Class Template unordered_multimap

boost::unordered_multimap — An unordered associative container that associates keys with another value. The same key can be stored multiple times.

Synopsis

// #include <boost/unordered/unordered_map.hpp>

namespace boost {
namespace unordered {

  template<class Key,
           class T,
           class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
  class unordered_multimap {
  public:
    // types
    using key_type             = Key;
    using mapped_type          = T;
    using value_type           = std::pair<const Key, T>;
    using hasher               = Hash;
    using key_equal            = Pred;
    using allocator_type       = Allocator;
    using pointer              = typename std::allocator_traits<Allocator>::pointer;
    using const_pointer        = typename std::allocator_traits<Allocator>::const_pointer;
    using reference            = value_type&;
    using const_reference      = const value_type&;
    using size_type            = std::size_t;
    using difference_type      = std::ptrdiff_t;

    using iterator             = implementation-defined;
    using const_iterator       = implementation-defined;
    using local_iterator       = implementation-defined;
    using const_local_iterator = implementation-defined;
    using node_type            = implementation-defined;

    // construct/copy/destroy
    unordered_multimap();
    explicit unordered_multimap(size_type n,
                                const hasher& hf = hasher(),
                                const key_equal& eql = key_equal(),
                                const allocator_type& a = allocator_type());
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l,
                         size_type n = implementation-defined,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& a = allocator_type());
    unordered_multimap(const unordered_multimap& other);
    unordered_multimap(unordered_multimap&& other);
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a);
    explicit unordered_multimap(const Allocator& a);
    unordered_multimap(const unordered_multimap& other, const Allocator& a);
    unordered_multimap(unordered_multimap&& other, const Allocator& a);
    unordered_multimap(std::initializer_list<value_type> il,
                       size_type n = implementation-defined,
                       const hasher& hf = hasher(),
                       const key_equal& eql = key_equal(),
                       const allocator_type& a = allocator_type());
    unordered_multimap(size_type n, const allocator_type& a);
    unordered_multimap(size_type n, const hasher& hf, const allocator_type& a);
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a);
    template<class InputIterator>
      unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                         const allocator_type& a);
    unordered_multimap(std::initializer_list<value_type> il, const allocator_type& a);
    unordered_multimap(std::initializer_list<value_type> il, size_type n,
                       const allocator_type& a);
    unordered_multimap(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                       const allocator_type& a);
    ~unordered_multimap();
    unordered_multimap& operator=(const unordered_multimap& other);
    unordered_multimap& operator=(unordered_multimap&& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_move_assignable_v<Hash> &&
               boost::is_nothrow_move_assignable_v<Pred>);
    unordered_multimap& operator=(std::initializer_list<value_type> il);
    allocator_type get_allocator() const noexcept;

    // iterators
    iterator       begin() noexcept;
    const_iterator begin() const noexcept;
    iterator       end() noexcept;
    const_iterator end() const noexcept;
    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;

    // capacity
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    // modifiers
    template<class... Args> iterator emplace(Args&&... args);
    template<class... Args> iterator emplace_hint(const_iterator position, Args&&... args);
    iterator insert(const value_type& obj);
    iterator insert(value_type&& obj);
    template<class P> iterator insert(P&& obj);
    iterator insert(const_iterator hint, const value_type& obj);
    iterator insert(const_iterator hint, value_type&& obj);
    template<class P> iterator insert(const_iterator hint, P&& obj);
    template<class InputIterator> void insert(InputIterator first, InputIterator last);
    void insert(std::initializer_list<value_type> il);

    node_type extract(const_iterator position);
    node_type extract(const key_type& k);
    template<class K> node_type extract(K&& k);
    iterator insert(node_type&& nh);
    iterator insert(const_iterator hint, node_type&& nh);

    iterator  erase(iterator position);
    iterator  erase(const_iterator position);
    size_type erase(const key_type& k);
    template<class K> size_type erase(K&& k);
    iterator  erase(const_iterator first, const_iterator last);
    void      quick_erase(const_iterator position);
    void      erase_return_void(const_iterator position);
    void      swap(unordered_multimap& other)
      noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
               boost::is_nothrow_swappable_v<Hash> &&
               boost::is_nothrow_swappable_v<Pred>);
    void      clear() noexcept;

