# <hash\_map>

<head><title>&lt;hash_map&gt;</title></head>

# `<hash_map>`

* * *

Include the [STL](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_index.html#STL)
standard header **`<hash_map>`** to define the
[container](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_cont.html#Containers)
template classes `hash_map` and
`hash_multimap`, and their supporting
templates.

    namespace std {
    template<class Key, class Pr>
        class hash_compare;
    template<class Key, class Ty, class Tr, class Alloc>
        class hash_map;
    template<class Key, class Ty, class Tr, class Alloc>
        class hash_multimap;
    
            // TEMPLATE FUNCTIONS
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator==(
            const hash_map<Key, Ty, Tr, Alloc>& left,
            const hash_map<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator==(
            const hash_multimap<Key, Ty, Tr, Alloc>& left,
            const hash_multimap<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator!=(
            const hash_map<Key, Ty, Tr, Alloc>& left,
            const hash_map<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator!=(
            const hash_multimap<Key, Ty, Tr, Alloc>& left,
            const hash_multimap<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator<(
            const hash_map<Key, Ty, Tr, Alloc>& left,
            const hash_map<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator<(
            const hash_multimap<Key, Ty, Tr, Alloc>& left,
            const hash_multimap<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator>(
            const hash_map<Key, Ty, Tr, Alloc>& left,
            const hash_map<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator>(
            const hash_multimap<Key, Ty, Tr, Alloc>& left,
            const hash_multimap<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator<=(
            const hash_map<Key, Ty, Tr, Alloc>& left,
            const hash_map<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator<=(
            const hash_multimap<Key, Ty, Tr, Alloc>& left,
            const hash_multimap<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator>=(
            const hash_map<Key, Ty, Tr, Alloc>& left,
            const hash_map<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator>=(
            const hash_multimap<Key, Ty, Tr, Alloc>& left,
            const hash_multimap<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        void swap(
            hash_map<Key, Ty, Tr, Alloc>& left,
            hash_map<Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        void swap(
            hash_multimap<Key, Ty, Tr, Alloc>& left,
            hash_multimap<Key, Ty, Tr, Alloc>& right);
    }  // namespace stdCopy to clipboard

## `hash_compare`

    template<class Key,
        class Pr = less<Key> >
        class hash_compare {
        Pr comp;
    public:
        const size_t bucket_size = 4;
        const size_t min_buckets = 8;
        hash_compare();
        hash_compare(Pr pred);
        size_t operator()(const Key& Key) const;
        bool operator()(const Key& keyval1,
            const Key& keyval2) const;
        };Copy to clipboard

The template class describes an object that can be used by
any of the containers
`hash_map`,
`hash_multimap`,
`hash_set`, or
`hash_multiset` as a
**hash traits** object
to order the sequence it controls.
Each of these stores hash traits object of type `Tr`
(a template parameter). You can derive a class from a specialization of
`hash_compare`, to selectively override certain functions
and objects. Or you can supply your own version of this class,
provided you meet certain minimum requirements.
Specifically, for an object `hash_comp` of type
`hash_compare<Key, Pr>`,
the following behavior is required by the above containers:

- For all values `keyval` of type `Key`,
the call `hash_comp(keyval)` serves as a
**hash function**,
which yields a distribution of values of type `size_t`.
The function supplied by `hash_compare` simply
returns `keyval`.
- For any value `keyval1` of
type `Key` that precedes `keyval2` in the sequence
and has the same hash value (value returned by the hash function),
`hash_comp(keyval2, keyval1)` is false. The function must impose a
[strict weak ordering](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#strict%20weak%20ordering)
on values of type `Key`.
The function supplied by `hash_compare` returns
`comp(keyval1, keyval2)` where `comp` is a stored
object of type `Tr` that you can specify when you construct
the object `hash_comp`. For the default `Pr` parameter type
`less<Key>`,
sort keys never decrease in value.
- The integer constant
`bucket_size`
specifies the mean number of elements per ``bucket'' (hash-table
entry) that the container should endeavor not to exceed. It must
be greater than zero. The value supplied by
`hash_compare` is 4.
- The integer constant
`min_buckets`
specifies the minimum number of buckets to maintain in the hash table.
It must be a power of two and greater than zero. The value supplied by
`hash_compare` is 8.

