
Category: containers  Component type: type 
Looking up an element in a hash_multimap by its key is efficient, so hash_multimap is useful for "dictionaries" where the order of elements is irrelevant. If it is important for the elements to be in a particular order, however, then multimap is more appropriate.
struct eqstr { bool operator()(const char* s1, const char* s2) const { return strcmp(s1, s2) == 0; } }; typedef hash_multimap<const char*, int, hash<const char*>, eqstr> map_type; void lookup(const map_type& Map, const char* str) { cout << str << ": "; pair<map_type::const_iterator, map_type::const_iterator> p = Map.equal_range(str); for (map_type::const_iterator i = p.first; i != p.second; ++i) cout << (*i).second << " "; cout << endl; } int main() { map_type M; M.insert(map_type::value_type("H", 1)); M.insert(map_type::value_type("H", 2)); M.insert(map_type::value_type("C", 12)); M.insert(map_type::value_type("C", 13)); M.insert(map_type::value_type("O", 16)); M.insert(map_type::value_type("O", 17)); M.insert(map_type::value_type("O", 18)); M.insert(map_type::value_type("I", 127)); lookup(M, "I"); lookup(M, "O"); lookup(M, "Rn"); }
Parameter  Description  Default 

Key  The hash_multimap's key type. This is also defined as hash_multimap::key_type.  
Data  The hash_multimap's data type. This is also defined as hash_multimap::data_type.  
HashFcn  The hash function used by the hash_multimap. This is also defined as hash_multimap::hasher.  hash<Key> 
EqualKey  The hash_multimap's key equality function: a binary predicate that determines whether two keys are equal. This is also defined as hash_multimap::key_equal.  equal_to<Key> 
Alloc  The hash_set's allocator, used for all internal memory management.  alloc 
Member  Where defined  Description 

key_type  Associative Container  The hash_multimap's key type, Key. 
data_type  Pair Associative Container  The type of object associated with the keys. 
value_type  Pair Associative Container  The type of object, pair<const key_type, data_type>, stored in the hash_multimap. 
hasher  Hashed Associative Container  The hash_multimap's hash function. 
key_equal  Hashed Associative Container  Function object that compares keys for equality. 
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 hash_multimap. [1] 
const_iterator  Container  Const iterator used to iterate through a hash_multimap. 
iterator begin()  Container  Returns an iterator pointing to the beginning of the hash_multimap. 
iterator end()  Container  Returns an iterator pointing to the end of the hash_multimap. 
const_iterator begin() const  Container  Returns an const_iterator pointing to the beginning of the hash_multimap. 
const_iterator end() const  Container  Returns an const_iterator pointing to the end of the hash_multimap. 
size_type size() const  Container  Returns the size of the hash_multimap. 
size_type max_size() const  Container  Returns the largest possible size of the hash_multimap. 
bool empty() const  Container  true if the hash_multimap's size is 0. 
size_type bucket_count() const  Hashed Associative Container  Returns the number of buckets used by the hash_multimap. 
void resize(size_type n)  Hashed Associative Container  Increases the bucket count to at least n. 
hasher hash_funct() const  Hashed Associative Container  Returns the hasher object used by the hash_multimap. 
key_equal key_eq() const  Hashed Associative Container  Returns the key_equal object used by the hash_multimap. 
hash_multimap()  Container  Creates an empty hash_multimap. 
hash_multimap(size_type n)  Hashed Associative Container  Creates an empty hash_multimap with at least n buckets. 
hash_multimap(size_type n, const hasher& h) 
Hashed Associative Container  Creates an empty hash_multimap with at least n buckets, using h as the hash function. 
hash_multimap(size_type n, const hasher& h, const key_equal& k) 
Hashed Associative Container  Creates an empty hash_multimap with at least n buckets, using h as the hash function and k as the key equal function. 
template <class InputIterator> hash_multimap(InputIterator, InputIterator) [2] 
Multiple Hashed Associative Container  Creates a hash_multimap with a copy of a range. 
template <class InputIterator> hash_multimap(InputIterator, InputIterator, size_type n) [2] 
Multiple Hashed Associative Container  Creates a hash_multimap with a copy of a range and a bucket count of at least n. 
template <class InputIterator> hash_multimap(InputIterator, InputIterator, size_type n, const hasher& h) [2] 
Multiple Hashed Associative Container  Creates a hash_multimap with a copy of a range and a bucket count of at least n, using h as the hash function. 
template <class InputIterator> hash_multimap(InputIterator, InputIterator, size_type n, const hasher& h, const key_equal& k) [2] 
Multiple Hashed Associative Container  Creates a hash_multimap with a copy of a range and a bucket count of at least n, using h as the hash function and k as the key equal function. 
hash_multimap(const hash_multimap&)  Container  The copy constructor. 
hash_multimap& operator=(const hash_multimap&)  Container  The assignment operator 
void swap(hash_multimap&)  Container  Swaps the contents of two hash_multimaps. 
iterator insert(const value_type& x)  Multiple Associative Container  Inserts x into the hash_multimap. 
template <class InputIterator> void insert(InputIterator, InputIterator) [2] 
Multiple Associative Container  Inserts a range into the hash_multimap. 
void erase(iterator pos)  Associative Container  Erases the element pointed to by pos. 
size_type erase(const key_type& k)  Associative Container  Erases the element whose key is k. 
void erase(iterator first, iterator last)  Associative Container  Erases all elements in a range. 
void clear()  Associative Container  Erases all of the elements. 
const_iterator find(const key_type& k) const  Associative Container  Finds an element whose key is k. 
iterator find(const key_type& k)  Associative Container  Finds an element whose key is k. 
size_type count(const key_type& k) const  Associative Container  Counts the number of elements whose key is k. 
pair<const_iterator, const_iterator> equal_range(const key_type& k) const 
Associative Container  Finds a range containing all elements whose key is k. 
pair<iterator, iterator> equal_range(const key_type& k) 
Associative Container  Finds a range containing all elements whose key is k. 
bool operator==(const hash_multimap&, const hash_multimap&) 
Hashed Associative Container  Tests two hash_multimaps for equality. This is a global function, not a member function. 
[1] Hash_multimap::iterator is not a mutable iterator, because hash_multimap::value_type is not Assignable. That is, if i is of type hash_multimap::iterator and p is of type hash_multimap::value_type, then *i = p is not a valid expression. However, hash_multimap::iterator isn't a constant iterator either, because it can be used to modify the object that it points to. Using the same notation as above, (*i).second = p is a valid expression.
[2] 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 input iterator. If your compiler does not yet support member templates, though, then the arguments must either be of type const value_type* or of type hash_multimap::const_iterator.
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