| |
|
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
{
}
};
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
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> | AssociativeContainer | Finds a range containing all elements whose key is k . |
| AssociativeContainer | Finds a range containing all elements whose key is k . |
data_type& operator[](const key_type& k) [3] | hash_map | See below. |
| 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