## `hash_map`

* * *

**`allocator_type· begin· bucket· bucket_count· bucket_size· clear· const_iterator· const_local_iterator· const_pointer· const_reference· const_reverse_iterator· count· difference_type· empty· end· equal_range· erase· find· get_allocator· insert· iterator· key_comp· key_compare· key_type· load_factor· local_iterator· lower_bound· mapped_type· max_bucket_count· max_load_factor· max_size· operator[]· hash_map· rbegin· reference· rehash· rend· reverse_iterator· size· size_type· swap· upper_bound· value_comp· value_compare· value_type`**

* * *

    template<class Key, class Ty,
        class Tr = hash_compare<Key, less<Key> >,
        class Alloc = allocator<pair<const Key, Ty> > >
        class hash_map {
    public:
        typedef Key key_type;
        typedef Ty mapped_type;
        typedef Tr key_compare;
        typedef Alloc allocator_type;
    
        typedef pair<const Key, Ty> value_type;
        class value_compare;
        typedef Alloc::pointer pointer;
        typedef Alloc::const_pointer const_pointer;
        typedef Alloc::reference reference;
        typedef Alloc::const_reference const_reference;
    
        typedef T0 iterator;
        typedef T1 const_iterator;
        typedef T2 size_type;
        typedef T3 difference_type;
        typedef T4 local_iterator;
        typedef T5 const_local_iterator;
        typedef reverse_iterator<const_iterator>
            const_reverse_iterator;
        typedef reverse_iterator<iterator> reverse_iterator;
    
        hash_map();
        explicit hash_map(const Tr& traits);
        hash_map(const Tr& traits, const Alloc& al);
        hash_map(const hash_map& right);
        template<class InIt>
            hash_map(InIt first, InIt last);
        template<class InIt>
            hash_map(InIt first, InIt last,
                const Tr& traits);
        template<class InIt>
            hash_map(InIt first, InIt last,
                const Tr& traits, const Alloc& al);
    
        iterator begin();
        const_iterator begin() const;
        local_iterator begin(size_type nbucket);
        const_local_iterator begin(size_type nbucket) const;
    
        iterator end();
        const_iterator end() const;
        local_iterator end(size_type nbucket);
        const_local_iterator end(size_type nbucket) const;
    
        reverse_iterator rbegin();
        const_reverse_iterator rbegin() const;
        reverse_iterator rend();
        const_reverse_iterator rend() const;
    
        size_type size() const;
        size_type max_size() const;
        bool empty() const;
    
        size_type bucket_count() const;
        size_type max_bucket_count() const;
        size_type bucket(const Key& keyval) const;
        size_type bucket_size(size_type nbucket) const;
    
        key_compare key_comp() const;
        value_compare value_comp() const;
        Alloc get_allocator() const;
    
        float load_factor() const;
        float max_load_factor() const;
        void max_load_factor(float factor);
        void rehash(size_type nbuckets);
    
       mapped_type& operator[](const Key& keyval);
    
        iterator insert(const value_type& val);
        iterator insert(iterator where, const value_type& val);
        template<class InIt>
            void insert(InIt first, InIt last);
    
        iterator erase(iterator where);
        iterator erase(iterator first, iterator last);
        size_type erase(const Key& keyval);
        void clear();
    
        void swap(hash_map& right);
    
        iterator find(const Key& keyval);
        const_iterator find(const Key& keyval) const;
        size_type count(const Key& keyval) const;
        iterator lower_bound(const Key& keyval);
        const_iterator lower_bound(const Key& keyval) const;
        iterator upper_bound(const Key& keyval);
        const_iterator upper_bound(const Key& keyval) const;
        pair<iterator, iterator> equal_range(const Key& keyval);
        pair<const_iterator, const_iterator>
            equal_range(const Key& keyval) const;
        };Copy to clipboard

The template class describes an object that controls a
varying-length sequence of elements of type
`pair<const Key, Ty>`.
The sequence is
[ordered by](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#sequence%20ordering) the
[hash traits](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20traits) object
`Tr`, which includes two functions:

- a one-operand **[hash function](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20function)**,
which partitions the sequence into an ordered set of subsequences called
**buckets**.
- a two-operand **comparison function** for imposing a
[strict weak ordering](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#strict%20weak%20ordering)
within each bucket.

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.

The object orders the sequence it controls by calling a stored
[hash traits](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20traits) object of type `Tr`.
You access this stored object by calling the member function
`key_comp()`.
Such a traits object must behave the same as an object of class
`hash_compare<Key, Pr>`.
Specifically, for all values `keyval` of type `Key`,
the call `key_comp()(keyval)` yields a distribution
of values of type `size_t`.
Moreover, class `Tr` imposes a
[strict weak ordering](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#strict%20weak%20ordering)
on sort keys of type `Key`.
For any element `X` that precedes
`Y` in the sequence and has the same hash value,
`key_comp()(Y.first,
X.first)` is false. (For the default function object
`less<Key>`,
sort keys never decrease in value.)
Unlike template class `hash_multimap`,
an object of template class `hash_map` does not ensure that
`key_comp()(X.first, Y.first)` is true.
(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 `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](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#equivalent%20ordering)
are adjacent in the controlled sequence.

The object allocates and frees storage for the sequence it controls
through a stored [allocator object](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_memory.html#allocator%20object)
of class `Alloc`. Such an allocator object must have
the same external interface as an object of template class
[`allocator`](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_memory.html#allocator).
Note that the stored allocator object is *not* copied when the container
object is assigned.

### `hash_map::allocator_type`

    typedef Alloc allocator_type;Copy to clipboard

The type is a synonym for the template parameter `Alloc`.

