unordered_multimap Class

Artikkeli
08/17/2021

The class template describes an object that controls a varying-length sequence of elements of type std::pair<const Key, Ty>. The sequence is weakly ordered by a hash function, which partitions the sequence into an ordered set of subsequences called buckets. Within each bucket a comparison function determines whether any pair of elements has equivalent ordering. Each element stores two objects, a sort key and a value. The sequence is represented in a way that permits lookup, insertion, and removal of an arbitrary element with a number of operations that can be independent of the number of elements in the sequence (constant time), at least when all buckets are of roughly equal length. In the worst case, when all of the elements are in one bucket, the number of operations is proportional to the number of elements in the sequence (linear time). Moreover, inserting an element invalidates no iterators, and removing an element invalidates only those iterators which point at the removed element.

Syntax

template <class Key,
    class Ty,
    class Hash = std::hash<Key>,
    class Pred = std::equal_to<Key>,
    class Alloc = std::allocator<Key>>
class unordered_multimap;

Parameters

Key
The key type.

Ty
The mapped type.

Hash
The hash function object type.

Pred
The equality comparison function object type.

Alloc
The allocator class.

Members

Type Definition	Description
allocator_type	The type of an allocator for managing storage.
const_iterator	The type of a constant iterator for the controlled sequence.
const_local_iterator	The type of a constant bucket iterator for the controlled sequence.
const_pointer	The type of a constant pointer to an element.
const_reference	The type of a constant reference to an element.
difference_type	The type of a signed distance between two elements.
hasher	The type of the hash function.
iterator	The type of an iterator for the controlled sequence.
key_equal	The type of the comparison function.
key_type	The type of an ordering key.
local_iterator	The type of a bucket iterator for the controlled sequence.
mapped_type	The type of a mapped value associated with each key.
pointer	The type of a pointer to an element.
reference	The type of a reference to an element.
size_type	The type of an unsigned distance between two elements.
value_type	The type of an element.

Member Function	Description
begin	Designates the beginning of the controlled sequence.
bucket	Gets the bucket number for a key value.
bucket_count	Gets the number of buckets.
bucket_size	Gets the size of a bucket.
cbegin	Designates the beginning of the controlled sequence.
cend	Designates the end of the controlled sequence.
clear	Removes all elements.
contains^C++20	Checks if there is an element with the specified key in the `unordered_multimap`.
count	Finds the number of elements matching a specified key.
emplace	Adds an element constructed in place.
emplace_hint	Adds an element constructed in place, with hint.
empty	Tests whether no elements are present.
end	Designates the end of the controlled sequence.
equal_range	Finds range that matches a specified key.
erase	Removes elements at specified positions.
find	Finds an element that matches a specified key.
get_allocator	Gets the stored allocator object.
hash_function	Gets the stored hash function object.
insert	Adds elements.
key_eq	Gets the stored comparison function object.
load_factor	Counts the average elements per bucket.
max_bucket_count	Gets the maximum number of buckets.
max_load_factor	Gets or sets the maximum elements per bucket.
max_size	Gets the maximum size of the controlled sequence.
rehash	Rebuilds the hash table.
size	Counts the number of elements.
swap	Swaps the contents of two containers.
unordered_multimap	Constructs a container object.

Operator	Description
unordered_multimap::operator=	Copies a hash table.

Remarks

The object orders the sequence it controls by calling two stored objects, a comparison function object of type unordered_multimap::key_equal and a hash function object of type unordered_multimap::hasher. You access the first stored object by calling the member function unordered_multimap::key_eq(); and you access the second stored object by calling the member function unordered_multimap::hash_function(). Specifically, for all values X and Y of type Key, the call key_eq()(X, Y) returns true only if the two argument values have equivalent ordering; the call hash_function()(keyval) yields a distribution of values of type size_t. Unlike class template unordered_map Class, an object of type unordered_multimap does not ensure that key_eq()(X, Y) is always false for any two elements of the controlled sequence. (Keys need not be unique.)

The object also stores a maximum load factor, which specifies the maximum desired average number of elements per bucket. If inserting an element causes unordered_multimap::load_factor() to exceed the maximum load factor, the container increases the number of buckets and rebuilds the hash table as needed.

The actual order of elements in the controlled sequence depends on the hash function, the comparison function, the order of insertion, the maximum load factor, and the current number of buckets. You cannot in general predict the order of elements in the controlled sequence. You can always be assured, however, that any subset of elements that have equivalent ordering are adjacent in the controlled sequence.

The object allocates and frees storage for the sequence it controls through a stored allocator object of type unordered_multimap::allocator_type. Such an allocator object must have the same external interface as an object of type allocator. Note that the stored allocator object is not copied when the container object is assigned.

Requirements

Header: <unordered_map>

Namespace: std

unordered_multimap::allocator_type

The type of an allocator for managing storage.

typedef Alloc allocator_type;

Remarks

The type is a synonym for the template parameter Alloc.

Example

// std__unordered_map__unordered_multimap_allocator_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
typedef std::allocator<std::pair<const char, int> > Myalloc;
int main()
    {
    Mymap c1;

    Mymap::allocator_type al = c1.get_allocator();
    std::cout << "al == std::allocator() is "
        << std::boolalpha << (al == Myalloc()) << std::endl;

    return (0);
    }

al == std::allocator() is true

unordered_multimap::begin

Designates the beginning of the controlled sequence or a bucket.

iterator begin();

const_iterator begin() const;

local_iterator begin(size_type nbucket);

const_local_iterator begin(size_type nbucket) const;

Parameters

nbucket
The bucket number.

