hash_multimap Class

Article
07/29/2015

Note

This API is obsolete. The alternative is unordered_multimap Class.

The container class hash_multimap is an extension of the Standard Template Library and is used for the storage and fast retrieval of data from a collection in which each element is a pair that has a sort key whose value need not be unique and an associated data value.

template <
   class Key, 
   class Type, 
   class Traits=hash_compare<Key, less<Key> >, 
   class Allocator=allocator<pair <const Key, Type> > 
>
class hash_multimap

Parameters

Key
The key data type to be stored in the hash_multimap.
Type
The element data type to be stored in the hash_multimap.
Traits
The type that includes two function objects, one of class Traits that is able to compare two element values as sort keys to determine their relative order and a hash function that is a unary predicate mapping key values of the elements to unsigned integers of type size_t. This argument is optional, and the hash_compare<Key, less<Key> > is the default value.
Allocator
The type that represents the stored allocator object that encapsulates details about the hash_multimap's allocation and deallocation of memory. This argument is optional, and the default value is allocator<pair <const Key, Type> >.

Remarks

The hash_multimap is:

An associative container, which a variable size container that supports the efficient retrieval of element values based on an associated key value.
Reversible, because it provides a bidirectional iterator to access its elements.
Hashed, because its elements are grouped into buckets based on the value of a hash function applied to the key values of the elements.
Multiple, because its elements do not need to have a unique keys, so that one key value may have many element data values associated with it.
A pair associative container, because its element values are distinct from its key values.
A template class, because the functionality it provides is generic and so independent of the specific type of data contained as elements or keys. The data types to be used for elements and keys are, instead, specified as parameters in the class template along with the comparison function and allocator.

The main advantage of hashing over sorting is greater efficiency; a successful hashing performs insertions, deletions, and finds in constant average time as compared with a time proportional to the logarithm of the number of elements in the container for sorting techniques. The value of an element in a hash_multimap, but not its associated key value, may be changed directly. Instead, key values associated with old elements must be deleted and new key values associated with new elements inserted.

The choice of container type should be based in general on the type of searching and inserting required by the application. Hashed associative containers are optimized for the operations of lookup, insertion and removal. The member functions that explicitly support these operations are efficient when used with a well-designed hash function, performing them in a time that is on average constant and not dependent on the number of elements in the container. A well-designed hash function produces a uniform distribution of hashed values and minimizes the number of collisions, where a collision is said to occur when distinct key values are mapped into the same hashed value. In the worst case, with the worst possible hash function, the number of operations is proportional to the number of elements in the sequence (linear time).

The hash_multimap should be the associative container of choice when the conditions associating the values with their keys are satisfied by the application. A model for this type of structure is an ordered list of key words with associated string values providing, say, definitions, where the words were not always uniquely defined. If, instead, the keywords were uniquely defined so that keys were unique, then a hash_map would be the container of choice. If, on the other hand, just the list of words were being stored, then a hash_set would be the correct container. If multiple occurrences of the words were allowed, then a hash_multiset would be the appropriate container structure.

The hash_multimap orders the sequence it controls by calling a stored hash Traits object of type value_compare. This stored object may be accessed by calling the member function key_comp. Such a function object must behave the same as an object of class hash_compare<Key, less<Key> >. Specifically, for all values Key of type Key, the call Traits (Key) yields a distribution of values of type size_t.

In general, the elements need be merely less than comparable to establish this order: so that, given any two elements, it may be determined either that they are equivalent (in the sense that neither is less than the other) or that one is less than the other. This results in an ordering between the non-equivalent elements. On a more technical note, the comparison function is a binary predicate that induces a strict weak ordering in the standard mathematical sense. A binary predicate f(x, y) is a function object that has two argument objects x and y and a return value of true or false. An ordering imposed on a hash_multimap is a strict weak ordering if the binary predicate is irreflexive, antisymmetric, and transitive and if equivalence is transitive, where two objects x and y are defined to be equivalent when both f(x, y) and f(y, x) are false. If the stronger condition of equality between keys replaces that of equivalence, then the ordering becomes total (in the sense that all the elements are ordered with respect to each other) and the keys matched will be indiscernible from each other.

The actual order of elements in the controlled sequence depends on the hash function, the ordering function, and the current size of the hash table stored in the container object. You cannot determine the current size of the hash table, so you cannot in general predict the order of elements in the controlled sequence. Inserting elements invalidates no iterators, and removing elements invalidates only those iterators that had specifically pointed at the removed elements.