### `hash_map::begin`

    iterator begin();
        const_iterator begin() const;
        local_iterator begin(size_type nbucket);
        const_local_iterator begin(size_type nbucket) const;Copy to clipboard

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).

### `hash_map::bucket`

    size_type bucket(const Key& keyval) const;Copy to clipboard

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

### `hash_map::bucke_countt`

    size_type bucket_count() const;Copy to clipboard

The member function returns the current number of buckets.

### `hash_map::bucket_size`

    size_type bucket_size(size_type nbucket) const;Copy to clipboard

The member functions returns the size of bucket number `nbucket`.

### `hash_map::clear`

    void clear();Copy to clipboard

The member function calls
`erase(
begin(),
end())`.

### `hash_map::const_iterator`

    typedef T1 const_iterator;Copy to clipboard

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

### `hash_map::const_local_iterator`

    typedef T5 const_local_iterator;Copy to clipboard

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`.

### `hash_map::const_pointer`

    typedef Alloc::const_pointer const_pointer;Copy to clipboard

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

### `hash_map::const_reference`

    typedef Alloc::const_reference const_reference;Copy to clipboard

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

### `hash_map::const_reverse_iterator`

    typedef reverse_iterator<const_iterator>
        const_reverse_iterator;Copy to clipboard

The type describes an object that can serve as a constant reverse
bidirectional iterator for the controlled sequence.

### `hash_map::count`

    size_type count(const Key& keyval) const;Copy to clipboard

The member function returns the number of elements in the range
`[lower_bound(keyval),
upper_bound(keyval)).`

### `hash_map::difference_type`

    typedef T3 difference_type;Copy to clipboard

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`.

### `hash_map::empty`

    bool empty() const;Copy to clipboard

The member function returns true for an empty controlled sequence.

### `hash_map::end`

    iterator end();
        const_iterator end() const;
        local_iterator end(size_type nbucket);
        const_local_iterator end(size_type nbucket) const;Copy to clipboard

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`.

### `hash_map::equal_range`

    pair<iterator, iterator> equal_range(const Key& keyval);
    pair<const_iterator, const_iterator>
        equal_range(const Key& keyval) const;Copy to clipboard

The member function returns a pair of iterators `X`
such that `X.first ==
lower_bound(keyval)`
and `X.second ==
upper_bound(keyval)`.

### `hash_map::erase`

    iterator erase(iterator where);
    iterator erase(iterator first, iterator last);
    size_type erase(const Key& keyval);Copy to clipboard

The first member function removes the element of the controlled
sequence pointed to by `where`.
The second member function removes the elements
in the range `[first, last)`.
Both return an iterator that designates the first element remaining
beyond any elements removed, or
`end()` if no such element exists.

The third member removes
the elements with sort keys in the range
`[lower_bound(keyval),
upper_bound(keyval)).`
It returns the number of elements it removes.

The member functions never throw an exception.

### `hash_map::find`

    iterator find(const Key& keyval);
    const_iterator find(const Key& keyval) const;Copy to clipboard

The member function returns
`lower_bound(keyval)`.

### `hash_map::get_allocator`

    Alloc get_allocator() const;Copy to clipboard

The member function returns the stored
[allocator object](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_memory.html#allocator%20object).

### `hash_map::hash_map`

    hash_map();
    explicit hash_map(const Tr& traits);
    hash_map(const Tr& traits, const Alloc& al);
    hash_map(const hash_map& right);
    template<class InIt>
        hash_map(InIt first, InIt last);
    template<class InIt>
        hash_map(InIt first, InIt last,
            const Tr& traits);
    template<class InIt>
        hash_map(InIt first, InIt last,
            const Tr& traits, const Alloc& al);Copy to clipboard

All constructors store an
[allocator object](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_memory.html#allocator%20object) and
initialize the controlled sequence. The allocator object is the argument
`al`, if present. For the copy constructor, it is
`right.get_allocator()`.
Otherwise, it is `Alloc()`.

All constructors also store a
[hash traits](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20traits) object that can later
be returned by calling
`key_comp()`.
The hash traits object is the argument `traits`, if present.
For the copy constructor, it is
`right.key_comp()`).
Otherwise, it is `Tr()`.

The first three constructors specify an
empty initial controlled sequence. The fourth constructor specifies
a copy of the sequence controlled by `right`.
The last three constructors specify the sequence of element values
`[first, last)`.

### `hash_map::insert`

    iterator insert(const value_type& val);
    iterator insert(iterator where, const value_type& val);
    template<class InIt>
        void insert(InIt first, InIt last);Copy to clipboard

The first member function inserts the element `val`
in the controlled sequence, then returns
the iterator that designates the inserted element.
The second member function returns `insert(val)`,
using `where` as a starting place within the controlled
sequence to search for the insertion point. (Insertion can
possibly occur somewhat faster, if the
insertion point immediately precedes or follows `where`.)
The third member function
inserts the sequence of element values,
for each `where` in the range `[first, last)`,
by calling `insert(*where)`.