Remarks

The first two member functions return a forward iterator that points at the first element of the sequence (or just beyond the end of an empty sequence). The last two member functions return a forward iterator that points at the first element of bucket nbucket (or just beyond the end of an empty bucket).

Example

// std__unordered_map__unordered_multimap_begin.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// inspect first two items " [c 3] [b 2]"
    Mymap::iterator it2 = c1.begin();
    std::cout << " [" << it2->first << ", " << it2->second << "]";
    ++it2;
    std::cout << " [" << it2->first << ", " << it2->second << "]";
    std::cout << std::endl;

// inspect bucket containing 'a'
    Mymap::const_local_iterator lit = c1.begin(c1.bucket('a'));
    std::cout << " [" << lit->first << ", " << lit->second << "]";

    return (0);
    }

[c, 3] [b, 2] [a, 1]
[c, 3] [b, 2]
[a, 1]

unordered_multimap::bucket

Gets the bucket number for a key value.

size_type bucket(const Key& keyval) const;

Parameters

keyval
The key value to map.

Remarks

The member function returns the bucket number currently corresponding to the key value keyval.

Example

// std__unordered_map__unordered_multimap_bucket.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// display buckets for keys
    Mymap::size_type bs = c1.bucket('a');
    std::cout << "bucket('a') == " << bs << std::endl;
    std::cout << "bucket_size(" << bs << ") == " << c1.bucket_size(bs)
        << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
bucket('a') == 7
bucket_size(7) == 1

unordered_multimap::bucket_count

Gets the number of buckets.

size_type bucket_count() const;

Remarks

The member function returns the current number of buckets.

Example

// std__unordered_map__unordered_multimap_bucket_count.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// inspect current parameters
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// change max_load_factor and redisplay
    c1.max_load_factor(0.10f);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// rehash and redisplay
    c1.rehash(100);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
bucket_count() == 8
load_factor() == 0.375
max_bucket_count() == 8
max_load_factor() == 4

bucket_count() == 8
load_factor() == 0.375
max_bucket_count() == 8
max_load_factor() == 0.1

bucket_count() == 128
load_factor() == 0.0234375
max_bucket_count() == 128
max_load_factor() == 0.1

unordered_multimap::bucket_size

Gets the size of a bucket

size_type bucket_size(size_type nbucket) const;

Parameters

nbucket
The bucket number.

Remarks

The member functions returns the size of bucket number nbucket.

Example

// std__unordered_map__unordered_multimap_bucket_size.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// display buckets for keys
    Mymap::size_type bs = c1.bucket('a');
    std::cout << "bucket('a') == " << bs << std::endl;
    std::cout << "bucket_size(" << bs << ") == " << c1.bucket_size(bs)
        << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
bucket('a') == 7
bucket_size(7) == 1

unordered_multimap::cbegin

Returns a const iterator that addresses the first element in the range.

const_iterator cbegin() const;

Return Value

A const forward-access iterator that points at the first element of the range, or the location just beyond the end of an empty range (for an empty range, cbegin() == cend()).

Remarks

With the return value of cbegin, the elements in the range cannot be modified.

You can use this member function in place of the begin() member function to guarantee that the return value is const_iterator. Typically, it's used in conjunction with the auto type deduction keyword, as shown in the following example. In the example, consider Container to be a modifiable (non- const) container of any kind that supports begin() and cbegin().

auto i1 = Container.begin();
// i1 is Container<T>::iterator
auto i2 = Container.cbegin();

// i2 is Container<T>::const_iterator

unordered_multimap::cend

Returns a const iterator that addresses the location just beyond the last element in a range.

const_iterator cend() const;

Return Value

A const forward-access iterator that points just beyond the end of the range.

Remarks

cend is used to test whether an iterator has passed the end of its range.

You can use this member function in place of the end() member function to guarantee that the return value is const_iterator. Typically, it's used in conjunction with the auto type deduction keyword, as shown in the following example. In the example, consider Container to be a modifiable (non- const) container of any kind that supports end() and cend().

auto i1 = Container.end();
// i1 is Container<T>::iterator
auto i2 = Container.cend();

// i2 is Container<T>::const_iterator

The value returned by cend should not be dereferenced.

unordered_multimap::clear

Removes all elements.

void clear();

Remarks

The member function calls unordered_multimap::erase( unordered_multimap::begin(), unordered_multimap::end()).

Example

// std__unordered_map__unordered_multimap_clear.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// clear the container and reinspect
    c1.clear();
    std::cout << "size == " << c1.size() << std::endl;
    std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;
    std::cout << std::endl;

    c1.insert(Mymap::value_type('d', 4));
    c1.insert(Mymap::value_type('e', 5));

// display contents " [e 5] [d 4]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    std::cout << "size == " << c1.size() << std::endl;
    std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
size == 0
empty() == true

[e, 5] [d, 4]
size == 2
empty() == false

unordered_multimap::const_iterator

The type of a constant iterator for the controlled sequence.

typedef T1 const_iterator;

Remarks

The type describes an object that can serve as a constant forward iterator for the controlled sequence. It is described here as a synonym for the implementation-defined type T1.

