::multimap

Defined inmap

Template parameters

pubKeyThe type of the stored keys.
pubValueThe type of the stored values.
pubCompareA comparator type satisfying Compare.
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
).
pubkey_compareCompare
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
pubreverse_iterator
std::reverse_iterator<iterator>
pubconst_reverse_iterator
std::reverse_iterator<const_iterator>
pubnode_type (since C++17)A specialization of node handle representing a container node.

Member classes

pubvalue_compare

Function object that compares two objects of type value_type.

Member functions

pub(constructors)

Constructs a new multimap.

pub(destructor)

Destructs a multimap.

puboperator=

Assigns one multimap to another.

pubget_allocator

Returns an associated allocator.

Iterators

pubbegin
cbegin (since C++11)

Returns an iterator to the beginning.

pubend
cend (since C++11)

Returns an iterator to the end.

pubrbegin
crbegin (since C++11)

Returns a reverse iterator to the beginning.

pubrend
crend (since C++11)

Returns a reverse iterator to the end.

Capacity

pubempty

Returns true if a multimap is empty, otherwise false.

pubsize

Returns the number of elements in a multimap.

pubmax_size

Returns the maximum possible number of elements.

Modifiers

pubclear

Clears the contents of a multimap.

pubinsert

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

pubemplace (since C++11)

Constructs a new element in place.

pubemplace_hint (since C++11)

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

puberase

Erases elements.

pubswap

Swaps two multimaps.

pubextract (since C++17)

Extracts nodes from a multimap (can be later inserted somewhere else).

pubmerge (since C++17)

Merges two 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 a multimap, otherwise false.

pubequal_range

Returns a range of elements matching a specific key.

publower_bound

Returns an iterator to the first element not less than the given key.

pubupper_bound

Returns an iterator to the first element greater than the given key.

Observers

pubkey_comp

Returns an internal function object that compares keys.

pubvalue_comp

Returns an internal function object that compares keys in objects of type value_type (std::pair<const Key, Value>).

Non-member functions

puboperator==
operator!= (removed in C++20)
operator< (removed in C++20)
operator> (removed in C++20)
operator<= (removed in C++20)
operator>= (removed in C++20)
operator<=> (since C++20)
Lexicographically compares the values in a multimap.
pubstd::swap (std::multimap)An overload for a std::swap algorithm.
pubstd::erase_if (std::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 >
multimap( InputIt, InputIt, Alloc )
-> multimap<iter_key_t<InputIt>, iter_val_t<InputIt>,
std::less<iter_key_t<InputIt>>, Alloc>;
// (2)
template< class InputIt,
class Comp = std::less<iter_key_t<InputIt>>,
class Alloc = std::allocator<iter_to_alloc_t<InputIt>> >
multimap( InputIt, InputIt, Comp = Comp(), Alloc = Alloc() )
-> multimap<iter_key_t<InputIt>, iter_val_t<InputIt>, Comp, Alloc>;

(1) and (2) allow for deduction from an iterator range

// (3)
template< class Key, class T, class Allocator >
multimap( std::initializer_list<std::pair<Key, T>>, Allocator )
-> multimap<Key, T, std::less<Key>, Allocator>;
// (4)
template< class Key,
class T,
class Comp = std::less<Key>,
class Alloc = std::allocator<std::pair<const Key, T>> >
multimap( std::initializer_list<std::pair<Key, T>>, Comp = Comp(), Alloc = Alloc() )
-> multimap<Key, T, Comp, Alloc>;

(3) and (4) allow for 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
>
note

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:

note

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

Creating map, inserting pairs, printing it out
#include <iostream>
#include <utility>
#include <string>
#include <map>

int main(){
std::multimap<std::string, int> potions { {"health", 30}, {"mana", 25}, {"mana", 50} };

potions.emplace("health", 200);
potions.insert(std::make_pair("speed", 10));

for (const auto& [key, value] : potions)
std::cout << key << " " << value << std::endl;

return 0;
}
Result
health 30
health 200
mana 25
mana 50
speed 10
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.
caution

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

Overview

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

The std::multimap is a container that stores key-value pairs in a specified order. It works almost as std::map, the only difference is that the keys can repeat themselves. If the keys of both elements are the same, they are ordered by their values.

Technical details

Technical definition of a multimap

Multimap is an associative container that contains a sorted list of key-value pairs, while permitting multiple entries with the same key.