If an exception is thrown during the
insertion of a single element, the container is left unaltered
and the exception is rethrown.
If an exception is thrown during the
insertion of multiple elements, the container is left in a stable
but unspecified state and the exception is rethrown.

### `hash_map::iterator`

    typedef T0 iterator;Copy to clipboard

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

### `hash_map::key_comp`

    key_compare key_comp() const;Copy to clipboard

The member function returns the stored
[hash traits](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20traits) object that
determines the order of elements in the controlled sequence.
In particular, the stored object defines the member function:

    bool operator()(const Key& left, const Key& right);Copy to clipboard

which returns true if `left` strictly
precedes `right` in the sort order.

### `hash_map::key_compare`

    typedef Tr key_compare;Copy to clipboard

The type describes a traits object that behaves much like an object of class
`hash_compare<Key, Pr>`.
In particular, it can compare two
sort keys to determine the relative order of two
elements in the controlled sequence.

### `hash_map::key_type`

    typedef Key key_type;Copy to clipboard

The type describes the sort key object stored in each
element of the controlled sequence.

### `hash_map::load_factor`

    float load_factor() const;Copy to clipboard

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

### `hash_map::local_iterator`

    typedef T4 local_iterator;Copy to clipboard

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`.

### `hash_map::lower_bound`

    iterator lower_bound(const Key& keyval);
    const_iterator lower_bound(const Key& keyval) const;Copy to clipboard

The member function returns an iterator that designates the
earliest element `X` in the controlled sequence for which
`key_comp()(X.
first, keyval)` is
false.
 If no such element exists, the function returns
`end()`.

### `hash_map::mapped_type`

    typedef Ty mapped_type;Copy to clipboard

The type is a synonym for the template parameter `Ty`.

### `hash_map::max_bucket_count`

    size_type max_bucket_count() const;Copy to clipboard

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

### `hash_map::max_load_factort`

    float max_load_factor() const;
    void max_load_factor(float factor);Copy to clipboard

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

### `hash_map::max_size`

    size_type max_size() const;Copy to clipboard

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

### `hash_map::operator[]`

    mapped_type& operator[](const Key& keyval);Copy to clipboard

The member function determines the iterator `where`
as the return value of
`insert(
value_type(keyval, Ty())`.
(It inserts an element with the specified key if no such element
exists.) It then returns a reference to
`(*where).second`.

### `hash_map::pointer`

    typedef Alloc::pointer pointer;Copy to clipboard

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

### `hash_map::rbegin`

    const_reverse_iterator rbegin() const;
    reverse_iterator rbegin();Copy to clipboard

The member function returns a reverse bidirectional
iterator that points just
beyond the end of the controlled sequence. Hence, it designates the
beginning of the reverse sequence.

### `hash_map::reference`

    typedef Alloc::reference reference;Copy to clipboard

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

### `hash_map::rehash`

    void rehash(size_type nbuckets);Copy to clipboard

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

### `hash_map::rend`

    const_reverse_iterator rend() const;
    reverse_iterator rend();Copy to clipboard

The member function returns a reverse bidirectional
iterator that points at the
first element of the sequence (or just beyond the end of an empty
sequence). Hence, it designates the end of the reverse sequence.

### `hash_map::reverse_iterator`

    typedef reverse_iterator<iterator> reverse_iterator;Copy to clipboard

The type describes an object that can serve as a reverse
bidirectional iterator for the controlled sequence.

### `hash_map::size`

    size_type size() const;Copy to clipboard

The member function returns the length of the controlled sequence.

### `hash_map::size_type`

    typedef T2 size_type;Copy to clipboard

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`.

### `hash_map::swap`

    void swap(hash_map& right);Copy to clipboard

The member function swaps the controlled sequences between
`*this` and `right`. If
`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.

### `hash_map::upper_bound`

    iterator upper_bound(const Key& keyval);
    const_iterator upper_bound(const Key& keyval) const;Copy to clipboard

The member function returns an iterator
just beyond the iterator that designates the
latest element `X` in the controlled sequence
for which `X.first` has
[equivalent ordering](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#equivalent%20ordering)
to `keyval`.
If no such element exists, the function returns
`end()`.

### `hash_map::value_comp`

    value_compare value_comp() const;Copy to clipboard

The member function returns a function object that
determines the order of elements in the controlled sequence.

### `hash_map::value_compare`

    class value_compare
        : public binary_function<value_type, value_type,
            bool> {
    public:
        bool operator()(const value_type& left,
            const value_type& right) const
            {return (comp(left.first, right.first)); }
    protected:
        value_compare(key_compare pr)
            : comp(pr) {}
        key_compare comp;
        };Copy to clipboard

The type describes a function object that can compare the
sort keys in two elements to determine their relative order
in the controlled sequence. The function object stores an object
**`comp`**
of type `key_type`.
The member function **`operator()`** uses this
object to compare the sort-key components of two element.