Example

// std__unordered_map__unordered_multimap_const_iterator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]

unordered_multimap::const_local_iterator

The type of a constant bucket iterator for the controlled sequence.

typedef T5 const_local_iterator;

Remarks

The type describes an object that can serve as a constant forward iterator for a bucket. It is described here as a synonym for the implementation-defined type T5.

Example

// std__unordered_map__unordered_multimap_const_local_iterator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// inspect bucket containing 'a'
    Mymap::const_local_iterator lit = c1.begin(c1.bucket('a'));
    std::cout << " [" << lit->first << ", " << lit->second << "]";

    return (0);
    }

[c, 3] [b, 2] [a, 1]
[a, 1]

unordered_multimap::const_pointer

The type of a constant pointer to an element.

typedef Alloc::const_pointer const_pointer;

Remarks

The type describes an object that can serve as a constant pointer to an element of the controlled sequence.

Example

// std__unordered_map__unordered_multimap_const_pointer.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::iterator it = c1.begin();
        it != c1.end(); ++it)
        {
        Mymap::const_pointer p = &*it;
        std::cout << " [" << p->first << ", " << p->second << "]";
        }
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]

unordered_multimap::const_reference

The type of a constant reference to an element.

typedef Alloc::const_reference const_reference;

Remarks

The type describes an object that can serve as a constant reference to an element of the controlled sequence.

Example

// std__unordered_map__unordered_multimap_const_reference.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::iterator it = c1.begin();
        it != c1.end(); ++it)
        {
        Mymap::const_reference ref = *it;
        std::cout << " [" << ref.first << ", " << ref.second << "]";
        }
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]

unordered_multimap::contains

Checks if there is an element with the specified key in the unordered_multimap.

bool contains(const Key& key) const;
template<class K> bool contains(const K& key) const;

Parameters

K
The type of the key.

key
The element's key value to look for.

Return Value

true if the element is found in the container; false otherwise.

Remarks

contains() is new in C++20. To use it, specify the /std:c++20 or later compiler option.

template<class K> bool contains(const K& key) const only participates in overload resolution if key_compare is transparent.

Example

// Requires /std:c++20 or /std:c++latest
#include <unordered_map>
#include <iostream>

int main()
{
    std::unordered_multimap<int, bool> theUnorderedMultimap = {{0, false}, {1,true}};

    std::cout << std::boolalpha; // so booleans show as 'true' or 'false'
    std::cout << theUnorderedMultimap.contains(1) << '\n';
    std::cout << theUnorderedMultimap.contains(2) << '\n';

    return 0;
}

true
false

unordered_multimap::count

Finds the number of elements matching a specified key.

size_type count(const Key& keyval) const;

Parameters

keyval
Key value to search for.

Remarks

The member function returns the number of elements in the range delimited by unordered_multimap::equal_range(keyval).

Example

// std__unordered_map__unordered_multimap_count.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    std::cout << "count('A') == " << c1.count('A') << std::endl;
    std::cout << "count('b') == " << c1.count('b') << std::endl;
    std::cout << "count('C') == " << c1.count('C') << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
count('A') == 0
count('b') == 1
count('C') == 0

unordered_multimap::difference_type

The type of a signed distance between two elements.

typedef T3 difference_type;

Remarks

The signed integer type describes an object that can represent the difference between the addresses of any two elements in the controlled sequence. It is described here as a synonym for the implementation-defined type T3.

Example

// std__unordered_map__unordered_multimap_difference_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// compute positive difference
    Mymap::difference_type diff = 0;
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        ++diff;
    std::cout << "end()-begin() == " << diff << std::endl;

// compute negative difference
    diff = 0;
    for (Mymap::const_iterator it = c1.end();
        it != c1.begin(); --it)
        --diff;
    std::cout << "begin()-end() == " << diff << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
end()-begin() == 3
begin()-end() == -3

unordered_multimap::emplace

Inserts an element constructed in place (no copy or move operations are performed), with a placement hint.

template <class... Args>
iterator emplace(Args&&... args);

Parameters

args
The arguments forwarded to construct an element to be inserted into the unordered_multimap.

Return Value

An iterator to the newly inserted element.

Remarks

No references to container elements are invalidated by this function, but it may invalidate all iterators to the container.

The value_type of an element is a pair, so that the value of an element will be an ordered pair with the first component equal to the key value and the second component equal to the data value of the element.

During the insertion, if an exception is thrown but does not occur in the container's hash function, the container is not modified. If the exception is thrown in the hash function, the result is undefined.

For a code example, see multimap::emplace.

unordered_multimap::emplace_hint

Inserts an element constructed in place (no copy or move operations are performed), with a placement hint.

template <class... Args>
iterator emplace_hint(
    const_iterator where,
    Args&&... args);

Parameters

args
The arguments forwarded to construct an element to be inserted into the unordered.

where
A hint regarding the place to start searching for the correct point of insertion.

Return Value

An iterator to the newly inserted element.

Remarks

No references to container elements are invalidated by this function, but it may invalidate all iterators to the container.