    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
    template<class H2, class P2>
      void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);

    // observers
    hasher hash_function() const;
    key_equal key_eq() const;

    // map operations
    iterator         find(const key_type& k);
    const_iterator   find(const key_type& k) const;
    template<class K>
      iterator       find(const K& k);
    template<class K>
      const_iterator find(const K& k) const;
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      iterator       find(CompatibleKey const& k, CompatibleHash const& hash,
                          CompatiblePredicate const& eq);
    template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
      const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
                          CompatiblePredicate const& eq) const;
    size_type        count(const key_type& k) const;
    template<class K>
      size_type      count(const K& k) const;
    bool             contains(const key_type& k) const;
    template<class K>
      bool           contains(const K& k) const;
    std::pair<iterator, iterator>               equal_range(const key_type& k);
    std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
    template<class K>
      std::pair<iterator, iterator>             equal_range(const K& k);
    template<class K>
      std::pair<const_iterator, const_iterator> equal_range(const K& k) const;

    // bucket interface
    size_type bucket_count() const noexcept;
    size_type max_bucket_count() const noexcept;
    size_type bucket_size(size_type n) const;
    size_type bucket(const key_type& k) const;
    template<class K> size_type bucket(const K& k) const;
    local_iterator begin(size_type n);
    const_local_iterator begin(size_type n) const;
    local_iterator end(size_type n);
    const_local_iterator end(size_type n) const;
    const_local_iterator cbegin(size_type n) const;
    const_local_iterator cend(size_type n) const;

    // hash policy
    float load_factor() const noexcept;
    float max_load_factor() const noexcept;
    void max_load_factor(float z);
    void rehash(size_type n);
    void reserve(size_type n);
  };

  // Deduction Guides
  template<class InputIterator,
           class Hash = boost::hash<iter-key-type<InputIterator>>,
           class Pred = std::equal_to<iter-key-type<InputIterator>>,
           class Allocator = std::allocator<iter-to-alloc-type<InputIterator>>>
    unordered_multimap(InputIterator, InputIterator, typename see below::size_type = see below,
                       Hash = Hash(), Pred = Pred(), Allocator = Allocator())
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                            Pred, Allocator>;

  template<class Key, class T, class Hash = boost::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<const Key, T>>>
    unordered_multimap(std::initializer_list<std::pair<Key, T>>,
                       typename see below::size_type = see below, Hash = Hash(),
                       Pred = Pred(), Allocator = Allocator())
      -> unordered_multimap<Key, T, Hash, Pred, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Allocator)
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            boost::hash<iter-key-type<InputIterator>>,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Allocator>
    unordered_multimap(InputIterator, InputIterator, Allocator)
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>,
                            boost::hash<iter-key-type<InputIterator>>,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class InputIterator, class Hash, class Allocator>
    unordered_multimap(InputIterator, InputIterator, typename see below::size_type, Hash,
                       Allocator)
      -> unordered_multimap<iter-key-type<InputIterator>, iter-mapped-type<InputIterator>, Hash,
                            std::equal_to<iter-key-type<InputIterator>>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_multimap(std::initializer_list<std::pair<Key, T>>, typename see below::size_type,
                       Allocator)
      -> unordered_multimap<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Allocator>
    unordered_multimap(std::initializer_list<std::pair<Key, T>>, Allocator)
      -> unordered_multimap<Key, T, boost::hash<Key>, std::equal_to<Key>, Allocator>;

  template<class Key, class T, class Hash, class Allocator>
    unordered_multimap(std::initializer_list<std::pair<Key, T>>, typename see below::size_type,
                       Hash, Allocator)
      -> unordered_multimap<Key, T, Hash, std::equal_to<Key>, Allocator>;

} // namespace unordered
} // namespace boost

Description

Template Parameters

Key

Key must be Erasable from the container (i.e. allocator_traits can destroy it).