### `hash_map::value_type`

    typedef pair<const Key, Ty> value_type;
    Copy to clipboard

The type describes an element of the controlled sequence.

## `hash_multimap`

* * *

**`allocator_type· begin· bucket· bucket_count· bucket_size· clear· const_iterator· const_local_iterator· const_pointer· const_reference· const_reverse_iterator· count· difference_type· empty· end· equal_range· erase· find· get_allocator· insert· iterator· key_comp· key_compare· key_type· load_factor· local_iterator· lower_bound· mapped_type· max_bucket_count· max_load_factor· max_size· hash_multimap· rbegin· reference· rehash· rend· reverse_iterator· size· size_type· swap· upper_bound· value_comp· value_compare· value_type`**

* * *

    template<class Key, class Ty,
        class Tr = hash_compare<Key, less<Key> >,
        class Alloc = allocator<pair<const Key, Ty> > >
        class hash_multimap {
    public:
        typedef Key key_type;
        typedef Ty mapped_type;
        typedef Tr key_compare;
        typedef Alloc allocator_type;
    
        typedef pair<const Key, Ty> value_type;
        class value_compare;
        typedef Alloc::pointer pointer;
        typedef Alloc::const_pointer const_pointer;
        typedef Alloc::reference reference;
        typedef Alloc::const_reference const_reference;
    
        typedef T0 iterator;
        typedef T1 const_iterator;
        typedef T2 size_type;
        typedef T3 difference_type;
        typedef T4 local_iterator;
        typedef T5 const_local_iterator;
        typedef reverse_iterator<const_iterator>
            const_reverse_iterator;
        typedef reverse_iterator<iterator> reverse_iterator;
    
        hash_multimap();
        explicit hash_multimap(const Tr& traits);
        hash_multimap(const Tr& traits, const Alloc& al);
        hash_multimap(const hash_multimap& right);
        template<class InIt>
            hash_multimap(InIt first, InIt last);
        template<class InIt>
            hash_multimap(InIt first, InIt last,
                const Tr& traits);
        template<class InIt>
            hash_multimap(InIt first, InIt last,
                const Tr& traits, const Alloc& al);
    
        iterator begin();
        const_iterator begin() const;
        local_iterator begin(size_type nbucket);
        const_local_iterator begin(size_type nbucket) const;
    
        iterator end();
        const_iterator end() const;
        local_iterator end(size_type nbucket);
        const_local_iterator end(size_type nbucket) const;
    
        reverse_iterator rbegin();
        const_reverse_iterator rbegin() const;
        reverse_iterator rend();
        const_reverse_iterator rend() const;
    
        size_type size() const;
        size_type max_size() const;
        bool empty() const;
    
        size_type bucket_count() const;
        size_type max_bucket_count() const;
        size_type bucket(const Key& keyval) const;
        size_type bucket_size(size_type nbucket) const;
    
        key_compare key_comp() const;
        value_compare value_comp() const;
        Alloc get_allocator() const;
    
        float load_factor() const;
        float max_load_factor() const;
        void max_load_factor(float factor);
        void rehash(size_type nbuckets);
    
        iterator insert(const value_type& val);
        iterator insert(iterator where, const value_type& val);
        template<class InIt>
            void insert(InIt first, InIt last);
    
        iterator erase(iterator where);
        iterator erase(iterator first, iterator last);
        size_type erase(const Key& keyval);
        void clear();
    
        void swap(hash_multimap& right);
    
        iterator find(const Key& keyval);
        const_iterator find(const Key& keyval) const;
        size_type count(const Key& keyval) const;
        iterator lower_bound(const Key& keyval);
        const_iterator lower_bound(const Key& keyval) const;
        iterator upper_bound(const Key& keyval);
        const_iterator upper_bound(const Key& keyval) const;
        pair<iterator, iterator> equal_range(const Key& keyval);
        pair<const_iterator, const_iterator>
            equal_range(const Key& keyval) const;
        };Copy to clipboard

The template class describes an object that controls a
varying-length sequence of elements of type
`pair<const Key, Ty>`.
The sequence is
[ordered by](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#sequence%20ordering) the
[hash traits](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20traits) object
`Tr`, which includes two functions:

- a one-operand **[hash function](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20function)**,
which partitions the sequence into an ordered set of subsequences called
**buckets**.
- a two-operand **comparison function** for imposing a
[strict weak ordering](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#strict%20weak%20ordering)
within each bucket.

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.

The object orders the sequence it controls by calling a stored
[hash traits](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20traits) object of type `Tr`.
You access this stored object by calling the member function
`key_comp()`.
Such a traits object must behave the same as an object of class
`hash_compare<Key, Pr>`.
Specifically, for all values `keyval` of type `Key`,
the call `key_comp()(keyval)` yields a distribution
of values of type `size_t`.
Moreover, class `Tr` imposes a
[strict weak ordering](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#strict%20weak%20ordering)
on sort keys of type `Key`.
For any element `X` that precedes
`Y` in the sequence and has the same hash value,
`key_comp()(Y.first,
X.first)` is false. (For the default function object
`less<Key>`,
sort keys never decrease in value.)
Unlike template class `hash_map`,
an object of template class `hash_multimap` does not ensure that
`key_comp()(X.first, Y.first)` is true.
(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 `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](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#equivalent%20ordering)
are adjacent in the controlled sequence.

