Apache C++ Standard Library Reference Guide

## multiset

Library: Containers

    Does not inherit

- Local Index
- Summary
- Synopsis
- Description
- Interface
- Constructors
- Destructors
- Assignment Operators
- Allocators
- Iterators
- Member Functions
- Nonmember Operators
- Specialized Algorithms
- Example
- Warnings
- See Also
- Standards Conformance

### Local Index

#### Members

begin() clear() count() empty() end() equal_range() | erase() find() get_allocator() insert() key_comp() lower_bound() | max_size() multiset() operator=() rbegin() rend() size() | swap() upper_bound() value_comp() ~multiset()

#### Non-Members

operator!=() operator>() | operator>=() operator<() | operator<=() operator==() |

### Summary

An associative container that allows fast access to stored key values. Storage of duplicate keys is allowed. A multiset supports bidirectional iterators.

### Synopsis

    #include <set>

    namespace std {
      template <class Key, class Compare = less<Key>,
                class Allocator = allocator<Key> >
      class multiset;
    }

### Description

multiset allows fast access to stored key values. The default operation for key comparison is the `<` operator. Insertion of duplicate keys is allowed with a multiset.

multiset uses bidirectional iterators that point to a stored key.

Any type used for the template parameter `Key` must include the following (where`T` is the `type`,`t` is a value of `T` and`u` is a `const` `value` of `T`):

Copy constructors | `T(t)` and `T(u)`
Destructor | `t.~T()`
Address of | `&t` and `&u` yielding `T*` and `const T*` respectively
Assignment | `t = a` where `a` is a (possibly `const`) value of `T`

The `type` used for the `Compare` template parameter must satisfy the requirements for binary functions.

### Interface

    namespace std {

      template <class Key, class Compare = less<Key>,
                class Allocator = allocator<Key> >
      class multiset {

      public:

        // typedefs
        typedef Key key_type;
        typedef Key value_type;
        typedef Compare key_compare;
        typedef Compare value_compare;
        typedef Allocator allocator_type;
        typedef typename
                Allocator::reference reference;
        typedef typename
                Allocator::const_reference const_reference;
        class iterator;
        class const_iterator;
        typedef typename
               Allocator::size_type size_type;
        typedef typename
               Allocator::difference_type  difference_type;
        typedef typename Allocator::pointer pointer;
        typedef typename Allocator::const_pointer const_pointer;
        typedef typename std::reverse_iterator<iterator>
                              reverse_iterator;
        typedef typename std::reverse_iterator<const_iterator>
                              const_reverse_iterator;

        // Construct/Copy/Destroy
        explicit multiset(const Compare& = Compare(),
                          const Allocator& = Allocator());
        template <class InputIterator>
        multiset(InputIterator, InputIterator,
                 const Compare& = Compare(),
                 const Allocator& = Allocator());
        multiset(const multiset<Key, Compare, Allocator>&);
        ~multiset();
        multiset<Key, Compare, Allocator>&
                operator=(const multiset<Key,
                           Compare, Allocator>&);

        // Iterators
        iterator begin();
        const_iterator begin() const;
        iterator end();
        const_iterator end() const;
        reverse_iterator rbegin();
        const_reverse_iterator rbegin() const;
        reverse_iterator rend();
        const_reverse_iterator rend() const;

        // Capacity
        bool empty() const;
        size_type size() const;
        size_type max_size() const;

        // Modifiers
        iterator insert(const value_type&);
        iterator insert(iterator, const value_type&);
        template <class InputIterator>
        void insert(InputIterator, InputIterator);

        void erase(iterator);
        size_type erase(const key_type&);
        void erase(iterator, iterator);
        void swap(multiset<Key, Compare, Allocator>&);
        void clear();

        // Observers
        key_compare key_comp () const;
        value_compare value_comp () const;

        // Multiset operations
        iterator find(const key_type&) const;
        size_type count(const key_type&) const;
        iterator lower_bound(const key_type&) const;
        iterator upper_bound(const key_type&) const;
        pair<iterator, iterator> equal_range
           (const key_type&) const;
       };

      // Nonmember Operators
      template <class Key, class Compare, class Allocator>
      bool operator==
        (const multiset<Key, Compare, Allocator>&,
         const multiset<Key, Compare, Allocator>&);

      template <class Key, class Compare, class Allocator>
      bool operator!=
        (const multiset<Key, Compare, Allocator>&,
         const multiset<Key, Compare, Allocator>&);

      template <class Key, class Compare, class Allocator>
      bool operator<
        (const multiset<Key, Compare, Allocator>&,
         const multiset<Key, Compare, Allocator>&);

      template <class Key, class Compare, class Allocator>
      bool operator>
        (const multiset<Key, Compare, Allocator>&,
         const multiset<Key, Compare, Allocator>&);

      template <class Key, class Compare, class Allocator>
      bool operator<=
        (const multiset<Key, Compare, Allocator>&,
         const multiset<Key, Compare, Allocator>&);

      template <class Key, class Compare, class Allocator>
      bool operator>=
        (const multiset<Key, Compare, Allocator>&,
         const multiset<Key, Compare, Allocator>&);

