hash_map Class

Stores and retrieves data quickly from a collection in which each element is a pair that has a sort key whose value is unique and an associated data value.

Syntax

template <class Key,
    class Type,
    class Traits=hash_compare<Key, less<Key>>,
    class Allocator=allocator<pair <const Key, Type>>>
class hash_map

Parameters

Key
The key data type to be stored in the hash_map.

Type
The element data type to be stored in the hash_map.

Traits
The type that includes two function objects, one of class compare able to compare two element values as sort keys to determine their relative order and a hash function that is a unary predicate mapping key values of the elements to unsigned integers of type size_t. This argument is optional, and hash_compare<Key, less<Key>> is the default value.

Allocator
The type that represents the stored allocator object that encapsulates details about the hash_map's allocation and deallocation of memory. This argument is optional, and the default value is allocator<pair <const Key, Type>>.

Remarks

The hash_map is:

• An associative container, which a variable size container that supports the efficient retrieval of element values based on an associated key value.

• Reversible, because it provides a bidirectional iterator to access its elements.

• Hashed, because its elements are grouped into buckets based on the value of a hash function applied to the key values of the elements.

• Unique in the sense that each of its elements must have a unique key.

• A pair associative container, because its element data values are distinct from its key values.

• A class template, because the functionality it provides is generic and so independent of the specific type of data contained as elements or keys. The data types to be used for elements and keys are, instead, specified as parameters in the class template along with the comparison function and allocator.

The main advantage of hashing over sorting is greater efficiency; a successful hashing performs insertions, deletions, and finds in constant average time as compared with a time proportional to the logarithm of the number of elements in the container for sorting techniques. The value of an element in a hash_map, but not its associated key value, may be changed directly. Instead, key values associated with old elements must be deleted and new key values associated with new elements inserted.

The choice of container type should be based in general on the type of searching and inserting required by the application. Hashed associative containers are optimized for the operations of lookup, insertion and removal. The member functions that explicitly support these operations are efficient when used with a well-designed hash function, performing them in a time that is on average constant and not dependent on the number of elements in the container. A well-designed hash function produces a uniform distribution of hashed values and minimizes the number of collisions, where a collision is said to occur when distinct key values are mapped into the same hashed value. In the worst case, with the worst possible hash function, the number of operations is proportional to the number of elements in the sequence (linear time).

The hash_map should be the associative container of choice when the conditions associating the values with their keys are satisfied by the application. A model for this type of structure is an ordered list of uniquely occurring keywords with associated string values providing, say, definitions. If, instead, the words had more than one correct definition, so that keys weren't unique, then a hash_multimap would be the container of choice. If, on the other hand, just the list of words were being stored, then a hash_set would be the correct container. If multiple occurrences of the words were allowed, then a hash_multiset would be the appropriate container structure.

The hash_map orders the sequence it controls by calling a stored hash Traits object of class value_compare. This stored object may be accessed by calling the member function key_comp. Such a function object must behave the same as an object of class hash_compare<Key, less<Key>>. Specifically, for all values Key of type Key, the call Traits( Key ) yields a distribution of values of type size_t. For more information, see hash_compare.

In general, the elements need be merely less than comparable to establish this order: so that, given any two elements, it may be determined either that they're equivalent (in the sense that neither is less than the other) or that one is less than the other. This results in an ordering between the nonequivalent elements. On a more technical note, the comparison function is a binary predicate that induces a strict weak ordering in the standard mathematical sense. A binary predicate f(x y) is a function object that has two argument objects x and y and a return value of true or false. An ordering imposed on a hash_map is a strict weak ordering if the binary predicate is irreflexive, antisymmetric, and transitive and if equivalence is transitive, where two objects x and y are defined to be equivalent when both f(x, y) and f(y, x) are false. If the stronger condition of equality between keys replaces that of equivalence, then the ordering becomes total (in the sense that all the elements are ordered with respect to each other) and the keys matched will be indiscernible from each other.

The actual order of elements in the controlled sequence depends on the hash function, the ordering function, and the current size of the hash table stored in the container object. You can't determine the current size of the hash table, so you can't in general predict the order of elements in the controlled sequence. Inserting elements invalidates no iterators, and removing elements invalidates only those iterators that had specifically pointed at the removed elements.

The iterator provided by the hash_map class is a bidirectional iterator, but the class member functions insert and hash_map have versions that take as template parameters a weaker input iterator, whose functionality requirements are more minimal than those guaranteed by the class of bidirectional iterators. The different iterator concepts form a family related by refinements in their functionality. Each iterator concept has its own set of requirements, and the algorithms that work with them must limit their assumptions to the requirements provided by that type of iterator. It may be assumed that an input iterator may be dereferenced to refer to some object and that it may be incremented to the next iterator in the sequence. This is a minimal set of functionality, but it's enough to be able to talk meaningfully about a range of iterators [First, Last) in the context of the class member functions.