For a code example, see map::emplace_hint.

unordered_multimap::empty

Tests whether no elements are present.

bool empty() const;

Remarks

The member function returns true for an empty controlled sequence.

Example

// std__unordered_map__unordered_multimap_empty.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// clear the container and reinspect
    c1.clear();
    std::cout << "size == " << c1.size() << std::endl;
    std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;
    std::cout << std::endl;

    c1.insert(Mymap::value_type('d', 4));
    c1.insert(Mymap::value_type('e', 5));

// display contents " [e 5] [d 4]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    std::cout << "size == " << c1.size() << std::endl;
    std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
size == 0
empty() == true

[e, 5] [d, 4]
size == 2
empty() == false

unordered_multimap::end

Designates the end of the controlled sequence.

iterator end();

const_iterator end() const;

local_iterator end(size_type nbucket);

const_local_iterator end(size_type nbucket) const;

Parameters

nbucket
The bucket number.

Remarks

The first two member functions return a forward iterator that points just beyond the end of the sequence. The last two member functions return a forward iterator that points just beyond the end of bucket nbucket.

Example

// std__unordered_map__unordered_multimap_end.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// inspect last two items " [a 1] [b 2]"
    Mymap::iterator it2 = c1.end();
    --it2;
    std::cout << " [" << it2->first << ", " << it2->second << "]";
    --it2;
    std::cout << " [" << it2->first << ", " << it2->second << "]";
    std::cout << std::endl;

// inspect bucket containing 'a'
    Mymap::const_local_iterator lit = c1.end(c1.bucket('a'));
    --lit;
    std::cout << " [" << lit->first << ", " << lit->second << "]";

    return (0);
    }

[c, 3] [b, 2] [a, 1]
[a, 1] [b, 2]
[a, 1]

unordered_multimap::equal_range

Finds range that matches a specified key.

std::pair<iterator, iterator>
    equal_range(const Key& keyval);

std::pair<const_iterator, const_iterator>
    equal_range(const Key& keyval) const;

Parameters

keyval
Key value to search for.

Remarks

The member function returns a pair of iterators X such that [X.first, X.second) delimits just those elements of the controlled sequence that have equivalent ordering with keyval. If no such elements exist, both iterators are end().

Example

// std__unordered_map__unordered_multimap_equal_range.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// display results of failed search
    std::pair<Mymap::iterator, Mymap::iterator> pair1 =
        c1.equal_range('x');
    std::cout << "equal_range('x'):";
    for (; pair1.first != pair1.second; ++pair1.first)
        std::cout << " [" << pair1.first->first
            << ", " << pair1.first->second << "]";
    std::cout << std::endl;

// display results of successful search
    pair1 = c1.equal_range('b');
    std::cout << "equal_range('b'):";
    for (; pair1.first != pair1.second; ++pair1.first)
        std::cout << " [" << pair1.first->first
            << ", " << pair1.first->second << "]";
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
equal_range('x'):
equal_range('b'): [b, 2]

unordered_multimap::erase

Removes an element or a range of elements in a unordered_multimap from specified positions or removes elements that match a specified key.

iterator erase(
    const_iterator Where);

iterator erase(
    const_iterator First,
    const_iterator Last);

size_type erase(
    const key_type& Key);

Parameters

Where
Position of the element to be removed.

First
Position of the first element to be removed.

Last
Position just beyond the last element to be removed.

Key
The key value of the elements to be removed.

Return Value

For the first two member functions, a bidirectional iterator that designates the first element remaining beyond any elements removed, or an element that is the end of the map if no such element exists.

For the third member function, returns the number of elements that have been removed from the unordered_multimap.

Remarks

For a code example, see map::erase.

unordered_multimap::find

Finds an element that matches a specified key.

const_iterator find(const Key& keyval) const;

Parameters

keyval
Key value to search for.

Remarks

The member function returns unordered_multimap::equal_range(keyval).first.

Example

// std__unordered_map__unordered_multimap_find.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// try to find and fail
    std::cout << "find('A') == "
        << std::boolalpha << (c1.find('A') != c1.end()) << std::endl;

// try to find and succeed
    Mymap::iterator it = c1.find('b');
    std::cout << "find('b') == "
        << std::boolalpha << (it != c1.end())
        << ": [" << it->first << ", " << it->second << "]" << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
find('A') == false
find('b') == true: [b, 2]

unordered_multimap::get_allocator

Gets the stored allocator object.

Alloc get_allocator() const;

Remarks

The member function returns the stored allocator object.

Example

// std__unordered_map__unordered_multimap_get_allocator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
typedef std::allocator<std::pair<const char, int> > Myalloc;
int main()
    {
    Mymap c1;

    Mymap::allocator_type al = c1.get_allocator();
    std::cout << "al == std::allocator() is "
        << std::boolalpha << (al == Myalloc()) << std::endl;

    return (0);
    }

al == std::allocator() is true

unordered_multimap::hash_function

Gets the stored hash function object.

Hash hash_function() const;

Remarks

The member function returns the stored hash function object.

Example

// std__unordered_map__unordered_multimap_hash_function.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    Mymap::hasher hfn = c1.hash_function();
    std::cout << "hfn('a') == " << hfn('a') << std::endl;
    std::cout << "hfn('b') == " << hfn('b') << std::endl;

    return (0);
    }

hfn('a') == 1630279
hfn('b') == 1647086

unordered_multimap::hasher

The type of the hash function.

typedef Hash hasher;

Remarks

The type is a synonym for the template parameter Hash.