T

T must be Erasable from the container (i.e. allocator_traits can destroy it).

Hash

A unary function object type that acts a hash function for a Key. It takes a single argument of type Key and returns a value of type std::size_t.

Pred

A binary function object that implements an equivalence relation on values of type Key. A binary function object that induces an equivalence relation on values of type Key. It takes two arguments of type Key and returns a value of type bool.

Allocator

An allocator whose value type is the same as the container’s value type. Allocators using fancy pointers are supported.

The elements are organized into buckets. Keys with the same hash code are stored in the same bucket.

The number of buckets can be automatically increased by a call to insert, or as the result of calling rehash.

Configuration macros

BOOST_UNORDERED_ENABLE_SERIALIZATION_COMPATIBILITY_V0

Globally define this macro to support loading of unordered_multimaps saved to a Boost.Serialization archive with a version of Boost prior to Boost 1.84.

Typedefs

typedef implementation-defined iterator;

An iterator whose value type is value_type.

The iterator category is at least a forward iterator.

Convertible to const_iterator.


typedef implementation-defined const_iterator;

A constant iterator whose value type is value_type.

The iterator category is at least a forward iterator.


typedef implementation-defined local_iterator;

An iterator with the same value type, difference type and pointer and reference type as iterator.

A local_iterator object can be used to iterate through a single bucket.


typedef implementation-defined const_local_iterator;

A constant iterator with the same value type, difference type and pointer and reference type as const_iterator.

A const_local_iterator object can be used to iterate through a single bucket.


typedef implementation-defined node_type;

See node_handle_map for details.


Constructors

Default Constructor

unordered_multimap();

Constructs an empty container using hasher() as the hash function, key_equal() as the key equality predicate, allocator_type() as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.


Bucket Count Constructor

explicit unordered_multimap(size_type n,
                            const hasher& hf = hasher(),
                            const key_equal& eql = key_equal(),
                            const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.


Iterator Range Constructor

template<class InputIterator>
unordered_multimap(InputIterator f, InputIterator l,
                   size_type n = implementation-defined,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.


Copy Constructor

unordered_multimap(const unordered_multimap& other);

The copy constructor. Copies the contained elements, hash function, predicate, maximum load factor and allocator.

If Allocator::select_on_container_copy_construction exists and has the right signature, the allocator will be constructed from its result.

Requires:

value_type is copy constructible


Move Constructor

unordered_multimap(unordered_multimap&& other);

The move constructor.

Notes:

This is implemented using Boost.Move.

Requires:

value_type is move-constructible.


Iterator Range Constructor with Allocator

template<class InputIterator>
  unordered_multimap(InputIterator f, InputIterator l, const allocator_type& a);

Constructs an empty container using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.


Allocator Constructor

explicit unordered_multimap(const Allocator& a);

Constructs an empty container, using allocator a.


Copy Constructor with Allocator

unordered_multimap(const unordered_multimap& other, const Allocator& a);

Constructs an container, copying other's contained elements, hash function, predicate, maximum load factor, but using allocator a.


Move Constructor with Allocator

unordered_multimap(unordered_multimap&& other, const Allocator& a);

Construct a container moving other's contained elements, and having the hash function, predicate and maximum load factor, but using allocate a.

Notes:

This is implemented using Boost.Move.

Requires:

value_type is move insertable.


Initializer List Constructor

unordered_multimap(std::initializer_list<value_type> il,
                   size_type n = implementation-defined,
                   const hasher& hf = hasher(),
                   const key_equal& eql = key_equal(),
                   const allocator_type& a = allocator_type());

Constructs an empty container with at least n buckets, using hf as the hash function, eql as the key equality predicate, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

If the defaults are used, hasher, key_equal and allocator_type need to be DefaultConstructible.