The object allocates and frees storage for the sequence it controls
through a stored [allocator object](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_memory.html#allocator%20object)
of class `Alloc`. Such an allocator object must have
the same external interface as an object of template class
[`allocator`](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_memory.html#allocator).
Note that the stored allocator object is *not* copied when the container
object is assigned.

### `hash_multimap::allocator_type`

    typedef Alloc allocator_type;Copy to clipboard

The type is a synonym for the template parameter `Alloc`.

### `hash_multimap::begin`

    iterator begin();
        const_iterator begin() const;
        local_iterator begin(size_type nbucket);
        const_local_iterator begin(size_type nbucket) const;Copy to clipboard

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).

### `hash_multimap::bucket`

    size_type bucket(const Key& keyval) const;Copy to clipboard

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

### `hash_multimap::bucke_countt`

    size_type bucket_count() const;Copy to clipboard

The member function returns the current number of buckets.

### `hash_multimap::bucket_size`

    size_type bucket_size(size_type nbucket) const;Copy to clipboard

The member functions returns the size of bucket number `nbucket`.

### `hash_multimap::clear`

    void clear();Copy to clipboard

The member function calls
`erase(
begin(),
end())`.

### `hash_multimap::const_iterator`

    typedef T1 const_iterator;Copy to clipboard

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

### `hash_multimap::const_local_iterator`

    typedef T5 const_local_iterator;Copy to clipboard

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`.

### `hash_multimap::const_pointer`

    typedef Alloc::const_pointer const_pointer;Copy to clipboard

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

### `hash_multimap::const_reference`

    typedef Alloc::const_reference const_reference;Copy to clipboard

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

### `hash_multimap::const_reverse_iterator`

    typedef reverse_iterator<const_iterator>
        const_reverse_iterator;Copy to clipboard

The type describes an object that can serve as a constant reverse
bidirectional iterator for the controlled sequence.

### `hash_multimap::count`

    size_type count(const Key& keyval) const;Copy to clipboard

The member function returns the number of elements in the range
`[lower_bound(keyval),
upper_bound(keyval)).`

### `hash_multimap::difference_type`

    typedef T3 difference_type;Copy to clipboard

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`.

### `hash_multimap::empty`

    bool empty() const;Copy to clipboard

The member function returns true for an empty controlled sequence.

### `hash_multimap::end`

    iterator end();
        const_iterator end() const;
        local_iterator end(size_type nbucket);
        const_local_iterator end(size_type nbucket) const;Copy to clipboard

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`.

### `hash_multimap::equal_range`

    pair<iterator, iterator> equal_range(const Key& keyval);
    pair<const_iterator, const_iterator>
        equal_range(const Key& keyval) const;Copy to clipboard

The member function returns a pair of iterators `X`
such that `X.first ==
lower_bound(keyval)`
and `X.second ==
upper_bound(keyval)`.

### `hash_multimap::erase`

    iterator erase(iterator where);
    iterator erase(iterator first, iterator last);
    size_type erase(const Key& keyval);Copy to clipboard

The first member function removes the element of the controlled
sequence pointed to by `where`.
The second member function removes the elements
in the range `[first, last)`.
Both return an iterator that designates the first element remaining
beyond any elements removed, or
`end()` if no such element exists.

The third member removes
the elements with sort keys in the range
`[lower_bound(keyval),
upper_bound(keyval)).`
It returns the number of elements it removes.

The member functions never throw an exception.

### `hash_multimap::find`

    iterator find(const Key& keyval);
    const_iterator find(const Key& keyval) const;Copy to clipboard

The member function returns
`lower_bound(keyval)`.

### `hash_multimap::get_allocator`

    Alloc get_allocator() const;Copy to clipboard

The member function returns the stored
[allocator object](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_memory.html#allocator%20object).

### `hash_multimap::hash_multimap`

    hash_multimap();
    explicit hash_multimap(const Tr& traits);
    hash_multimap(const Tr& traits, const Alloc& al);
    hash_multimap(const hash_multimap& right);
    template<class InIt>
        hash_multimap(InIt first, InIt last);
    template<class InIt>
        hash_multimap(InIt first, InIt last,
            const Tr& traits);
    template<class InIt>
        hash_multimap(InIt first, InIt last,
            const Tr& traits, const Alloc& al);Copy to clipboard

All constructors store an
[allocator object](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_memory.html#allocator%20object) and
initialize the controlled sequence. The allocator object is the argument
`al`, if present. For the copy constructor, it is
`right.get_allocator()`.
Otherwise, it is `Alloc()`.