Constructors

Constructor	Description
hash_map	Constructs a hash_map that is empty or that is a copy of all or part of some other hash_map.

Typedefs

Type name	Description
allocator_type	A type that represents the allocator class for the hash_map object.
const_iterator	A type that provides a bidirectional iterator that can read a const element in the hash_map.
const_pointer	A type that provides a pointer to a const element in a hash_map.
const_reference	A type that provides a reference to a const element stored in a hash_map for reading and performing const operations.
const_reverse_iterator	A type that provides a bidirectional iterator that can read any const element in the hash_map.
difference_type	A signed integer type that can be used to represent the number of elements of a hash_map in a range between elements pointed to by iterators.
iterator	A type that provides a bidirectional iterator that can read or modify any element in a hash_map.
key_compare	A type that provides a function object that can compare two sort keys to determine the relative order of two elements in the hash_map.
key_type	A type describes the sort key object that constitutes each element of the hash_map.
mapped_type	A type that represents the data type stored in a hash_map.
pointer	A type that provides a pointer to an element in a hash_map.
reference	A type that provides a reference to an element stored in a hash_map.
reverse_iterator	A type that provides a bidirectional iterator that can read or modify an element in a reversed hash_map.
size_type	An unsigned integer type that can represent the number of elements in a hash_map.
value_type	A type that provides a function object that can compare two elements as sort keys to determine their relative order in the hash_map.

Member functions

Member function	Description
at	Finds an element in a hash_map with a specified key value.
begin	Returns an iterator addressing the first element in the hash_map.
cbegin	Returns a const iterator addressing the first element in the hash_map.
cend	Returns a const iterator that addresses the location succeeding the last element in a hash_map.
clear	Erases all the elements of a hash_map.
count	Returns the number of elements in a hash_map whose key matches a parameter-specified key.
crbegin	Returns a const iterator addressing the first element in a reversed hash_map.
crend	Returns a const iterator that addresses the location succeeding the last element in a reversed hash_map.
emplace	Inserts an element constructed in place into a hash_map.
emplace_hint	Inserts an element constructed in place into a hash_map, with a placement hint.
empty	Tests if a hash_map is empty.
end	Returns an iterator that addresses the location succeeding the last element in a hash_map.
equal_range	Returns a pair of iterators, respectively, to the first element in a hash_map with a key that is greater than a specified key and to the first element in the hash_map with a key that is equal to or greater than the key.
erase	Removes an element or a range of elements in a hash_map from specified positions
find	Returns an iterator addressing the location of an element in a hash_map that has a key equivalent to a specified key.
get_allocator	Returns a copy of the allocator object used to construct the hash_map.
insert	Inserts an element or a range of elements into a hash_map.
key_comp	Returns an iterator to the first element in a hash_map with a key value that is equal to or greater than that of a specified key.
lower_bound	Returns an iterator to the first element in a hash_map with a key value that is equal to or greater than that of a specified key.
max_size	Returns the maximum length of the hash_map.
rbegin	Returns an iterator addressing the first element in a reversed hash_map.
rend	Returns an iterator that addresses the location succeeding the last element in a reversed hash_map.
size	Returns the number of elements in the hash_map.
swap	Exchanges the elements of two hash_maps.
upper_bound	Returns an iterator to the first element in a hash_map that with a key value that is greater than that of a specified key.
value_comp	Retrieves a copy of the comparison object used to order element values in a hash_map.

Operators

Operator	Description
operator[]	Inserts an element into a hash_map with a specified key value.
hash_map::operator=	Replaces the elements of a hash_map with a copy of another hash_map.

Requirements

Header: <hash_map>

Namespace: stdext

hash_map::allocator_type

A type that represents the allocator class for the hash_map object.

typedef list<typename Traits::value_type, typename Traits::allocator_type>::allocator_type allocator_type;

Example

See example for get_allocator for an example using allocator_type.

hash_map::at

Finds an element in a hash_map with a specified key value.

Type& at(const Key& key);

const Type& at(const Key& key) const;

Parameters

key
The key value of the element that is to be found.

Return Value

A reference to the data value of the element found.

Remarks

If the argument key value isn't found, then the function throws an object of class out_of_range Class.

Example

// hash_map_at.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   typedef pair <const int, int> cInt2Int;
   hash_map <int, int> hm1;

   // Insert data values
   hm1.insert ( cInt2Int ( 1, 10 ) );
   hm1.insert ( cInt2Int ( 2, 20 ) );
   hm1.insert ( cInt2Int ( 3, 30 ) );

   cout  << "The values of the mapped elements are:";
   for ( int i = 1 ; i <= hm1.size() ; i++ )
      cout << " " << hm1.at(i);
   cout << "." << endl;
}

hash_map::begin

Returns an iterator addressing the first element in the hash_map.

const_iterator begin() const;

iterator begin();

Return Value

A bidirectional iterator addressing the first element in the hash_map or the location succeeding an empty hash_map.

