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hash_map<Key, Data, HashFcn, EqualKey, Alloc>

Category: containers Component type: type

Description

Hash_map is a Hashed Associative Container that associates objects of type Key with objects of type Data. Hash_map is a Pair Associative Container, meaning that its value type is pair<const Key, Data>. It is also a Unique Associative Container, 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 hash function used by the hash_map. This is also defined as hash_map::hasher. hash<Key>
EqualKey The hash_map key equality function: a binary predicate 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. alloc

Model of

Unique Hashed Associative Container, Pair Associative Container

Type requirements

Public base classes

None.

Members

Member Where defined Description
key_type Associative Container The hash_map'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_map.
hasher Hashed Associative Container The hash_map'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_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 Hashed Associative Container Returns the number of buckets used by the hash_map.
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_map.
key_equal key_eq() const Hashed Associative Container Returns the key_equal object used by the hash_map.
hash_map() Container Creates an empty hash_map.
hash_map(size_type n) Hashed Associative Container Creates an empty hash_map with at least n buckets.
hash_map(size_type n, 
         const hasher& h)
Hashed Associative Container 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)
Hashed Associative Container 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]
Unique Hashed Associative Container Creates a hash_map with a copy of a range.
template <class InputIterator>
hash_map(InputIterator f, InputIterator l,
         size_type n)
[2]
Unique Hashed Associative Container 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]
Unique Hashed Associative Container 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]
Unique Hashed Associative Container 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)
Unique Associative Container Inserts x into the hash_map.
template <class InputIterator>
void insert(InputIterator f, InputIterator l)
[2]
Unique Associative Container Inserts a range into the hash_map.
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 Unique 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.
data_type& 
operator[](const key_type& k) [3]
hash_map See below.
bool operator==(const hash_map&, 
                const hash_map&)
Hashed Associative Container Tests two hash_maps for equality. This is a global function, not a member function.

New members

These members are not defined in the Unique Hashed Associative Container and Pair Associative Container 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 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_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

Associative Container, Hashed Associative Container, Pair Associative Container, Unique Hashed Associative Container, set, map multiset, multimap, hash_set, hash_multiset, hash_multimap


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