Example

// std__unordered_map__unordered_multimap_hasher.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    Mymap::hasher hfn = c1.hash_function();
    std::cout << "hfn('a') == " << hfn('a') << std::endl;
    std::cout << "hfn('b') == " << hfn('b') << std::endl;

    return (0);
    }

hfn('a') == 1630279
hfn('b') == 1647086

unordered_multimap::insert

Inserts an element or a range of elements into an unordered_multimap.

// (1) single element
pair<iterator, bool> insert(
    const value_type& Val);

// (2) single element, perfect forwarded
template <class ValTy>
pair<iterator, bool>
insert(
    ValTy&& Val);

// (3) single element with hint
iterator insert(
    const_iterator Where,
    const value_type& Val);

// (4) single element, perfect forwarded, with hint
template <class ValTy>
iterator insert(
    const_iterator Where,
    ValTy&& Val);

// (5) range
template <class InputIterator>
void insert(
    InputIterator First,
    InputIterator Last);

// (6) initializer list
void insert(
    initializer_list<value_type>
IList);

Parameters

Val
The value of an element to be inserted into the unordered_multimap.

Where
The place to start searching for the correct point of insertion.

ValTy
Template parameter that specifies the argument type that the unordered_multimap can use to construct an element of value_type, and perfect-forwards Val as an argument.

First
The position of the first element to be copied.

Last
The position just beyond the last element to be copied.

InputIterator
Template function argument that meets the requirements of an input iterator that points to elements of a type that can be used to construct value_type objects.

IList
The initializer_list from which to copy the elements.

Return Value

The single-element-insert member functions, (1) and (2), return an iterator to the position where the new element was inserted into the unordered_multimap.

The single-element-with-hint member functions, (3) and (4), return an iterator that points to the position where the new element was inserted into the unordered_multimap.

Remarks

No pointers or references are invalidated by this function, but it may invalidate all iterators to the container.

During the insertion of just one element, if an exception is thrown but does not occur in the container's hash function, the container's state is not modified. If the exception is thrown in the hash function, the result is undefined. During the insertion of multiple elements, if an exception is thrown, the container is left in an unspecified but valid state.

The value_type of a container is a typedef that belongs to the container, and for map, map<K, V>::value_type is pair<const K, V>. The value of an element is an ordered pair in which the first component is equal to the key value and the second component is equal to the data value of the element.

The range member function (5) inserts the sequence of element values into an unordered_multimap that corresponds to each element addressed by an iterator in the range [First, Last); therefore, Last does not get inserted. The container member function end() refers to the position just after the last element in the container—for example, the statement m.insert(v.begin(), v.end()); inserts all elements of v into m.

The initializer list member function (6) uses an initializer_list to copy elements into the unordered_multimap.

For insertion of an element constructed in place—that is, no copy or move operations are performed—see unordered_multimap::emplace and unordered_multimap::emplace_hint.

For a code example, see multimap::insert.

unordered_multimap::iterator

The type of an iterator for the controlled sequence.

typedef T0 iterator;

Remarks

The type describes an object that can serve as a forward iterator for the controlled sequence. It is described here as a synonym for the implementation-defined type T0.

Example

// std__unordered_map__unordered_multimap_iterator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]

unordered_multimap::key_eq

Gets the stored comparison function object.

Pred key_eq() const;

Remarks

The member function returns the stored comparison function object.

Example

// std__unordered_map__unordered_multimap_key_eq.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    Mymap::key_equal cmpfn = c1.key_eq();
    std::cout << "cmpfn('a', 'a') == "
        << std::boolalpha << cmpfn('a', 'a') << std::endl;
    std::cout << "cmpfn('a', 'b') == "
        << std::boolalpha << cmpfn('a', 'b') << std::endl;

    return (0);
    }

cmpfn('a', 'a') == true
cmpfn('a', 'b') == false

unordered_multimap::key_equal

The type of the comparison function.

typedef Pred key_equal;

Remarks

The type is a synonym for the template parameter Pred.

Example

// std__unordered_map__unordered_multimap_key_equal.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    Mymap::key_equal cmpfn = c1.key_eq();
    std::cout << "cmpfn('a', 'a') == "
        << std::boolalpha << cmpfn('a', 'a') << std::endl;
    std::cout << "cmpfn('a', 'b') == "
        << std::boolalpha << cmpfn('a', 'b') << std::endl;

    return (0);
    }

cmpfn('a', 'a') == true
cmpfn('a', 'b') == false

unordered_multimap::key_type

The type of an ordering key.

typedef Key key_type;

Remarks

The type is a synonym for the template parameter Key.

Example

// std__unordered_map__unordered_multimap_key_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// add a value and reinspect
    Mymap::key_type key = 'd';
    Mymap::mapped_type mapped = 4;
    Mymap::value_type val = Mymap::value_type(key, mapped);
    c1.insert(val);

    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
[d, 4] [c, 3] [b, 2] [a, 1]

unordered_multimap::load_factor

Counts the average elements per bucket.

float load_factor() const;

Remarks

The member function returns (float)unordered_multimap::size() / (float)unordered_multimap::bucket_count(), the average number of elements per bucket.