Bucket Count Constructor with Allocator

unordered_multimap(size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default hash function and key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0

Requires:

hasher and key_equal need to be DefaultConstructible.


Bucket Count Constructor with Hasher and Allocator

unordered_multimap(size_type n, const hasher& hf, const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, the default key equality predicate, a as the allocator and a maximum load factor of 1.0.

Postconditions:

size() == 0

Requires:

key_equal needs to be DefaultConstructible.


Iterator Range Constructor with Bucket Count and Allocator

template<class InputIterator>
  unordered_multimap(InputIterator f, InputIterator l, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator, with the default hash function and key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

hasher, key_equal need to be DefaultConstructible.


Iterator Range Constructor with Bucket Count and Hasher

template<class InputIterator>
  unordered_multimap(InputIterator f, InputIterator l, size_type n, const hasher& hf,
                     const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator, with the default key equality predicate and a maximum load factor of 1.0 and inserts the elements from [f, l) into it.

Requires:

key_equal needs to be DefaultConstructible.


initializer_list Constructor with Allocator

unordered_multimap(std::initializer_list<value_type> il, const allocator_type& a);

Constructs an empty container using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.


initializer_list Constructor with Bucket Count and Allocator

unordered_multimap(std::initializer_list<value_type> il, size_type n, const allocator_type& a);

Constructs an empty container with at least n buckets, using a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

hasher and key_equal need to be DefaultConstructible.


initializer_list Constructor with Bucket Count and Hasher and Allocator

unordered_multimap(std::initializer_list<value_type> il, size_type n, const hasher& hf,
                   const allocator_type& a);

Constructs an empty container with at least n buckets, using hf as the hash function, a as the allocator and a maximum load factor of 1.0 and inserts the elements from il into it.

Requires:

key_equal needs to be DefaultConstructible.


Destructor

~unordered_multimap();
Note:

The destructor is applied to every element, and all memory is deallocated


Assignment

Copy Assignment

unordered_multimap& operator=(const unordered_multimap& other);

The assignment operator. Copies the contained elements, hash function, predicate and maximum load factor but not the allocator.

If Alloc::propagate_on_container_copy_assignment exists and Alloc::propagate_on_container_copy_assignment::value is true, the allocator is overwritten, if not the copied elements are created using the existing allocator.

Requires:

value_type is copy constructible


Move Assignment

unordered_multimap& operator=(unordered_multimap&& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_move_assignable_v<Hash> &&
           boost::is_nothrow_move_assignable_v<Pred>);

The move assignment operator.

If Alloc::propagate_on_container_move_assignment exists and Alloc::propagate_on_container_move_assignment::value is true, the allocator is overwritten, if not the moved elements are created using the existing allocator.

Requires:

value_type is move constructible.


Initializer List Assignment

unordered_multimap& operator=(std::initializer_list<value_type> il);

Assign from values in initializer list. All existing elements are either overwritten by the new elements or destroyed.

Requires:

value_type is CopyInsertable into the container and CopyAssignable.

Iterators

begin

iterator begin() noexcept;
const_iterator begin() const noexcept;
Returns:

An iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.


end

iterator       end() noexcept;
const_iterator end() const noexcept;
Returns:

An iterator which refers to the past-the-end value for the container.


cbegin

const_iterator cbegin() const noexcept;
Returns:

A const_iterator referring to the first element of the container, or if the container is empty the past-the-end value for the container.


cend

const_iterator cend() const noexcept;
Returns:

A const_iterator which refers to the past-the-end value for the container.


Size and Capacity

empty

[[nodiscard]] bool empty() const noexcept;
Returns:

size() == 0


size

size_type size() const noexcept;
Returns:

std::distance(begin(), end())


max_size

size_type max_size() const noexcept;
Returns:

size() of the largest possible container.


Modifiers

emplace

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

Inserts an object, constructed with the arguments args, in the container.

Requires:

value_type is EmplaceConstructible into X from args.

Returns:

An iterator pointing to the inserted element.

Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.


emplace_hint

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

Inserts an object, constructed with the arguments args, in the container.

position is a suggestion to where the element should be inserted.

Requires:

value_type is EmplaceConstructible into X from args.

Returns:

An iterator pointing to the inserted element.

Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.


Copy Insert

iterator insert(const value_type& obj);

Inserts obj in the container.

Requires:

value_type is CopyInsertable.

Returns:

An iterator pointing to the inserted element.

Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.


Move Insert

iterator insert(value_type&& obj);

Inserts obj in the container.

Requires:

value_type is MoveInsertable.

Returns:

An iterator pointing to the inserted element.

Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.


Emplace Insert

template<class P> iterator insert(P&& obj);

Inserts an element into the container by performing emplace(std::forward<P>(value)).

Only participates in overload resolution if std::is_constructible<value_type, P&&>::value is true.

Returns:

An iterator pointing to the inserted element.


Copy Insert with Hint

iterator insert(const_iterator hint, const value_type& obj);

Inserts obj in the container.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is CopyInsertable.

Returns:

An iterator pointing to the inserted element.

Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.


Move Insert with Hint

iterator insert(const_iterator hint, value_type&& obj);

Inserts obj in the container.

hint is a suggestion to where the element should be inserted.

Requires:

value_type is MoveInsertable.

Returns:

An iterator pointing to the inserted element.

Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.


Emplace Insert with Hint

template<class P> iterator insert(const_iterator hint, P&& obj);

Inserts an element into the container by performing emplace_hint(hint, std::forward<P>(value)).

Only participates in overload resolution if std::is_constructible<value_type, P&&>::value is true.

hint is a suggestion to where the element should be inserted.

Returns:

An iterator pointing to the inserted element.

Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.


Insert Iterator Range

template<class InputIterator> void insert(InputIterator first, InputIterator last);

Inserts a range of elements into the container.

Requires:

value_type is EmplaceConstructible into X from *first.

Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.


Insert Initializer List

void insert(std::initializer_list<value_type> il);

Inserts a range of elements into the container.

Requires:

value_type is CopyInsertable into the container.

Throws:

When inserting a single element, if an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.


Extract by Iterator

node_type extract(const_iterator position);

Removes the element pointed to by position.

Returns:

A node_type owning the element.

Notes:

A node extracted using this method can be inserted into a compatible unordered_map.


Extract by Key

node_type extract(const key_type& k);
template<class K> node_type extract(K&& k);

Removes an element with key equivalent to k.

Returns:

A node_type owning the element if found, otherwise an empty node_type.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

Notes:

A node extracted using this method can be inserted into a compatible unordered_map.

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.


Insert with node_handle

iterator insert(node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.

Returns:

If nh was empty, returns end().

Otherwise returns an iterator pointing to the newly inserted element.

Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This can be used to insert a node extracted from a compatible unordered_map.


Insert with Hint and node_handle

iterator insert(const_iterator hint, node_type&& nh);

If nh is empty, has no effect.

Otherwise inserts the element owned by nh.

hint is a suggestion to where the element should be inserted.

Requires:

nh is empty or nh.get_allocator() is equal to the container’s allocator.

Returns:

If nh was empty, returns end().

Otherwise returns an iterator pointing to the newly inserted element.

Throws:

If an exception is thrown by an operation other than a call to hasher the function has no effect.

Notes:

The standard is fairly vague on the meaning of the hint. But the only practical way to use it, and the only way that Boost.Unordered supports is to point to an existing element with the same key.

Can invalidate iterators, but only if the insert causes the load factor to be greater to or equal to the maximum load factor.

Pointers and references to elements are never invalidated.

This can be used to insert a node extracted from a compatible unordered_map.


Erase by Position

iterator erase(iterator position);
iterator erase(const_iterator position);

Erase the element pointed to by position.

Returns:

The iterator following position before the erasure.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

Notes:

In older versions this could be inefficient because it had to search through several buckets to find the position of the returned iterator. The data structure has been changed so that this is no longer the case, and the alternative erase methods have been deprecated.