The iterator provided by the hash_multimap class is a bidirectional iterator, but the class member functions insert and hash_multimap have versions that take as template parameters a weaker input iterator, whose functionality requirements are more minimal than those guaranteed by the class of bidirectional iterators. The different iterator concepts form a family related by refinements in their functionality. Each iterator concept has its own hash_multimap of requirements, and the algorithms that work with them must limit their assumptions to the requirements provided by that type of iterator. It may be assumed that an input iterator may be dereferenced to refer to some object and that it may be incremented to the next iterator in the sequence. This is a minimal hash_multimap of functionality, but it is enough to be able to talk meaningfully about a range of iterators [First, Last) in the context of the member functions.

In Visual C++ .NET 2003, members of the <hash_map> and <hash_set> header files are no longer in the std namespace, but rather have been moved into the stdext namespace. See The stdext Namespace for more information.

Constructors

hash_multimap

Constructs a list of a specific size or with elements of a specific value or with a specific allocator or as a copy of some other hash_multimap.

Typedefs

allocator_type	A type that represents the allocator class for the hash_multimap object.
const_iterator	A type that provides a bidirectional iterator that can read a const element in the hash_multimap.
const_pointer	A type that provides a pointer to a const element in a hash_multimap.
const_reference	A type that provides a reference to a const element stored in a hash_multimap for reading and performing const operations.
const_reverse_iterator	A type that provides a bidirectional iterator that can read any const element in the hash_multimap.
difference_type	A signed integer type that can be used to represent the number of elements of a hash_multimap in a range between elements pointed to by iterators.
iterator	A type that provides a bidirectional iterator that can read or modify any element in a hash_multimap.
key_compare	A type that provides a function object that can compare two sort keys to determine the relative order of two elements in the hash_multimap.
key_type	A type that describes the sort key object that constitutes each element of the hash_multimap.
mapped_type	A type that represents the data type stored in a hash_multimap.
pointer	A type that provides a pointer to an element in a hash_multimap.
reference	A type that provides a reference to an element stored in a hash_multimap.
reverse_iterator	A type that provides a bidirectional iterator that can read or modify an element in a reversed hash_multimap.
size_type	An unsigned integer type that can represent the number of elements in a hash_multimap.
value_type	A type that provides a function object that can compare two elements as sort keys to determine their relative order in the hash_multimap.

Member Functions

begin	Returns an iterator addressing the first element in the hash_multimap.
hash_multimap::cbegin	Returns a const iterator addressing the first element in the hash_multimap.
hash_multimap::cend	Returns a const iterator that addresses the location succeeding the last element in a hash_multimap.
clear	Erases all the elements of a hash_multimap.
count	Returns the number of elements in a hash_multimap whose key matches a parameter-specified key.
hash_multimap::crbegin	Returns a const iterator addressing the first element in a reversed hash_multimap.
hash_multimap::crend	Returns a const iterator that addresses the location succeeding the last element in a reversed hash_multimap.
hash_multimap::emplace	Inserts an element constructed in place into a hash_multimap.
hash_multimap::emplace_hint	Inserts an element constructed in place into a hash_multimap, with a placement hint.
empty	Tests if a hash_multimap is empty.
end	Returns an iterator that addresses the location succeeding the last element in a hash_multimap.
equal_range	Returns an iterator that addresses the location succeeding the last element in a hash_multimap.
erase	Removes an element or a range of elements in a hash_multimap from specified positions
find	Returns an iterator addressing the location of an element in a hash_multimap that has a key equivalent to a specified key.
get_allocator	Returns a copy of the allocator object used to construct the hash_multimap.
insert	Inserts an element or a range of elements into the hash_multimap at a specified position.
key_comp	Retrieves a copy of the comparison object used to order keys in a hash_multimap.
lower_bound	Returns an iterator to the first element in a hash_multimap that with a key value that is equal to or greater than that of a specified key.
max_size	Returns the maximum length of the hash_multimap.
rbegin	Returns an iterator addressing the first element in a reversed hash_multimap.
rend	Returns an iterator that addresses the location succeeding the last element in a reversed hash_multimap.
size	Specifies a new size for a hash_multimap.
swap	Exchanges the elements of two hash_multimaps.
upper_bound	Returns an iterator to the first element in a hash_multimap that with a key value that is greater than that of a specified key.
value_comp	Retrieves a copy of the comparison object used to order element values in a hash_multimap.

Operators

hash_multimap::operator=

Replaces the elements of a hash_multimap with a copy of another hash_multimap.

Requirements

Header: <hash_map>

Namespace: stdext

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