Example

// std__unordered_map__unordered_multimap_load_factor.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// inspect current parameters
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// change max_load_factor and redisplay
    c1.max_load_factor(0.10f);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// rehash and redisplay
    c1.rehash(100);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

    return (0);
    }

unordered_multimap::local_iterator

The type of a bucket iterator.

typedef T4 local_iterator;

Remarks

The type describes an object that can serve as a forward iterator for a bucket. It is described here as a synonym for the implementation-defined type T4.

Example

// std__unordered_map__unordered_multimap_local_iterator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// inspect bucket containing 'a'
    Mymap::local_iterator lit = c1.begin(c1.bucket('a'));
    std::cout << " [" << lit->first << ", " << lit->second << "]";

    return (0);
    }

[c, 3] [b, 2] [a, 1]
[a, 1]

unordered_multimap::mapped_type

The type of a mapped value associated with each key.

typedef Ty mapped_type;

Remarks

The type is a synonym for the template parameter Ty.

Example

// std__unordered_map__unordered_multimap_mapped_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// add a value and reinspect
    Mymap::key_type key = 'd';
    Mymap::mapped_type mapped = 4;
    Mymap::value_type val = Mymap::value_type(key, mapped);
    c1.insert(val);

    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
[d, 4] [c, 3] [b, 2] [a, 1]

unordered_multimap::max_bucket_count

Gets the maximum number of buckets.

size_type max_bucket_count() const;

Remarks

The member function returns the maximum number of buckets currently permitted.

Example

// std__unordered_map__unordered_multimap_max_bucket_count.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// inspect current parameters
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// change max_load_factor and redisplay
    c1.max_load_factor(0.10f);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// rehash and redisplay
    c1.rehash(100);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
bucket_count() == 8
load_factor() == 0.375
max_bucket_count() == 8
max_load_factor() == 4

bucket_count() == 8
load_factor() == 0.375
max_bucket_count() == 8
max_load_factor() == 0.1

bucket_count() == 128
load_factor() == 0.0234375
max_bucket_count() == 128
max_load_factor() == 0.1

unordered_multimap::max_load_factor

Gets or sets the maximum elements per bucket.

float max_load_factor() const;

void max_load_factor(float factor);

Parameters

factor
The new maximum load factor.

Remarks

The first member function returns the stored maximum load factor. The second member function replaces the stored maximum load factor with factor.

Example

// std__unordered_map__unordered_multimap_max_load_factor.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// inspect current parameters
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// change max_load_factor and redisplay
    c1.max_load_factor(0.10f);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// rehash and redisplay
    c1.rehash(100);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_bucket_count() == "
        << c1.max_bucket_count() << std::endl;
    std::cout << "max_load_factor() == "
        << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
bucket_count() == 8
load_factor() == 0.375
max_bucket_count() == 8
max_load_factor() == 4

bucket_count() == 8
load_factor() == 0.375
max_bucket_count() == 8
max_load_factor() == 0.1

bucket_count() == 128
load_factor() == 0.0234375
max_bucket_count() == 128
max_load_factor() == 0.1

unordered_multimap::max_size

Gets the maximum size of the controlled sequence.

size_type max_size() const;

Remarks

The member function returns the length of the longest sequence that the object can control.

Example

// std__unordered_map__unordered_multimap_max_size.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    std::cout << "max_size() == " << c1.max_size() << std::endl;

    return (0);
    }

max_size() == 536870911

unordered_multimap::operator=

Copies a hash table.

unordered_multimap& operator=(const unordered_multimap& right);

unordered_multimap& operator=(unordered_multimap&& right);

Parameters

right
The unordered_multimap being copied into the unordered_multimap.

Remarks

After erasing any existing elements in a unordered_multimap, operator= either copies or moves the contents of right into the unordered_multimap.

Example

// unordered_multimap_operator_as.cpp
// compile with: /EHsc
#include <unordered_multimap>
#include <iostream>

int main( )
   {
   using namespace std;
   unordered_multimap<int, int> v1, v2, v3;
   unordered_multimap<int, int>::iterator iter;

   v1.insert(pair<int, int>(1, 10));

   cout << "v1 = " ;
   for (iter = v1.begin(); iter != v1.end(); iter++)
      cout << iter->second << " ";
   cout << endl;

   v2 = v1;
   cout << "v2 = ";
   for (iter = v2.begin(); iter != v2.end(); iter++)
      cout << iter->second << " ";
   cout << endl;

// move v1 into v2
   v2.clear();
   v2 = move(v1);
   cout << "v2 = ";
   for (iter = v2.begin(); iter != v2.end(); iter++)
      cout << iter->second << " ";
   cout << endl;
   }

unordered_multimap::pointer

The type of a pointer to an element.

typedef Alloc::pointer pointer;

Remarks

The type describes an object that can serve as a pointer to an element of the controlled sequence.

Example

// std__unordered_map__unordered_multimap_pointer.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::iterator it = c1.begin();
        it != c1.end(); ++it)
        {
        Mymap::pointer p = &*it;
        std::cout << " [" << p->first << ", " << p->second << "]";
        }
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]

unordered_multimap::reference

The type of a reference to an element.

typedef Alloc::reference reference;

Remarks

The type describes an object that can serve as a reference to an element of the controlled sequence.