All constructors also store a
[hash traits](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20traits) object that can later
be returned by calling
`key_comp()`.
The hash traits object is the argument `traits`, if present.
For the copy constructor, it is
`right.key_comp()`).
Otherwise, it is `Tr()`.

The first three constructors specify an
empty initial controlled sequence. The fourth constructor specifies
a copy of the sequence controlled by `right`.
The last three constructors specify the sequence of element values
`[first, last)`.

### `hash_multimap::insert`

    iterator insert(const value_type& val);
    iterator insert(iterator where, const value_type& val);
    template<class InIt>
        void insert(InIt first, InIt last);Copy to clipboard

The first member function inserts the element `val`
in the controlled sequence, then returns
the iterator that designates the inserted element.
The second member function returns `insert(val)`,
using `where` as a starting place within the controlled
sequence to search for the insertion point. (Insertion can
possibly occur somewhat faster, if the
insertion point immediately precedes or follows `where`.)
The third member function
inserts the sequence of element values,
for each `where` in the range `[first, last)`,
by calling `insert(*where)`.

If an exception is thrown during the
insertion of a single element, the container is left unaltered
and the exception is rethrown.
If an exception is thrown during the
insertion of multiple elements, the container is left in a stable
but unspecified state and the exception is rethrown.

### `hash_multimap::iterator`

    typedef T0 iterator;Copy to clipboard

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

### `hash_multimap::key_comp`

    key_compare key_comp() const;Copy to clipboard

The member function returns the stored
[hash traits](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash%20traits) object that
determines the order of elements in the controlled sequence.
In particular, the stored object defines the member function:

    bool operator()(const Key& left, const Key& right);Copy to clipboard

which returns true if `left` strictly
precedes `right` in the sort order.

### `hash_multimap::key_compare`

    typedef Tr key_compare;Copy to clipboard

The type describes a traits object that behaves much like an object of class
`hash_compare<Key, Pr>`.
In particular, it can compare two
sort keys to determine the relative order of two
elements in the controlled sequence.

### `hash_multimap::key_type`

    typedef Key key_type;Copy to clipboard

The type describes the sort key object stored in each
element of the controlled sequence.

### `hash_multimap::load_factor`

    float load_factor() const;Copy to clipboard

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

### `hash_multimap::local_iterator`

    typedef T4 local_iterator;Copy to clipboard

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`.

### `hash_multimap::lower_bound`

    iterator lower_bound(const Key& keyval);
    const_iterator lower_bound(const Key& keyval) const;Copy to clipboard

The member function returns an iterator that designates the
earliest element `X` in the controlled sequence for which
`key_comp()(X.
first, keyval)` is
false.
 If no such element exists, the function returns
`end()`.

### `hash_multimap::mapped_type`

    typedef Ty mapped_type;Copy to clipboard

The type is a synonym for the template parameter `Ty`.

### `hash_multimap::max_bucket_count`

    size_type max_bucket_count() const;Copy to clipboard

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

### `hash_multimap::max_load_factort`

    float max_load_factor() const;
    void max_load_factor(float factor);Copy to clipboard

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

### `hash_multimap::max_size`

    size_type max_size() const;Copy to clipboard

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

### `hash_multimap::pointer`

    typedef Alloc::pointer pointer;Copy to clipboard

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

### `hash_multimap::rbegin`

    const_reverse_iterator rbegin() const;
    reverse_iterator rbegin();Copy to clipboard

The member function returns a reverse bidirectional
iterator that points just
beyond the end of the controlled sequence. Hence, it designates the
beginning of the reverse sequence.

### `hash_multimap::reference`

    typedef Alloc::reference reference;Copy to clipboard

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

### `hash_multimap::rehash`

    void rehash(size_type nbuckets);Copy to clipboard

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

### `hash_multimap::rend`

    const_reverse_iterator rend() const;
    reverse_iterator rend();Copy to clipboard

The member function returns a reverse bidirectional
iterator that points at the
first element of the sequence (or just beyond the end of an empty
sequence). Hence, it designates the end of the reverse sequence.

### `hash_multimap::reverse_iterator`

    typedef reverse_iterator<iterator> reverse_iterator;Copy to clipboard

The type describes an object that can serve as a reverse
bidirectional iterator for the controlled sequence.

### `hash_multimap::size`

    size_type size() const;Copy to clipboard

The member function returns the length of the controlled sequence.

### `hash_multimap::size_type`

    typedef T2 size_type;Copy to clipboard

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`.

### `hash_multimap::swap`

    void swap(hash_multimap& right);Copy to clipboard

The member function swaps the controlled sequences between
`*this` and `right`. If
`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.

### `hash_multimap::upper_bound`

    iterator upper_bound(const Key& keyval);
    const_iterator upper_bound(const Key& keyval) const;Copy to clipboard

The member function returns an iterator
just beyond the iterator that designates the
latest element `X` in the controlled sequence
for which `X.first` has
[equivalent ordering](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_lib_stl.html#equivalent%20ordering)
to `keyval`.
If no such element exists, the function returns
`end()`.