Erase by Key

size_type erase(const key_type& k);
template<class K> size_type erase(K&& k);

Erase all elements with key equivalent to k.

Returns:

The number of elements erased.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs and neither iterator nor const_iterator are implicitly convertible from K. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.


Erase Range

iterator erase(const_iterator first, const_iterator last);

Erases the elements in the range from first to last.

Returns:

The iterator following the erased elements - i.e. last.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.


quick_erase

void quick_erase(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.

Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.


erase_return_void

void erase_return_void(const_iterator position);

Erase the element pointed to by position.

Throws:

Only throws an exception if it is thrown by hasher or key_equal.

In this implementation, this overload doesn’t call either function object’s methods so it is no throw, but this might not be true in other implementations.

Notes:

This method was implemented because returning an iterator to the next element from erase was expensive, but the container has been redesigned so that is no longer the case. So this method is now deprecated.


swap

void swap(unordered_multimap& other)
  noexcept(boost::allocator_traits<Allocator>::is_always_equal::value &&
           boost::is_nothrow_swappable_v<Hash> &&
           boost::is_nothrow_swappable_v<Pred>);

Swaps the contents of the container with the parameter.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.

Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.


clear

void clear() noexcept;

Erases all elements in the container.

Postconditions:

size() == 0

Throws:

Never throws an exception.


merge

template<class H2, class P2>
  void merge(unordered_multimap<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_multimap<Key, T, H2, P2, Allocator>&& source);
template<class H2, class P2>
  void merge(unordered_map<Key, T, H2, P2, Allocator>& source);
template<class H2, class P2>
  void merge(unordered_map<Key, T, H2, P2, Allocator>&& source);

Attempt to "merge" two containers by iterating source and extracting all nodes in source and inserting them into *this.

Because source can have a different hash function and key equality predicate, the key of each node in source is rehashed using this->hash_function() and then, if required, compared using this->key_eq().

The behavior of this function is undefined if this->get_allocator() != source.get_allocator().

This function does not copy or move any elements and instead simply relocates the nodes from source into *this.

Notes:
  • Pointers and references to transferred elements remain valid.

  • Invalidates iterators to transferred elements.

  • Invalidates iterators belonging to *this.

  • Iterators to non-transferred elements in source remain valid.


Observers

get_allocator

allocator_type get_allocator() const;

hash_function

hasher hash_function() const;
Returns:

The container’s hash function.


key_eq

key_equal key_eq() const;
Returns:

The container’s key equality predicate


Lookup

find

iterator         find(const key_type& k);
const_iterator   find(const key_type& k) const;
template<class K>
  iterator       find(const K& k);
template<class K>
  const_iterator find(const K& k) const;
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  iterator       find(CompatibleKey const& k, CompatibleHash const& hash,
                      CompatiblePredicate const& eq);
template<typename CompatibleKey, typename CompatibleHash, typename CompatiblePredicate>
  const_iterator find(CompatibleKey const& k, CompatibleHash const& hash,
                      CompatiblePredicate const& eq) const;
Returns:

An iterator pointing to an element with key equivalent to k, or b.end() if no such element exists.

Notes:

The templated overloads containing CompatibleKey, CompatibleHash and CompatiblePredicate are non-standard extensions which allow you to use a compatible hash function and equality predicate for a key of a different type in order to avoid an expensive type cast. In general, its use is not encouraged and instead the K member function templates should be used.

The template<class K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.


count

size_type        count(const key_type& k) const;
template<class K>
  size_type      count(const K& k) const;
Returns:

The number of elements with key equivalent to k.

Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.


contains

bool             contains(const key_type& k) const;
template<class K>
  bool           contains(const K& k) const;
Returns:

A boolean indicating whether or not there is an element with key equal to key in the container

Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.


equal_range

std::pair<iterator, iterator>               equal_range(const key_type& k);
std::pair<const_iterator, const_iterator>   equal_range(const key_type& k) const;
template<class K>
  std::pair<iterator, iterator>             equal_range(const K& k);
template<class K>
  std::pair<const_iterator, const_iterator> equal_range(const K& k) const;
Returns:

A range containing all elements with key equivalent to k. If the container doesn’t contain any such elements, returns std::make_pair(b.end(), b.end()).