Sorting is done according to the comparison function Compare, applied to the keys. Search, insertion, and removal operations have logarithmic complexity.

The order of the key-value pairs whose keys compare equivalent is the order of insertion and does not change. (since C++11)

Everywhere the standard library uses the Compare requirements, equivalence is determined by using the equivalence relation as described on Compare.

In imprecise terms, two objects a and b are considered equivalent if neither compares less than the other: !comp(a, b) && !comp(b, a).

std::multimap

Defined inmap

Template parameters

pubKeyThe type of the stored keys.
pubValueThe type of the stored values.
pubCompareA comparator type satisfying Compare.
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
).
pubkey_compareCompare
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
pubreverse_iterator
std::reverse_iterator<iterator>
pubconst_reverse_iterator
std::reverse_iterator<const_iterator>
pubnode_type (since C++17)A specialization of node handle representing a container node.

Member classes

pubvalue_compare

Function object that compares two objects of type value_type.

Member functions

pub(constructors)

Constructs a new multimap.

pub(destructor)

Destructs a multimap.

puboperator=

Assigns one multimap to another.

pubget_allocator

Returns an associated allocator.

Iterators

pubbegin
cbegin (since C++11)

Returns an iterator to the beginning.

pubend
cend (since C++11)

Returns an iterator to the end.

pubrbegin
crbegin (since C++11)

Returns a reverse iterator to the beginning.

pubrend
crend (since C++11)

Returns a reverse iterator to the end.

Capacity

pubempty

Returns true if a multimap is empty, otherwise false.

pubsize

Returns the number of elements in a multimap.

pubmax_size

Returns the maximum possible number of elements.

Modifiers

pubclear

Clears the contents of a multimap.

pubinsert

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

pubemplace (since C++11)

Constructs a new element in place.

pubemplace_hint (since C++11)

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

puberase

Erases elements.

pubswap

Swaps two multimaps.

pubextract (since C++17)

Extracts nodes from a multimap (can be later inserted somewhere else).

pubmerge (since C++17)

Merges two 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 a multimap, otherwise false.

pubequal_range

Returns a range of elements matching a specific key.

publower_bound

Returns an iterator to the first element not less than the given key.

pubupper_bound

Returns an iterator to the first element greater than the given key.

Observers

pubkey_comp

Returns an internal function object that compares keys.

pubvalue_comp

Returns an internal function object that compares keys in objects of type value_type (std::pair<const Key, Value>).

Non-member functions

puboperator==
operator!= (removed in C++20)
operator< (removed in C++20)
operator> (removed in C++20)
operator<= (removed in C++20)
operator>= (removed in C++20)
operator<=> (since C++20)
Lexicographically compares the values in a multimap.
pubstd::swap (std::multimap)An overload for a std::swap algorithm.
pubstd::erase_if (std::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 >
multimap( InputIt, InputIt, Alloc )
-> multimap<iter_key_t<InputIt>, iter_val_t<InputIt>,
std::less<iter_key_t<InputIt>>, Alloc>;
// (2)
template< class InputIt,
class Comp = std::less<iter_key_t<InputIt>>,
class Alloc = std::allocator<iter_to_alloc_t<InputIt>> >
multimap( InputIt, InputIt, Comp = Comp(), Alloc = Alloc() )
-> multimap<iter_key_t<InputIt>, iter_val_t<InputIt>, Comp, Alloc>;

(1) and (2) allow for deduction from an iterator range

// (3)
template< class Key, class T, class Allocator >
multimap( std::initializer_list<std::pair<Key, T>>, Allocator )
-> multimap<Key, T, std::less<Key>, Allocator>;
// (4)
template< class Key,
class T,
class Comp = std::less<Key>,
class Alloc = std::allocator<std::pair<const Key, T>> >
multimap( std::initializer_list<std::pair<Key, T>>, Comp = Comp(), Alloc = Alloc() )
-> multimap<Key, T, Comp, Alloc>;

(3) and (4) allow for 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
>
note

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:

note

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

Creating map, inserting pairs, printing it out
#include <iostream>
#include <utility>
#include <string>
#include <map>

int main(){
std::multimap<std::string, int> potions { {"health", 30}, {"mana", 25}, {"mana", 50} };

potions.emplace("health", 200);
potions.insert(std::make_pair("speed", 10));

for (const auto& [key, value] : potions)
std::cout << key << " " << value << std::endl;

return 0;
}
Result
health 30
health 200
mana 25
mana 50
speed 10
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.