Example

// hash_map_begin.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;

   hash_map <int, int> :: iterator hm1_Iter;
   hash_map <int, int> :: const_iterator hm1_cIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 0, 0 ) );
   hm1.insert ( Int_Pair ( 1, 1 ) );
   hm1.insert ( Int_Pair ( 2, 4 ) );

   hm1_cIter = hm1.begin ( );
   cout << "The first element of hm1 is "
        << hm1_cIter -> first << "." << endl;

   hm1_Iter = hm1.begin ( );
   hm1.erase ( hm1_Iter );

   // The following 2 lines would err because the iterator is const
   // hm1_cIter = hm1.begin ( );
   // hm1.erase ( hm1_cIter );

   hm1_cIter = hm1.begin( );
   cout << "The first element of hm1 is now "
        << hm1_cIter -> first << "." << endl;
}

The first element of hm1 is 0.
The first element of hm1 is now 1.

hash_map::cbegin

Returns a const iterator addressing the first element in the hash_map.

const_iterator cbegin() const;

Return Value

A const bidirectional iterator addressing the first element in the hash_map or the location succeeding an empty hash_map.

Example

// hash_map_cbegin.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;

   hash_map <int, int> :: const_iterator hm1_cIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 2, 4 ) );

   hm1_cIter = hm1.cbegin ( );
   cout << "The first element of hm1 is "
        << hm1_cIter -> first << "." << endl;
   }

The first element of hm1 is 2.

hash_map::cend

Returns a const iterator that addresses the location succeeding the last element in a hash_map.

const_iterator cend() const;

Return Value

A const bidirectional iterator that addresses the location succeeding the last element in a hash_map. If the hash_map is empty, then hash_map::cend == hash_map::begin.

Remarks

cend is used to test whether an iterator has reached the end of its hash_map.

The value returned by cend shouldn't be dereferenced.

Example

// hash_map_cend.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;

   hash_map <int, int> :: const_iterator hm1_cIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 3, 30 ) );

   hm1_cIter = hm1.cend( );
   hm1_cIter--;
   cout << "The value of last element of hm1 is "
        << hm1_cIter -> second << "." << endl;
   }

The value of last element of hm1 is 30.

hash_map::clear

Erases all the elements of a hash_map.

void clear();

Remarks

Example

The following example demonstrates the use of the hash_map::clear member function.

// hash_map_clear.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
    using namespace std;
    using namespace stdext;
    hash_map<int, int> hm1;
    hash_map<int, int>::size_type i;
    typedef pair<int, int> Int_Pair;

    hm1.insert(Int_Pair(1, 1));
    hm1.insert(Int_Pair(2, 4));

    i = hm1.size();
    cout << "The size of the hash_map is initially "
         << i << "." << endl;

    hm1.clear();
    i = hm1.size();
    cout << "The size of the hash_map after clearing is "
         << i << "." << endl;
}

The size of the hash_map is initially 2.
The size of the hash_map after clearing is 0.

hash_map::const_iterator

A type that provides a bidirectional iterator that can read a const element in the hash_map.

typedef list<typename Traits::value_type, typename Traits::allocator_type>::const_iterator const_iterator;

Remarks

A type const_iterator can't be used to modify the value of an element.

The const_iterator defined by hash_map points to elements that are objects of value_type, that is of type pair< const Key, Type >, whose first member is the key to the element and whose second member is the mapped datum held by the element.

To dereference a const_iterator cIter pointing to an element in a hash_map, use the -> operator.

To access the value of the key for the element, use cIter->first, which is equivalent to (*cIter).first. To access the value of the mapped datum for the element, use cIter->second, which is equivalent to (*cIter).second.

Example

See example for begin for an example using const_iterator.

hash_map::const_pointer

A type that provides a pointer to a const element in a hash_map.

typedef list<typename _Traits::value_type, typename _Traits::allocator_type>::const_pointer const_pointer;

Remarks

A type const_pointer can't be used to modify the value of an element.

In most cases, an iterator should be used to access the elements in a hash_map object.

hash_map::const_reference

A type that provides a reference to a const element stored in a hash_map for reading and performing const operations.

typedef list<typename _Traits::value_type, typename _Traits::allocator_type>::const_reference const_reference;

Remarks

Example

// hash_map_const_ref.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map<int, int> hm1;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );

   // Declare and initialize a const_reference &Ref1
   // to the key of the first element
   const int &Ref1 = ( hm1.begin( ) -> first );

   // The following line would cause an error because the
   // non-const_reference cannot be used to access the key
   // int &Ref1 = ( hm1.begin( ) -> first );

   cout << "The key of the first element in the hash_map is "
        << Ref1 << "." << endl;

   // Declare and initialize a reference &Ref2
   // to the data value of the first element
   int &Ref2 = ( hm1.begin( ) -> second );

   cout << "The data value of the first element in the hash_map is "
        << Ref2 << "." << endl;
}

The key of the first element in the hash_map is 1.
The data value of the first element in the hash_map is 10.

hash_map::const_reverse_iterator

A type that provides a bidirectional iterator that can read any const element in the hash_map.

typedef list<typename Traits::value_type, typename Traits::allocator_type>::const_reverse)iterator const_reverse_iterator;

Remarks

A type const_reverse_iterator can't modify the value of an element and is used to iterate through the hash_map in reverse.