Notes:

The template<class K> overloads only participate in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.


Bucket Interface

bucket_count

size_type bucket_count() const noexcept;
Returns:

The number of buckets.


max_bucket_count

size_type max_bucket_count() const noexcept;
Returns:

An upper bound on the number of buckets.


bucket_size

size_type bucket_size(size_type n) const;
Requires:

n < bucket_count()

Returns:

The number of elements in bucket n.


bucket

size_type bucket(const key_type& k) const;
template<class K> size_type bucket(const K& k) const;
Returns:

The index of the bucket which would contain an element with key k.

Postconditions:

The return value is less than bucket_count().

Notes:

The template<class K> overload only participates in overload resolution if Hash::is_transparent and Pred::is_transparent are valid member typedefs. The library assumes that Hash is callable with both K and Key and that Pred is transparent. This enables heterogeneous lookup which avoids the cost of instantiating an instance of the Key type.


begin

local_iterator begin(size_type n);
const_local_iterator begin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).

Returns:

A local iterator pointing the first element in the bucket with index n.


end

local_iterator end(size_type n);
const_local_iterator end(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).

Returns:

A local iterator pointing the 'one past the end' element in the bucket with index n.


cbegin

const_local_iterator cbegin(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).

Returns:

A constant local iterator pointing the first element in the bucket with index n.


cend

const_local_iterator cend(size_type n) const;
Requires:

n shall be in the range [0, bucket_count()).

Returns:

A constant local iterator pointing the 'one past the end' element in the bucket with index n.


Hash Policy

load_factor

float load_factor() const noexcept;
Returns:

The average number of elements per bucket.


max_load_factor

float max_load_factor() const noexcept;
Returns:

Returns the current maximum load factor.


Set max_load_factor

void max_load_factor(float z);
Effects:

Changes the container’s maximum load factor, using z as a hint.


rehash

void rehash(size_type n);

Changes the number of buckets so that there are at least n buckets, and so that the load factor is less than or equal to the maximum load factor. When applicable, this will either grow or shrink the bucket_count() associated with the container.

When size() == 0, rehash(0) will deallocate the underlying buckets array.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.


reserve

void reserve(size_type n);

Equivalent to a.rehash(ceil(n / a.max_load_factor())), or a.rehash(1) if n > 0 and a.max_load_factor() == std::numeric_limits<float>::infinity().

Similar to rehash, this function can be used to grow or shrink the number of buckets in the container.

Invalidates iterators, and changes the order of elements. Pointers and references to elements are not invalidated.

Throws:

The function has no effect if an exception is thrown, unless it is thrown by the container’s hash function or comparison function.


Deduction Guides

A deduction guide will not participate in overload resolution if any of the following are true:

  • It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

  • It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

  • It has a Hash template parameter and an integral type or a type that qualifies as an allocator is deduced for that parameter.

  • It has a Pred template parameter and a type that qualifies as an allocator is deduced for that parameter.

A size_­type parameter type in a deduction guide refers to the size_­type member type of the container type deduced by the deduction guide. Its default value coincides with the default value of the constructor selected.

iter-value-type

template<class InputIterator>
  using iter-value-type =
    typename std::iterator_traits<InputIterator>::value_type; // exposition only

iter-key-type

template<class InputIterator>
  using iter-key-type = std::remove_const_t<
    std::tuple_element_t<0, iter-value-type<InputIterator>>>; // exposition only

iter-mapped-type

template<class InputIterator>
  using iter-mapped-type =
    std::tuple_element_t<1, iter-value-type<InputIterator>>;  // exposition only

iter-to-alloc-type

template<class InputIterator>
  using iter-to-alloc-type = std::pair<
    std::add_const_t<std::tuple_element_t<0, iter-value-type<InputIterator>>>,
    std::tuple_element_t<1, iter-value-type<InputIterator>>>; // exposition only