Example

// std__unordered_map__unordered_multimap_reference.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::iterator it = c1.begin();
        it != c1.end(); ++it)
        {
        Mymap::reference ref = *it;
        std::cout << " [" << ref.first << ", " << ref.second << "]";
        }
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]

unordered_multimap::rehash

Rebuilds the hash table.

void rehash(size_type nbuckets);

Parameters

nbuckets
The requested number of buckets.

Remarks

The member function alters the number of buckets to be at least nbuckets and rebuilds the hash table as needed.

Example

// std__unordered_map__unordered_multimap_rehash.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// inspect current parameters
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_load_factor() == " << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// change max_load_factor and redisplay
    c1.max_load_factor(0.10f);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_load_factor() == " << c1.max_load_factor() << std::endl;
    std::cout << std::endl;

// rehash and redisplay
    c1.rehash(100);
    std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
    std::cout << "load_factor() == " << c1.load_factor() << std::endl;
    std::cout << "max_load_factor() == " << c1.max_load_factor() << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
bucket_count() == 8
load_factor() == 0.375
max_load_factor() == 4

bucket_count() == 8
load_factor() == 0.375
max_load_factor() == 0.1

bucket_count() == 128
load_factor() == 0.0234375
max_load_factor() == 0.1

unordered_multimap::size

Counts the number of elements.

size_type size() const;

Remarks

The member function returns the length of the controlled sequence.

Example

// std__unordered_map__unordered_multimap_size.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// clear the container and reinspect
    c1.clear();
    std::cout << "size == " << c1.size() << std::endl;
    std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;
    std::cout << std::endl;

    c1.insert(Mymap::value_type('d', 4));
    c1.insert(Mymap::value_type('e', 5));

// display contents " [e 5] [d 4]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    std::cout << "size == " << c1.size() << std::endl;
    std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
size == 0
empty() == true

[e, 5] [d, 4]
size == 2
empty() == false

unordered_multimap::size_type

The type of an unsigned distance between two elements.

typedef T2 size_type;

Remarks

The unsigned integer type describes an object that can represent the length of any controlled sequence. It is described here as a synonym for the implementation-defined type T2.

Example

// std__unordered_map__unordered_multimap_size_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;
    Mymap::size_type sz = c1.size();

    std::cout << "size == " << sz << std::endl;

    return (0);
    }

size == 0

unordered_multimap::swap

Swaps the contents of two containers.

void swap(unordered_multimap& right);

Parameters

right
The container to swap with.

Remarks

The member function swaps the controlled sequences between *this and right. If unordered_multimap::get_allocator() == right.get_allocator(), it does so in constant time, it throws an exception only as a result of copying the stored traits object of type Tr, and it invalidates no references, pointers, or iterators that designate elements in the two controlled sequences. Otherwise, it performs a number of element assignments and constructor calls proportional to the number of elements in the two controlled sequences.

Example

// std__unordered_map__unordered_multimap_swap.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    Mymap c2;

    c2.insert(Mymap::value_type('d', 4));
    c2.insert(Mymap::value_type('e', 5));
    c2.insert(Mymap::value_type('f', 6));

    c1.swap(c2);

// display contents " [f 6] [e 5] [d 4]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    swap(c1, c2);

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
[f, 6] [e, 5] [d, 4]
[c, 3] [b, 2] [a, 1]

unordered_multimap::unordered_multimap

Constructs a container object.

unordered_multimap(
    const unordered_multimap& Right);

explicit unordered_multimap(
    size_type Bucket_count = N0,
    const Hash& Hash = Hash(),
    const Comp& Comp = Pred(),
    const Allocator& Al = Alloc());

unordered_multimap(
    unordered_multimap&& Right);

unordered_multimap(
    initializer_list<Type> IList);

unordered_multimap(
    initializer_list<Type> IList,
    size_type Bucket_count);

unordered_multimap(
    initializer_list<Type> IList,
    size_type Bucket_count,
    const Hash& Hash);

unordered_multimap(
    initializer_list<Type> IList,
    size_type Bucket_count,
    const Hash& Hash,
    const Key& Key);

unordered_multimap(
    initializer_list<Type> IList,
    size_type Bucket_count,
    const Hash& Hash,
    const Key& Key,
    const Allocator& Al);

template <class InputIterator>
unordered_multimap(
    InputIterator first,
    InputIterator last,
    size_type Bucket_count = N0,
    const Hash& Hash = Hash(),
    const Comp& Comp = Pred(),
    const Allocator& Al = Alloc());

Parameters

InputIterator
The iterator type.

Al
The allocator object to store.

Comp
The comparison function object to store.

Hash
The hash function object to store.

Bucket_count
The minimum number of buckets.

Right
The container to copy.

IList
The initializer_list from which to copy the elements.

Remarks

The first constructor specifies a copy of the sequence controlled by Right. The second constructor specifies an empty controlled sequence. The third constructor. specifies a copy of the sequence by moving Right. The fourth, fifth, sixth, seventh, and eighth constructors use an initializer_list for the members. The ninth constructor inserts the sequence of element values [First, Last).

All constructors also initialize several stored values. For the copy constructor, the values are obtained from Right. Otherwise:

The minimum number of buckets is the argument Bucket_count, if present; otherwise it is a default value described here as the implementation-defined value N0.