The const_reverse_iterator defined by hash_map points to elements that are objects of value_type, that is of type pair< const Key, Type >, whose first member is the key to the element and whose second member is the mapped datum held by the element.

To dereference a const_reverse_iterator crIter pointing to an element in a hash_map, use the -> operator.

To access the value of the key for the element, use crIter->first, which is equivalent to (*crIter).first. To access the value of the mapped datum for the element, use crIter->second, which is equivalent to (*crIter).first.

Example

See the example for rend for an example of how to declare and use the const_reverse_iterator.

hash_map::count

Returns the number of elements in a hash_map whose key matches a parameter-specified key.

size_type count(const Key& key) const;

Parameters

key
The key value of the elements to be matched from the hash_map.

Return Value

1 if the hash_map contains an element whose sort key matches the parameter key; 0 if the hash_map doesn't contain an element with a matching key.

Remarks

The member function returns the number of elements x in the range

lower_bound(key), upper_bound(key)

which is 0 or 1 in the case of hash_map, which is a unique associative container.

Example

The following example demonstrates the use of the hash_map::count member function.

// hash_map_count.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main()
{
    using namespace std;
    using namespace stdext;
    hash_map<int, int> hm1;
    hash_map<int, int>::size_type i;
    typedef pair<int, int> Int_Pair;

    hm1.insert(Int_Pair (1, 1));
    hm1.insert(Int_Pair (2, 1));
    hm1.insert(Int_Pair (1, 4));
    hm1.insert(Int_Pair (2, 1));

    // Keys must be unique in hash_map, so duplicates are ignored
    i = hm1.count(1);
    cout << "The number of elements in hm1 with a sort key of 1 is: "
         << i << "." << endl;

    i = hm1.count(2);
    cout << "The number of elements in hm1 with a sort key of 2 is: "
         << i << "." << endl;

    i = hm1.count(3);
    cout << "The number of elements in hm1 with a sort key of 3 is: "
         << i << "." << endl;
}

The number of elements in hm1 with a sort key of 1 is: 1.
The number of elements in hm1 with a sort key of 2 is: 1.
The number of elements in hm1 with a sort key of 3 is: 0.

hash_map::crbegin

Returns a const iterator addressing the first element in a reversed hash_map.

const_reverse_iterator crbegin() const;

Return Value

A const reverse bidirectional iterator addressing the first element in a reversed hash_map or addressing what had been the last element in the unreversed hash_map.

Remarks

crbegin is used with a reversed hash_map just as begin is used with a hash_map.

With the return value of crbegin, the hash_map object can't be modified.

crbegin can be used to iterate through a hash_map backwards.

Example

// hash_map_crbegin.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;

   hash_map <int, int> :: const_reverse_iterator hm1_crIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 3, 30 ) );

   hm1_crIter = hm1.crbegin( );
   cout << "The first element of the reversed hash_map hm1 is "
        << hm1_crIter -> first << "." << endl;
}

The first element of the reversed hash_map hm1 is 3.

hash_map::crend

Returns a const iterator that addresses the location succeeding the last element in a reversed hash_map.

const_reverse_iterator crend() const;

Return Value

A const reverse bidirectional iterator that addresses the location succeeding the last element in a reversed hash_map (the location that had preceded the first element in the unreversed hash_map).

Remarks

crend is used with a reversed hash_map just as hash_map::end is used with a hash_map.

With the return value of crend, the hash_map object can't be modified.

crend can be used to test to whether a reverse iterator has reached the end of its hash_map.

The value returned by crend shouldn't be dereferenced.

Example

// hash_map_crend.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;

   hash_map <int, int> :: const_reverse_iterator hm1_crIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 3, 30 ) );

   hm1_crIter = hm1.crend( );
   hm1_crIter--;
   cout << "The last element of the reversed hash_map hm1 is "
        << hm1_crIter -> first << "." << endl;
}

The last element of the reversed hash_map hm1 is 3.

hash_map::difference_type

A signed integer type that can be used to represent the number of elements of a hash_map in a range between elements pointed to by iterators.

typedef list<typename _Traits::value_type, typename _Traits::allocator_type>::difference_type difference_type;

Example

// hash_map_diff_type.cpp
// compile with: /EHsc
#include <iostream>
#include <hash_map>
#include <algorithm>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 2, 20 ) );
   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 3, 20 ) );

