
Category: containers  Component type: type 
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.
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; }
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 
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. 
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] 
[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.
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