      // Specialized Algorithms

      template <class Key, class Compare, class Allocator>
      void swap(multiset<Key, Compare, Allocator>&,
                multiset<Key, Compare, Allocator>&);
    }

### Constructors

    explicit multiset(const Compare& comp = Compare(),
                       const Allocator& alloc = Allocator());

Constructs an empty multiset that uses the optional relation `comp` to order keys, if it is supplied, and the allocator `alloc` for all storage management.

    template <class InputIterator>
    multiset(InputIterator start, InputIterator finish,
              const Compare& = Compare(),
              const Allocator& = Allocator());

Constructs a multiset containing values in the range `[start, finish).`

    multiset(const multiset<Key, Compare, Allocator>& x);

Creates a new multiset by copying all key values from `x`.

### Destructors

    ~multiset();

Releases any allocated memory for this multiset.

### Assignment Operators

    multiset<Key, Compare, Allocator>&
    operator=(const multiset<Key, Compare, Allocator>& x);

Replaces the contents of `*this` with a copy of the contents of `x`.

### Allocators

    allocator_type
    get_allocator() const;

Returns a copy of the allocator used by self for storage management.

### Iterators

    iterator
    begin();

Returns an `iterator` pointing to the first element stored in the multiset. "First" is defined by the multiset's comparison operator, `Compare`.

    const_iterator
    begin();

Returns a `const_iterator` pointing to the first element stored in the multiset.

    iterator
    end();

Returns an `iterator` pointing to the last element stored in the multiset (in other words, the off-the-end value).

    const_iterator
    end();

Returns a `const_iterator` pointing to the last element stored in the multiset (in other words, the off-the-end value).

    reverse_iterator
    rbegin();

Returns a `reverse_iterator` pointing to the first element stored in the multiset. "First" is defined by the multiset's comparison operator, `Compare`.

    const_reverse_iterator
    rbegin();

Returns a `const_reverse_iterator` pointing to the first element stored in the multiset.

    reverse_iterator
    rend();

Returns a `reverse_iterator` pointing to the last element stored in the multiset (in other words, the off-the-end value).

    const_reverse_iterator
    rend();

Returns a `const_reverse_iterator` pointing to the last element stored in the multiset (in other words, the off-the-end value).

### Member Functions

    void
    clear();

Erases all elements from the self.

    size_type
    count(const key_type& x) const;

Returns the number of elements in the multiset with the key value `x`.

    bool
    empty() const;

Returns `true` if the multiset is empty, `false` otherwise.

    pair<iterator,iterator>
    equal_range(const key_type& x)const;

Returns the pair `(lower_bound(x), upper_bound(x))`.

    size_type
    erase(const key_type& x);

Deletes all elements with the key value `x` from the multiset, if any exist. Returns the number of deleted elements.

    void
    erase(iterator position);

Deletes the multiset element pointed to by the iterator `position`.

    void
    erase(iterator start, iterator finish);

If the iterators `start` and `finish` point to the same multiset and last is reachable from first, all elements in the range `[start, finish)` are deleted from the multiset.

    iterator
    find(const key_type& x) const;

Searches the multiset for a key value `x` and returns an iterator to that key if it is found. If such a value is not found, the iterator `end()` is returned.

    iterator
    insert(const value_type& x);
    iterator
    insert(iterator position, const value_type& x);

`x` is inserted into the multiset. A position may be supplied as a hint regarding where to do the insertion. If the insertion is done right after position, then it takes amortized constant time. Otherwise, it takes `O(log N)` time.

    template <class InputIterator>
    void
    insert(InputIterator start, InputIterator finish);

Copies of each element in the range `[start, finish)` are inserted into the multiset. This `insert` takes approximately `O(N*log(size()+N))` time.

    key_compare
    key_comp() const;

Returns a function object capable of comparing key values using the comparison operation, `Compare`, of the current multiset.

    iterator
    lower_bound(const key_type& x) const;

Returns an iterator to the first element whose key is greater than or equal to `x`. If no such element exists, `end()` is returned.

    size_type
    max_size() const;

Returns the maximum possible size of the multiset `size_type.`

    size_type
    size() const;

Returns the number of elements in the multiset.

    void
    swap(multiset<Key, Compare, Allocator>& x);

Swaps the contents of the multiset `x` with the current multiset, `*this`.

    iterator
    upper_bound(const key_type& x) const;

Returns an iterator to the first element whose key is smaller than or equal to `x.` If no such element exists, then `end()` is returned.

    value_compare
    value_comp() const;

Returns a function object capable of comparing key values using the comparison operation, `Compare`, of the current multiset.