   // The following won't insert, because map keys are unique
   hm1.insert ( Int_Pair ( 2, 30 ) );

   hash_map <int, int>::iterator hm1_Iter, hm1_bIter, hm1_eIter;
   hm1_bIter = hm1.begin( );
   hm1_eIter = hm1.end( );

   // Count the number of elements in a hash_map
   hash_map <int, int>::difference_type  df_count = 0;
   hm1_Iter = hm1.begin( );
   while ( hm1_Iter != hm1_eIter)
   {
      df_count++;
      hm1_Iter++;
   }

   cout << "The number of elements in the hash_map hm1 is: "
        << df_count << "." << endl;

   cout  << "The keys of the mapped elements are:";
   for ( hm1_Iter= hm1.begin( ) ; hm1_Iter!= hm1.end( ) ;
         hm1_Iter++)
      cout << " " << hm1_Iter-> first;
   cout << "." << endl;

   cout  << "The values of the mapped elements are:";
   for ( hm1_Iter= hm1.begin( ) ; hm1_Iter!= hm1.end( ) ;
         hm1_Iter++)
      cout << " " << hm1_Iter-> second;
   cout << "." << endl;
}

The number of elements in the hash_map hm1 is: 3.
The keys of the mapped elements are: 1 2 3.
The values of the mapped elements are: 10 20 20.

hash_map::emplace

Inserts an element constructed in place into a hash_map.

template <class ValTy>
pair <iterator, bool>
emplace(
    ValTy&& val);

Parameters

val
The value used to move construct an element to be inserted into the hash_map unless the hash_map already contains that element (or, more generally, an element whose key is equivalently ordered).

Return Value

The emplace member function returns a pair whose bool component returns true if an insertion was made and false if the hash_map already contained an element whose key had an equivalent value in the ordering, and whose iterator component returns the address where a new element was inserted or where the element was already located.

To access the iterator component of a pair pr returned by this member function, use pr.first, and to dereference it, use *(pr.first). To access the bool component of a pair pr returned by this member function, use pr.second, and to dereference it, use *(pr.second).

Remarks

The hash_map::value_type of an element is a pair, so that the value of an element will be an ordered pair with the first component equal to the key value and the second component equal to the data value of the element.

Example

// hash_map_emplace.cpp
// compile with: /EHsc
#include<hash_map>
#include<iostream>
#include <string>

int main()
{
    using namespace std;
    using namespace stdext;
    hash_map<int, string> hm1;
    typedef pair<int, string> is1(1, "a");

    hm1.emplace(move(is1));
    cout << "After the emplace insertion, hm1 contains:" << endl
      << " " << hm1.begin()->first
      << " => " << hm1.begin()->second
      << endl;
}

After the emplace insertion, hm1 contains:
1 => a

hash_map::emplace_hint

Inserts an element constructed in place into a hash_map, with a placement hint.

template <class ValTy>
iterator emplace_hint(
    const_iterator _Where,
    ValTy&& val);

Parameters

val
The value used to move construct an element to be inserted into the hash_map unless the hash_map already contains that element (or, more generally, an element whose key is equivalently ordered).

c
A hint regarding the place to start searching for the correct point of insertion.

Return Value

The hash_multimap::emplace member function returns an iterator that points to the position where the new element was inserted into the hash_map, or where the existing element with equivalent ordering is located.

Remarks

The hash_map::value_type of an element is a pair, so that the value of an element will be an ordered pair with the first component equal to the key value and the second component equal to the data value of the element.

Insertion can occur in amortized constant time, instead of logarithmic time, if the insertion point immediately follows _Where.

Example

// hash_map_emplace_hint.cpp
// compile with: /EHsc
#include<hash_map>
#include<iostream>
#include <string>

int main()
{
    using namespace std;
    using namespace stdext;
    hash_map<int, string> hm1;
    typedef pair<int, string> is1(1, "a");

    hm1.emplace(hm1.begin(), move(is1));
    cout << "After the emplace, hm1 contains:" << endl
      << " " << hm1.begin()->first
      << " => " << hm1.begin()->second
      << endl;
}

After the emplace insertion, hm1 contains:
1 => a

hash_map::empty

Tests if a hash_map is empty.

bool empty() const;

Return Value

true if the hash_map is empty; false if the hash_map is nonempty.

Remarks

Example

// hash_map_empty.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1, hm2;

   typedef pair <int, int> Int_Pair;
   hm1.insert ( Int_Pair ( 1, 1 ) );

   if ( hm1.empty( ) )
      cout << "The hash_map hm1 is empty." << endl;
   else
      cout << "The hash_map hm1 is not empty." << endl;

   if ( hm2.empty( ) )
      cout << "The hash_map hm2 is empty." << endl;
   else
      cout << "The hash_map hm2 is not empty." << endl;
}

The hash_map hm1 is not empty.
The hash_map hm2 is empty.

hash_map::end

Returns an iterator that addresses the location succeeding the last element in a hash_map.

const_iterator end() const;

iterator end();

Return Value

A bidirectional iterator that addresses the location succeeding the last element in a hash_map. If the hash_map is empty, then hash_map::end == hash_map::begin.