Equality Comparisons

operator==

template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator==(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_multimap<Key, T, Hash, Pred, Alloc>& y);

Return true if x.size() == y.size() and for every equivalent key group in x, there is a group in y for the same key, which is a permutation (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.


operator!=

template<class Key, class T, class Hash, class Pred, class Alloc>
  bool operator!=(const unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
                  const unordered_multimap<Key, T, Hash, Pred, Alloc>& y);

Return false if x.size() == y.size() and for every equivalent key group in x, there is a group in y for the same key, which is a permutation (using operator== to compare the value types).

Notes:

Behavior is undefined if the two containers don’t have equivalent equality predicates.


Swap

template<class Key, class T, class Hash, class Pred, class Alloc>
  void swap(unordered_multimap<Key, T, Hash, Pred, Alloc>& x,
            unordered_multimap<Key, T, Hash, Pred, Alloc>& y)
    noexcept(noexcept(x.swap(y)));

Swaps the contents of x and y.

If Allocator::propagate_on_container_swap is declared and Allocator::propagate_on_container_swap::value is true then the containers' allocators are swapped. Otherwise, swapping with unequal allocators results in undefined behavior.

Effects:

x.swap(y)

Throws:

Doesn’t throw an exception unless it is thrown by the copy constructor or copy assignment operator of key_equal or hasher.

Notes:

The exception specifications aren’t quite the same as the C++11 standard, as the equality predicate and hash function are swapped using their copy constructors.


erase_if

template<class K, class T, class H, class P, class A, class Predicate>
  typename unordered_multimap<K, T, H, P, A>::size_type
    erase_if(unordered_multimap<K, T, H, P, A>& c, Predicate pred);

Traverses the container c and removes all elements for which the supplied predicate returns true.

Returns:

The number of erased elements.

Notes:

Equivalent to:

auto original_size = c.size();
for (auto i = c.begin(), last = c.end(); i != last; ) {
  if (pred(*i)) {
    i = c.erase(i);
  } else {
    ++i;
  }
}
return original_size - c.size();

Serialization

unordered_multimaps can be archived/retrieved by means of Boost.Serialization using the API provided by this library. Both regular and XML archives are supported.

Saving an unordered_multimap to an archive

Saves all the elements of an unordered_multimap x to an archive (XML archive) ar.

Requires:

std::remove_const<key_type>::type and std::remove_const<mapped_type>::type are serializable (XML serializable), and they do support Boost.Serialization save_construct_data/load_construct_data protocol (automatically suported by DefaultConstructible types).


Loading an unordered_multimap from an archive

Deletes all preexisting elements of an unordered_multimap x and inserts from an archive (XML archive) ar restored copies of the elements of the original unordered_multimap other saved to the storage read by ar.

Requires:

value_type is EmplaceConstructible from (std::remove_const<key_type>::type&&, std::remove_const<mapped_type>::type&&). x.key_equal() is functionally equivalent to other.key_equal().

Note:

If the archive was saved using a release of Boost prior to Boost 1.84, the configuration macro BOOST_UNORDERED_ENABLE_SERIALIZATION_COMPATIBILITY_V0 has to be globally defined for this operation to succeed; otherwise, an exception is thrown.


Saving an iterator/const_iterator to an archive

Saves the positional information of an iterator (const_iterator) it to an archive (XML archive) ar. it can be and end() iterator.

Requires:

The unordered_multimap x pointed to by it has been previously saved to ar, and no modifying operations have been issued on x between saving of x and saving of it.


Loading an iterator/const_iterator from an archive

Makes an iterator (const_iterator) it point to the restored position of the original iterator (const_iterator) saved to the storage read by an archive (XML archive) ar.

Requires:

If x is the unordered_multimap it points to, no modifying operations have been issued on x between loading of x and loading of it.