hash_map

Category: containers		Component type: type

Description

Hash_map is a HashedAssociativeContainer that associates objects of type Key with objects of type Data. Hash_map is a PairAssociativeContainer, meaning that its value type is pair<const Key, Data>. It is also a UniqueAssociativeContainer, meaning that no two elements have keys that compare equal using EqualKey.

Looking up an element in a hash_map by its key is efficient, so hash_map 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 Map is more appropriate.

Example

struct eqstr
{
  bool operator()(const char* s1, const char* s2) const
  {
    return strcmp(s1, s2) == 0;
  }
};

int main()
{
  hash_map<const char*, int, hash<const char*>, eqstr> months;
  
  months["january"] = 31;
  months["february"] = 28;
  months["march"] = 31;
  months["april"] = 30;
  months["may"] = 31;
  months["june"] = 30;
  months["july"] = 31;
  months["august"] = 31;
  months["september"] = 30;
  months["october"] = 31;
  months["november"] = 30;
  months["december"] = 31;
  
  cout << "september -> " << months["september"] << endl;
  cout << "april     -> " << months["april"] << endl;
  cout << "june      -> " << months["june"] << endl;
  cout << "november  -> " << months["november"] << endl;
}

Definition

Defined in the header hash_map, and in the backward-compatibility header hash_map.h. This class is an SGI extension; it is not part of the C++ standard.

Template parameters

Parameter	Description	Default
Key	The hash_map's key type. This is also defined as hash_map::key_type.
Data	The hash_map's data type. This is also defined as hash_map::data_type.
HashFcn	The HashFunction used by the hash_map. This is also defined as hash_map::hasher.	hash<Key>
EqualKey	The hash_map key equality function: a BinaryPredicate that determines whether two keys are equal. This is also defined as hash_map::key_equal.	equal_to<Key>
Alloc	The hash_map's allocator, used for all internal memory management.	Allocators

Model of

UniqueHashedAssociativeContainer, PairAssociativeContainer

Type requirements

• Key is Assignable.
• EqualKey is a BinaryPredicate whose argument type is Key.
• EqualKey is an equivalence relation.
• Alloc is an Allocators.

Public base classes

None.

Members

Member	Where defined	Description
key_type	AssociativeContainer	The hash_map's key type, Key.
data_type	PairAssociativeContainer	The type of object associated with the keys.
value_type	PairAssociativeContainer	The type of object, pair<const key_type, data_type>, stored in the hash_map.
hasher	HashedAssociativeContainer	The hash_map's HashFunction.
key_equal	HashedAssociativeContainer	functors 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_map. [1]
const_iterator	Container	Const iterator used to iterate through a hash_map.
iterator begin()	Container	Returns an iterator pointing to the beginning of the hash_map.
iterator end()	Container	Returns an iterator pointing to the end of the hash_map.
const_iterator begin() const	Container	Returns an const_iterator pointing to the beginning of the hash_map.
const_iterator end() const	Container	Returns an const_iterator pointing to the end of the hash_map.
size_type size() const	Container	Returns the size of the hash_map.
size_type max_size() const	Container	Returns the largest possible size of the hash_map.
bool empty() const	Container	true if the hash_map's size is 0.
size_type bucket_count() const	HashedAssociativeContainer	Returns the number of buckets used by the hash_map.
void resize(size_type n)	HashedAssociativeContainer	Increases the bucket count to at least n.
hasher hash_funct() const	HashedAssociativeContainer	Returns the hasher object used by the hash_map.
key_equal key_eq() const	HashedAssociativeContainer	Returns the key_equal object used by the hash_map.
hash_map()	Container	Creates an empty hash_map.
hash_map(size_type n)	HashedAssociativeContainer	Creates an empty hash_map with at least n buckets.
hash_map(size_type n, const hasher& h)	HashedAssociativeContainer	Creates an empty hash_map with at least n buckets, using h as the hash function.
hash_map(size_type n, const hasher& h, const key_equal& k)	HashedAssociativeContainer	Creates an empty hash_map with at least n buckets, using h as the hash function and k as the key equal function.
template <class InputIterator> hash_map(InputIterator f, InputIterator l) [2]	UniqueHashedAssociativeContainer	Creates a hash_map with a copy of a range.
template <class InputIterator> hash_map(InputIterator f, InputIterator l, size_type n) [2]	UniqueHashedAssociativeContainer	Creates a hash_map with a copy of a range and a bucket count of at least n.
template <class InputIterator> hash_map(InputIterator f, InputIterator l, size_type n, const hasher& h) [2]	UniqueHashedAssociativeContainer	Creates a hash_map with a copy of a range and a bucket count of at least n, using h as the hash function.
template <class InputIterator> hash_map(InputIterator f, InputIterator l, size_type n, const hasher& h, const key_equal& k) [2]	UniqueHashedAssociativeContainer	Creates a hash_map 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_map(const hash_map&)	Container	The copy constructor.
hash_map& operator=(const hash_map&)	Container	The assignment operator
void swap(hash_map&)	Container	Swaps the contents of two hash_maps.
pair<iterator, bool> insert(const value_type& x)	UniqueAssociativeContainer	Inserts x into the hash_map.
template <class InputIterator> void insert(InputIterator f, InputIterator l) [2]	UniqueAssociativeContainer	Inserts a range into the hash_map.
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.
const_iterator find(const key_type& k) const	AssociativeContainer	Finds an element whose key is k.
iterator find(const key_type& k)	AssociativeContainer	Finds an element whose key is k.
size_type count(const key_type& k) const	UniqueAssociativeContainer	Counts the number of elements whose key is k.
pair<const_iterator, const_iterator> equal_range(const key_type& k) const	AssociativeContainer	Finds a range containing all elements whose key is k.
pair<iterator, iterator> equal_range(const key_type& k)	AssociativeContainer	Finds a range containing all elements whose key is k.
data_type& operator[](const key_type& k) [3]	hash_map	See below.
bool operator==(const hash_map&, const hash_map&)	HashedAssociativeContainer	Tests two hash_maps for equality. This is a global function, not a member function.

New members

These members are not defined in the UniqueHashedAssociativeContainer and PairAssociativeContainer requirements, but are specific to hash_map.

Member	Description
data_type& operator[](const key_type& k) [3]	Returns a reference to the object that is associated with a particular key. If the hash_map does not already contain such an object, operator[] inserts the default object data_type(). [3]

Notes

[1] Hash_map::iterator is not a mutable iterator, because hash_map::value_type is not Assignable. That is, if i is of type hash_map::iterator and p is of type hash_map::value_type, then *i = p is not a valid expression. However, hash_map::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 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 hash_map::const_iterator.

[3] Since operator[] might insert a new element into the hash_map, it can't possibly be a const member function. Note that the definition of operator[] is extremely simple: m[k] is equivalent to (*((m.insert(value_type(k, data_type()))).first)).second. Strictly speaking, this member function is unnecessary: it exists only for convenience.

See also

AssociativeContainer, HashedAssociativeContainer, PairAssociativeContainer, UniqueHashedAssociativeContainer, set, Map multiset, Multimap, hash_set, hash_multiset, hash_multimap