Remarks

end is used to test whether an iterator has reached the end of its hash_map.

The value returned by end shouldn't be dereferenced.

Example

// hash_map_end.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;

   hash_map <int, int> :: iterator hm1_Iter;
   hash_map <int, int> :: const_iterator hm1_cIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );
   hm1.insert ( Int_Pair ( 3, 30 ) );

   hm1_cIter = hm1.end( );
   hm1_cIter--;
   cout << "The value of last element of hm1 is "
        << hm1_cIter -> second << "." << endl;

   hm1_Iter = hm1.end( );
   hm1_Iter--;
   hm1.erase ( hm1_Iter );

   // The following 2 lines would err because the iterator is const
   // hm1_cIter = hm1.end ( );
   // hm1_cIter--;
   // hm1.erase ( hm1_cIter );

   hm1_cIter = hm1.end( );
   hm1_cIter--;
   cout << "The value of last element of hm1 is now "
        << hm1_cIter -> second << "." << endl;
}

The value of last element of hm1 is 30.
The value of last element of hm1 is now 20.

hash_map::equal_range

Returns a pair of iterators respectively to the first element in a hash_map with a key that is greater than a specified key and to the first element in the hash_map with a key that is equal to or greater than the key.

pair <const_iterator, const_iterator> equal_range (const Key& key) const;

pair <iterator, iterator> equal_range (const Key& key);

Parameters

key
The argument key value to be compared with the sort key of an element from the hash_map being searched.

Return Value

A pair of iterators such that the first is the lower_bound of the key and the second is the upper_bound of the key.

To access the first iterator of a pair pr returned by the member function, use pr.first and to dereference the lower bound iterator, use *(pr.first). To access the second iterator of a pair pr returned by the member function, use pr.second and to dereference the upper bound iterator, use *(pr.second).

Remarks

Example

// hash_map_equal_range.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   typedef hash_map <int, int> IntMap;
   IntMap hm1;
   hash_map <int, int> :: const_iterator hm1_RcIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );
   hm1.insert ( Int_Pair ( 3, 30 ) );

   pair <IntMap::const_iterator, IntMap::const_iterator> p1, p2;
   p1 = hm1.equal_range( 2 );

   cout << "The lower bound of the element with "
        << "a key of 2 in the hash_map hm1 is: "
        << p1.first -> second << "." << endl;

   cout << "The upper bound of the element with "
        << "a key of 2 in the hash_map hm1 is: "
        << p1.second -> second << "." << endl;

   // Compare the upper_bound called directly
   hm1_RcIter = hm1.upper_bound( 2 );

   cout << "A direct call of upper_bound( 2 ) gives "
        << hm1_RcIter -> second << "," << endl
        << "matching the 2nd element of the pair"
        << " returned by equal_range( 2 )." << endl;

   p2 = hm1.equal_range( 4 );

   // If no match is found for the key,
   // both elements of the pair return end( )
   if ( ( p2.first == hm1.end( ) ) && ( p2.second == hm1.end( ) ) )
      cout << "The hash_map hm1 doesn't have an element "
             << "with a key less than 40." << endl;
   else
      cout << "The element of hash_map hm1 with a key >= 40 is: "
           << p1.first -> first << "." << endl;
}

The lower bound of the element with a key of 2 in the hash_map hm1 is: 20.
The upper bound of the element with a key of 2 in the hash_map hm1 is: 30.
A direct call of upper_bound( 2 ) gives 30,
matching the 2nd element of the pair returned by equal_range( 2 ).
The hash_map hm1 doesn't have an element with a key less than 40.

hash_map::erase

Removes an element or a range of elements in a hash_map from specified positions or removes elements that match a specified key.

iterator erase(iterator _Where);

iterator erase(iterator first, iterator last);

size_type erase(const key_type& key);

Parameters

_Where
Position of the element to be removed from the hash_map.

first
Position of the first element removed from the hash_map.

last
Position just beyond the last element removed from the hash_map.

key
The key value of the elements to be removed from the hash_map.

Return Value

For the first two member functions, a bidirectional iterator that designates the first element remaining beyond any elements removed, or a pointer to the end of the hash_map if no such element exists.

For the third member function, returns the number of elements that have been removed from the hash_map.

Remarks

The member functions never throw an exception.

Example

The following example demonstrates the use of the hash_map::erase member function.