The hash function object is the argument Hash, if present; otherwise it is Hash().

The comparison function object is the argument Comp, if present; otherwise it is Pred().

The allocator object is the argument Al, if present; otherwise, it is Alloc().

Example

// std__unordered_map__unordered_multimap_construct.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

using namespace std;

using  Mymap = unordered_multimap<char, int> ;
int main()
{
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

    // display contents " [c 3] [b 2] [a 1]"
    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    Mymap c2(8,
        hash<char>(),
        equal_to<char>(),
        allocator<pair<const char, int> >());

    c2.insert(Mymap::value_type('d', 4));
    c2.insert(Mymap::value_type('e', 5));
    c2.insert(Mymap::value_type('f', 6));

    // display contents " [f 6] [e 5] [d 4]"
    for (const auto& c : c2) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    Mymap c3(c1.begin(),
        c1.end(),
        8,
        hash<char>(),
        equal_to<char>(),
        allocator<pair<const char, int> >());

    // display contents " [c 3] [b 2] [a 1]"
    for (const auto& c : c3) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    Mymap c4(move(c3));

    // display contents " [c 3] [b 2] [a 1]"
    for (const auto& c : c4) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    // Construct with an initializer_list
    unordered_multimap<int, char> c5({ { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } });
    for (const auto& c : c5) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    // Initializer_list plus size
    unordered_multimap<int, char> c6({ { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } }, 4);
    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    // Initializer_list plus size and hash
    unordered_multimap<int, char, hash<char>> c7(
        { { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } },
        4,
        hash<char>()
    );

    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    // Initializer_list plus size, hash, and key_equal
    unordered_multimap<int, char, hash<char>, equal_to<char>> c8(
        { { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } },
        4,
        hash<char>(),
        equal_to<char>()
    );

    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;

    // Initializer_list plus size, hash, key_equal, and allocator
    unordered_multimap<int, char, hash<char>, equal_to<char>> c9(
        { { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } },
        4,
        hash<char>(),
        equal_to<char>(),
        allocator<pair<const char, int> >()
    );

    for (const auto& c : c1) {
        cout << " [" << c.first << ", " << c.second << "]";
    }
    cout << endl;
}

[a, 1] [b, 2] [c, 3] [d, 4] [e, 5] [f, 6] [a, 1] [b, 2] [c, 3] [a, 1] [b, 2] [c, 3] [5, g] [6, h] [7, i] [8, j] [a, 1] [b, 2] [c, 3] [a, 1] [b, 2] [c, 3] [a, 1] [b, 2] [c, 3] [a, 1] [b, 2] [c, 3] [c, 3] [b, 2] [a, 1]
[f, 6] [e, 5] [d, 4]
[c, 3] [b, 2] [a, 1]
[c, 3] [b, 2] [a, 1]

unordered_multimap::value_type

The type of an element.

typedef std::pair<const Key, Ty> value_type;

Remarks

The type describes an element of the controlled sequence.

Example

// std__unordered_map__unordered_multimap_value_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>

typedef std::unordered_multimap<char, int> Mymap;
int main()
    {
    Mymap c1;

    c1.insert(Mymap::value_type('a', 1));
    c1.insert(Mymap::value_type('b', 2));
    c1.insert(Mymap::value_type('c', 3));

// display contents " [c 3] [b 2] [a 1]"
    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

// add a value and reinspect
    Mymap::key_type key = 'd';
    Mymap::mapped_type mapped = 4;
    Mymap::value_type val = Mymap::value_type(key, mapped);
    c1.insert(val);

    for (Mymap::const_iterator it = c1.begin();
        it != c1.end(); ++it)
        std::cout << " [" << it->first << ", " << it->second << "]";
    std::cout << std::endl;

    return (0);
    }

[c, 3] [b, 2] [a, 1]
[d, 4] [c, 3] [b, 2] [a, 1]

Jaa

unordered_multimap Class

Syntax

Parameters

Members

Remarks

Requirements

unordered_multimap::allocator_type

Remarks

Example

unordered_multimap::begin

Parameters

Remarks

Example

unordered_multimap::bucket

Parameters

Remarks

Example

unordered_multimap::bucket_count

Remarks

Example

unordered_multimap::bucket_size

Parameters

Remarks

Example

unordered_multimap::cbegin

Return Value

Remarks

unordered_multimap::cend

Return Value

Remarks

unordered_multimap::clear

Remarks

Example

unordered_multimap::const_iterator

Remarks

Example

unordered_multimap::const_local_iterator

Remarks

Example

unordered_multimap::const_pointer

Remarks

Example

unordered_multimap::const_reference

Remarks

Example

unordered_multimap::contains

Parameters

Return Value

Remarks

Example

unordered_multimap::count

Parameters

Remarks

Example

unordered_multimap::difference_type

Remarks

Example

unordered_multimap::emplace

Parameters

Return Value

Remarks

unordered_multimap::emplace_hint

Parameters

Return Value

Remarks

unordered_multimap::empty

Remarks

Example

unordered_multimap::end

Parameters

Remarks

Example

unordered_multimap::equal_range

Parameters

Remarks

Example

unordered_multimap::erase

Parameters

Return Value