### Nonmember Operators

    template <class Key, class Compare, class Allocator>
    operator==(const multiset<Key, Compare, Allocator>& x,
                const multiset<Key, Compare, Allocator>& y);

Returns `true` if all elements in `x` are element-wise equal to all elements in `y`, using `(T::operator==)`. Otherwise it returns `false`.

    template <class Key, class Compare, class Allocator>
    operator!=(const multiset<Key, Compare, Allocator>& x,
                const multiset<Key, Compare, Allocator>& y);

Returns `!(x==y)`.

    template <class Key, class Compare, class Allocator>
    operator<(const multiset<Key, Compare, Allocator>& x,
               const multiset<Key, Compare, Allocator>& y);

Returns `true` if `x` is lexicographically less than `y`. Otherwise, it returns `false`.

    template <class Key, class Compare, class Allocator>
    operator>(const multiset<Key, Compare, Allocator>& x,
               const multiset<Key, Compare, Allocator>& y);

Returns `y < x`.

    template <class Key, class Compare, class Allocator>
    operator<=(const multiset<Key, Compare, Allocator>& x,
               const multiset<Key, Compare, Allocator>& y);

Returns `!(y < x)`.

    template <class Key, class Compare, class Allocator>
    operator>=(const multiset<Key, Compare, Allocator>& x,
               const multiset<Key, Compare, Allocator>& y);

Returns `!(x < y)`.

### Specialized Algorithms

    template <class Key, class Compare, class Allocator>
    void swap(multiset<Key,Compare,Allocator>& a,
              multiset<Key,Compare,Allocator>&b);

Swaps the contents of `a` and `b`.

### Example

    //
    //  multiset.cpp
    //

    #include <algorithm>   // for set_intersection, set_union
    #include <iostream>    // for cout, endl
    #include <set>         // for set

    typedef std::multiset<int, std::less<int>,
                          std::allocator<int> > set_type;

    inline std::ostream& operator<< (std::ostream& out,
                                     const set_type& s)
    {
        typedef std::ostream_iterator<set_type::value_type, char,
            std::char_traits<char> > os_iter;

        std::copy (s.begin (), s.end (), os_iter (std::cout, " "));

        return out;
    }

    int main ()
    {
        // Create a multiset of integers.
        set_type  si;

        for (set_type::value_type j = 0; j < 2; j++) {
            for (set_type::value_type i = 0; i < 10; ++i)
                // Insert values with a hint.
                si.insert (si.begin (), i);
        }

        // Print out the multiset.
        std::cout << si << std::endl;

        // Make another multiset and an empty multiset.
        set_type si2, result;
        for (set_type::value_type i = 0; i < 10; i++)
            si2.insert (i + 5);

        std::cout << si2 << std::endl;

        // Try a couple of set algorithms.
        std::set_union (si.begin (), si.end (),
                        si2.begin (), si2.end (),
                        std::inserter (result, result.begin ()));

        std::cout << "Union:\n" << result << std::endl;

        result.erase (result.begin (), result.end ());

        std::set_intersection (si.begin (), si.end (),
                           si2.begin (), si2.end (),
                           std::inserter(result, result.begin()));

        std::cout << "Intersection:\n" << result << std::endl;

        return 0;
    }

    Program Output:

    0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9
    5 6 7 8 9 10 11 12 13 14
    Union:
    0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 11 12 13 14
    Intersection:
    5 6 7 8 9

### Warnings

Member function templates are used in all containers included in the Standard Template Library. For example, the constructor for multiset takes two templatized iterators:

    template <class InputIterator>
    multiset (InputIterator, InputIterator,
              const Compare& = Compare(),
              const Allocator& = Allocator());

multiset also has an `insert` function of this type. These functions, when not restricted by compiler limitations, allow you to use any type of input iterator as arguments. For compilers that do not support this feature, substitute functions allow you to use an iterator obtained from the same type of container as the one you are constructing (or calling a member function on). You can also use a pointer to the type of element you have in the container.

For example, if your compiler does not support member function templates, you can construct a multiset in the following two ways:

    int intarray[10];
    multiset<int> first_multiset(intarray, intarray +10);
    multiset<int> second_multiset(first_multiset.begin(),
                  first_multiset.end());

You cannot construct a multiset this way:

    multiset<long>
     long_multiset(first_multiset.begin(),first_multiset.end());

since the `long_multiset` and`first_multiset` are not the same type.

If your compiler is does not support default template parameters, you must always supply the `Compare` template argument and the `Allocator` template argument. For instance, you must write:

`multiset<int, less<int>, allocator<int> >`

instead of:

`multiset<int>`

### See Also

allocator, Containers, Iterators, set

### Standards Conformance

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 23.3.4