// hash_map_erase.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main()
{
    using namespace std;
    using namespace stdext;
    hash_map<int, int> hm1, hm2, hm3;
    hash_map<int, int> :: iterator pIter, Iter1, Iter2;
    int i;
    hash_map<int, int>::size_type n;
    typedef pair<int, int> Int_Pair;

    for (i = 1; i < 5; i++)
    {
        hm1.insert(Int_Pair (i, i));
        hm2.insert(Int_Pair (i, i*i));
        hm3.insert(Int_Pair (i, i-1));
    }

    // The 1st member function removes an element at a given position
    Iter1 = ++hm1.begin();
    hm1.erase(Iter1);

    cout << "After the 2nd element is deleted, the hash_map hm1 is:";
    for (pIter = hm1.begin(); pIter != hm1.end(); pIter++)
        cout << " " << pIter -> second;
    cout << "." << endl;

    // The 2nd member function removes elements
    // in the range [ first,  last)
    Iter1 = ++hm2.begin();
    Iter2 = --hm2.end();
    hm2.erase(Iter1, Iter2);

    cout << "After the middle two elements are deleted, "
         << "the hash_map hm2 is:";
    for (pIter = hm2.begin(); pIter != hm2.end(); pIter++)
        cout << " " << pIter -> second;
    cout << "." << endl;

    // The 3rd member function removes elements with a given  key
    n = hm3.erase(2);

    cout << "After the element with a key of 2 is deleted,\n"
         << "the hash_map hm3 is:";
    for (pIter = hm3.begin(); pIter != hm3.end(); pIter++)
        cout << " " << pIter -> second;
    cout << "." << endl;

    // The 3rd member function returns the number of elements removed
    cout << "The number of elements removed from hm3 is: "
         << n << "." << endl;

    // The dereferenced iterator can also be used to specify a key
    Iter1 = ++hm3.begin();
    hm3.erase(Iter1);

    cout << "After another element with a key equal to that"
         << endl;
    cout  << "of the 2nd element is deleted, "
          << "the hash_map hm3 is:";
    for (pIter = hm3.begin(); pIter != hm3.end(); pIter++)
        cout << " " << pIter -> second;
    cout << "." << endl;
}

After the 2nd element is deleted, the hash_map hm1 is: 1 3 4.
After the middle two elements are deleted, the hash_map hm2 is: 1 16.
After the element with a key of 2 is deleted,
the hash_map hm3 is: 0 2 3.
The number of elements removed from hm3 is: 1.
After another element with a key equal to that
of the 2nd element is deleted, the hash_map hm3 is: 0 3.

hash_map::find

Returns an iterator addressing the location of an element in a hash_map that has a key equivalent to a specified key.

iterator find(const Key& key);

const_iterator find(const Key& key) const;

Parameters

key
The key value to be matched by the sort key of an element from the hash_map being searched.

Return Value

An iterator that addresses the location of an element with a specified key, or the location succeeding the last element in the hash_map if no match is found for the key.

Remarks

find returns an iterator that addresses an element in the hash_map whose sort key is equivalent to the argument key under a binary predicate that induces an ordering based on a less than comparability relation.

If the return value of find is assigned to a const_iterator, the hash_map object can't be modified. If the return value of find is assigned to an iterator, the hash_map object can be modified

Example

// hash_map_find.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;
   hash_map <int, int> :: const_iterator hm1_AcIter, hm1_RcIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );
   hm1.insert ( Int_Pair ( 3, 30 ) );

   hm1_RcIter = hm1.find( 2 );
   cout << "The element of hash_map hm1 with a key of 2 is: "
        << hm1_RcIter -> second << "." << endl;

   // If no match is found for the key, end( ) is returned
   hm1_RcIter = hm1.find( 4 );

   if ( hm1_RcIter == hm1.end( ) )
      cout << "The hash_map hm1 doesn't have an element "
           << "with a key of 4." << endl;
   else
      cout << "The element of hash_map hm1 with a key of 4 is: "
           << hm1_RcIter -> second << "." << endl;

   // The element at a specific location in the hash_map can be found
   // using a dereferenced iterator addressing the location
   hm1_AcIter = hm1.end( );
   hm1_AcIter--;
   hm1_RcIter = hm1.find( hm1_AcIter -> first );
   cout << "The element of hm1 with a key matching "
        << "that of the last element is: "
        << hm1_RcIter -> second << "." << endl;
}

The element of hash_map hm1 with a key of 2 is: 20.
The hash_map hm1 doesn't have an element with a key of 4.
The element of hm1 with a key matching that of the last element is: 30.

hash_map::get_allocator

Returns a copy of the allocator object used to construct the hash_map.

Allocator get_allocator() const;

Return Value

The allocator used by the hash_map.

Remarks

Allocators for the hash_map class specify how the class manages storage. The default allocators supplied with C++ Standard Library container classes are sufficient for most programming needs. Writing and using your own allocator class is an advanced C++ topic.