### `hash_multimap::value_comp`

    value_compare value_comp() const;Copy to clipboard

The member function returns a function object that
determines the order of elements in the controlled sequence.

### `hash_multimap::value_compare`

    class value_compare
        : public binary_function<value_type, value_type,
            bool> {
    public:
        bool operator()(const value_type& left,
            const value_type& right) const
            {return (comp(left.first, right.first)); }
    protected:
        value_compare(key_compare pr)
            : comp(pr) {}
        key_compare comp;
        };Copy to clipboard

The type describes a function object that can compare the
sort keys in two elements to determine their relative order
in the controlled sequence. The function object stores an object
**`comp`**
of type `key_type`.
The member function **`operator()`** uses this
object to compare the sort-key components of two element.

### `hash_multimap::value_type`

    typedef pair<const Key, Ty> value_type;
    Copy to clipboard

The type describes an element of the controlled sequence.

## `operator!=`

    template<class Key, class Ty, class Tr, class Alloc>
        bool operator!=(
            const hash_map <Key, Ty, Tr, Alloc>& left,
            const hash_map <Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator!=(
            const hash_multimap <Key, Ty, Tr, Alloc>& left,
            const hash_multimap <Key, Ty, Tr, Alloc>& right);Copy to clipboard

The template function returns `!(left == right)`.

## `operator==`

    template<class Key, class Ty, class Tr, class Alloc>
        bool operator==(
            const hash_map <Key, Ty, Tr, Alloc>& left,
            const hash_map <Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator==(
            const hash_multimap <Key, Ty, Tr, Alloc>& left,
            const hash_multimap <Key, Ty, Tr, Alloc>& right);Copy to clipboard

The first template function overloads `operator==`
to compare two objects of template class
[`hash_map`](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash_map).
The second template function overloads `operator==`
to compare two objects of template class
[`hash_multimap`](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash_multimap).
Both functions return
`left.size() == right.size() &&
equal(left.
begin(), left.
end(), right.begin())`.

## `operator<`

    template<class Key, class Ty, class Tr, class Alloc>
        bool operator<(
            const hash_map <Key, Ty, Tr, Alloc>& left,
            const hash_map <Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator<(
            const hash_multimap <Key, Ty, Tr, Alloc>& left,
            const hash_multimap <Key, Ty, Tr, Alloc>& right);Copy to clipboard

The first template function overloads `operator<`
to compare two objects of template class
[`hash_map`](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash_map).
The second template function overloads `operator<`
to compare two objects of template class
[`hash_multimap`](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_hash_map.html#hash_multimap).
Both functions return
`lexicographical_compare(left.
begin(), left.
end(), right.begin(), right.end(),
left.value_comp())`.

## `operator<=`

    template<class Key, class Ty, class Tr, class Alloc>
        bool operator<=(
            const hash_map <Key, Ty, Tr, Alloc>& left,
            const hash_map <Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator<=(
            const hash_multimap <Key, Ty, Tr, Alloc>& left,
            const hash_multimap <Key, Ty, Tr, Alloc>& right);Copy to clipboard

The template function returns `!(right < left)`.

## `operator>`

    template<class Key, class Ty, class Tr, class Alloc>
        bool operator>(
            const hash_map <Key, Ty, Tr, Alloc>& left,
            const hash_map <Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator>(
            const hash_multimap <Key, Ty, Tr, Alloc>& left,
            const hash_multimap <Key, Ty, Tr, Alloc>& right);Copy to clipboard

The template function returns `right < left`.

## `operator>=`

    template<class Key, class Ty, class Tr, class Alloc>
        bool operator>=(
            const hash_map <Key, Ty, Tr, Alloc>& left,
            const hash_map <Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        bool operator!=(
            const hash_multimap <Key, Ty, Tr, Alloc>& left,
            const hash_multimap <Key, Ty, Tr, Alloc>& right);Copy to clipboard

The template function returns `!(left < right)`.

## `swap`

    template<class Key, class Ty, class Tr, class Alloc>
        void swap(
            hash_map <Key, Ty, Tr, Alloc>& left,
            hash_map <Key, Ty, Tr, Alloc>& right);
    template<class Key, class Ty, class Tr, class Alloc>
        void swap(
            hash_multimap <Key, Ty, Tr, Alloc>& left,
            hash_multimap <Key, Ty, Tr, Alloc>& right);Copy to clipboard

The template function executes
`left.swap(right)`.

* * *

See also the
**[Table of Contents](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_index.html#Table%20of%20Contents)** and the
**[Index](https://docs.qualcomm.com/doc/80-N3040-23/topic/_cpp_index.html)**.

*[Copyright](https://docs.qualcomm.com/doc/80-N3040-23/topic/cpp_crit_pjp.html) © 1992-2006by P.J. Plauger. All rights reserved.*

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Last Published: Oct 06, 2026

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