multiset

Category: containers		Component type: type

Description

Multiset is a SortedAssociativeContainer that stores objects of type Key. Multiset is a SimpleAssociativeContainer, meaning that its value type, as well as its key type, is Key. It is also a MultipleAssociativeContainer, meaning that two or more elements may be identical.

set and multiset are particularly well suited to the set algorithms includes, set_union, set_intersection, set_difference, and set_symmetric_difference. The reason for this is twofold. First, the set algorithms require their arguments to be sorted ranges, and, since set and multiset are SortedAssociativeContainer, their elements are always sorted in ascending order. Second, the output range of these algorithms is always sorted, and inserting a sorted range into a set or multiset is a fast operation: the UniqueSortedAssociativeContainer and MultipleSortedAssociativeContainer requirements guarantee that inserting a range takes only linear time if the range is already sorted.

Multiset has the important property that inserting a new element into a multiset does not invalidate iterators that point to existing elements. Erasing an element from a multiset also does not invalidate any iterators, except, of course, for iterators that actually point to the element that is being erased.

Example

int main()
{
  const int N = 10;
  int a[N] = {4, 1, 1, 1, 1, 1, 0, 5, 1, 0};
  int b[N] = {4, 4, 2, 4, 2, 4, 0, 1, 5, 5};

  multiset<int> A(a, a + N);
  multiset<int> B(b, b + N);
  multiset<int> C;

  cout << "Set A: ";
  copy(A.begin(), A.end(), ostream_iterator<int>(cout, " "));
  cout << endl;
  cout << "Set B: ";
  copy(B.begin(), B.end(), ostream_iterator<int>(cout, " "));   
  cout << endl;

  cout << "Union: ";
  set_union(A.begin(), A.end(), B.begin(), B.end(),
            ostream_iterator<int>(cout, " "));
  cout << endl;

  cout << "Intersection: ";
  set_intersection(A.begin(), A.end(), B.begin(), B.end(),
                   ostream_iterator<int>(cout, " "));
  cout << endl;  

  set_difference(A.begin(), A.end(), B.begin(), B.end(),
                 insert_iterator(C, C.begin()));
  cout << "Set C (difference of A and B): ";
  copy(C.begin(), C.end(), ostream_iterator<int>(cout, " "));
  cout << endl;
}

Definition

Defined in the standard header set, and in the nonstandard backward-compatibility header multiset.h.

Template parameters

Parameter	Description	Default
Key	The set's key type and value type. This is also defined as multiset::key_type and multiset::value_type
Compare	The key comparison function, a StrictWeakOrdering whose argument type is key_type; it returns true if its first argument is less than its second argument, and false otherwise. This is also defined as multiset::key_compare and multiset::value_compare.	less<Key>
Alloc	The multiset's allocator, used for all internal memory management.	Allocators

Model of

MultipleSortedAssociativeContainer, SimpleAssociativeContainer

Type requirements

• Key is Assignable.
• Compare is a StrictWeakOrdering whose argument type is Key.
• Alloc is an Allocators.

Public base classes

None.

Members

Member	Where defined	Description
value_type	Container	The type of object, T, stored in the multiset.
key_type	AssociativeContainer	The key type associated with value_type.
key_compare	SortedAssociativeContainer	functors that compares two keys for ordering.
value_compare	SortedAssociativeContainer	functors that compares two values for ordering.
pointer	Container	Pointer to T.
reference	Container	Reference to T
const_reference	Container	Const reference to T
size_type	Container	An unsigned integral type.
difference_type	Container	A signed integral type.
iterator	Container	Iterator used to iterate through a multiset.
const_iterator	Container	Const iterator used to iterate through a multiset. (Iterator and const_iterator are the same type.)
reverse_iterator	ReversibleContainer	Iterator used to iterate backwards through a multiset.
const_reverse_iterator	ReversibleContainer	Const iterator used to iterate backwards through a multiset. (Reverse_iterator and const_reverse_iterator are the same type.)
iterator begin() const	Container	Returns an iterator pointing to the beginning of the multiset.
iterator end() const	Container	Returns an iterator pointing to the end of the multiset.
reverse_iterator rbegin() const	ReversibleContainer	Returns a reverse_iterator pointing to the beginning of the reversed multiset.
reverse_iterator rend() const	ReversibleContainer	Returns a reverse_iterator pointing to the end of the reversed multiset.
size_type size() const	Container	Returns the size of the multiset.
size_type max_size() const	Container	Returns the largest possible size of the multiset.
bool empty() const	Container	true if the multiset's size is 0.
key_compare key_comp() const	SortedAssociativeContainer	Returns the key_compare object used by the multiset.
value_compare value_comp() const	SortedAssociativeContainer	Returns the value_compare object used by the multiset.
multiset()	Container	Creates an empty multiset.
multiset(const key_compare& comp)	SortedAssociativeContainer	Creates an empty multiset, using comp as the key_compare object.
template <class InputIterator> multiset(InputIterator f, InputIterator l) [1]	MultipleSortedAssociativeContainer	Creates a multiset with a copy of a range.
template <class InputIterator> multiset(InputIterator f, InputIterator l, const key_compare& comp) [1]	MultipleSortedAssociativeContainer	Creates a multiset with a copy of a range, using comp as the key_compare object.
multiset(const multiset&)	Container	The copy constructor.
multiset& operator=(const multiset&)	Container	The assignment operator
void swap(multiset&)	Container	Swaps the contents of two multisets.
iterator insert(const value_type& x)	MultipleAssociativeContainer	Inserts x into the multiset.
iterator insert(iterator pos, const value_type& x)	MultipleSortedAssociativeContainer	Inserts x into the multiset, using pos as a hint to where it will be inserted.
template <class InputIterator> void insert(InputIterator, InputIterator) [1]	MultipleSortedAssociativeContainer	Inserts a range into the multiset.
void erase(iterator pos)	AssociativeContainer	Erases the element pointed to by pos.
size_type erase(const key_type& k)	AssociativeContainer	Erases the element whose key is k.
void erase(iterator first, iterator last)	AssociativeContainer	Erases all elements in a range.
void clear()	AssociativeContainer	Erases all of the elements.
iterator find(const key_type& k) const	AssociativeContainer	Finds an element whose key is k.
size_type count(const key_type& k) const	AssociativeContainer	Counts the number of elements whose key is k.
iterator lower_bound(const key_type& k) const	SortedAssociativeContainer	Finds the first element whose key is not less than k.
iterator upper_bound(const key_type& k) const	SortedAssociativeContainer	Finds the first element whose key greater than k.
pair<iterator, iterator> equal_range(const key_type& k) const	SortedAssociativeContainer	Finds a range containing all elements whose key is k.
bool operator==(const multiset&, const multiset&)	ForwardContainer	Tests two multisets for equality. This is a global function, not a member function.
bool operator<(const multiset&, const multiset&)	ForwardContainer	Lexicographical comparison. This is a global function, not a member function.

New members

All of multiset's members are defined in the MultipleSortedAssociativeContainer and SimpleAssociativeContainer requirements. Multiset does not introduce any new members.

Notes

[1] This member function relies on member template functions, which at present (early 1998) are not supported by all compilers. If your compiler supports member templates, you can call this function with any type of InputIterator. If your compiler does not yet support member templates, though, then the arguments must either be of type const value_type* or of type multiset::const_iterator.

See also

AssociativeContainer, SortedAssociativeContainer, SimpleAssociativeContainer, MultipleSortedAssociativeContainer, set, Map, Multimap, hash_set, hash_map, hash_multiset, hash_multimap