Example

// hash_map_get_allocator.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int>::allocator_type hm1_Alloc;
   hash_map <int, int>::allocator_type hm2_Alloc;
   hash_map <int, double>::allocator_type hm3_Alloc;
   hash_map <int, int>::allocator_type hm4_Alloc;

   // The following lines declare objects
   // that use the default allocator.
   hash_map <int, int> hm1;
   hash_map <int, int> hm2;
   hash_map <int, double> hm3;

   hm1_Alloc = hm1.get_allocator( );
   hm2_Alloc = hm2.get_allocator( );
   hm3_Alloc = hm3.get_allocator( );

   cout << "The number of integers that can be allocated"
        << endl << "before free memory is exhausted: "
        << hm2.max_size( ) << "." << endl;

   cout << "The number of doubles that can be allocated"
        << endl << "before free memory is exhausted: "
        << hm3.max_size( ) <<  "." << endl;

   // The following line creates a hash_map hm4
   // with the allocator of hash_map hm1.
   hash_map <int, int> hm4( less<int>( ), hm1_Alloc );

   hm4_Alloc = hm4.get_allocator( );

   // Two allocators are interchangeable if
   // storage allocated from each can be
   // deallocated with the other
   if( hm1_Alloc == hm4_Alloc )
   {
      cout << "The allocators are interchangeable."
           << endl;
   }
   else
   {
      cout << "The allocators are not interchangeable."
           << endl;
   }
}

hash_map::hash_map

Constructs a hash_map that is empty or is a copy of all or part of some other hash_map.

hash_map();

explicit hash_map(
    const Traits& Comp);

hash_map(
    const Traits& Comp,
    const Allocator& Al);

hash_map(
    const hash_map& Right);

hash_map(
    hash_map&& Right);

hash_map(
    initializer_list<Type> IList);hash_map(initializer_list<Type> IList,
    const key_compare& Comp);

hash_map(
    initializer_list<Type> IList,
    const key_compare& Comp,
    const allocator_type& Al);

template <class InputIterator>
hash_map(
    InputIterator First,
    InputIterator Last);

template <class InputIterator>
hash_map(
    InputIterator First,
    InputIterator Last,
    const Traits& Comp);

template <class InputIterator>
hash_map(
    InputIterator First,
    InputIterator Last,
    const Traits& Comp,
    const Allocator& Al

Parameters

Al
The storage allocator class to be used for this hash_map object, which defaults to Allocator.

Comp
The comparison function of type const Traits used to order the elements in the hash_map, which defaults to hash_compare.

Right
The hash_map of which the constructed map is to be a copy.

First
The position of the first element in the range of elements to be copied.

Last
The position of the first element beyond the range of elements to be copied.

IList
The initializer_list

Remarks

All constructors store a type of allocator object that manages memory storage for the hash_map and can later be returned by calling get_allocator. The allocator parameter is often omitted in the class declarations and preprocessing macros used to substitute alternative allocators.

All constructors initialize their hash_map.

All constructors store a function object of type Traits that is used to establish an order among the keys of the hash_map and that can later be returned by calling key_comp.

The first three constructors specify an empty initial hash_map, in addition, the second specifies the type of comparison function (Comp) to be used in establishing the order of the elements and the third explicitly specifies the allocator type (Al) to be used. The keyword explicit suppresses certain kinds of automatic type conversion.

The fourth constructor specifies a copy of the hash_map Right.

The next three constructors copy the range [First, Last) of a hash_map with increasing explicitness in specifying the type of comparison function of class Traits and allocator.

The last constructor moves the hash_map Right.

hash_map::insert

Inserts an element or a range of elements into a hash_map.

pair <iterator, bool> insert(
    const value_type& val);

iterator insert(
    const_iterator _Where,
    const value_type& val);

template <class InputIterator>
void insert(
    InputIterator first,
    InputIterator last);

template <class ValTy>
pair <iterator, bool>
insert(
    ValTy&& val);

template <class ValTy>
iterator insert(
    const_iterator _Where,
    ValTy&& val);

Parameters

val
The value of an element to be inserted into the hash_map unless the hash_map already contains that element (or, more generally, an element whose key is equivalently ordered).

_Where
A hint regarding the place to start searching for the correct point of insertion.

first
The position of the first element to be copied from a hash_map.

last
The position just beyond the last element to be copied from a hash_map.

Return Value

The first insert member function returns a pair whose bool component returns true if an insertion was made and false if the hash_map already contained an element whose key had an equivalent value in the ordering, and whose iterator component returns the address where a new element was inserted or where the element was already located.

To access the iterator component of a pair pr returned by this member function, use pr.first, and to dereference it, use (pr.first). To access the bool component of a pair pr returned by this member function, use pr.second, and to dereference it, use \(pr.second).

The second insert member function, the hint version, returns an iterator that points to the position where the new element was inserted into the hash_map.

The last two insert member functions behave the same as the first two, except that they move construct the inserted value.

Remarks

The value_type of an element is a pair, so that the value of an element will be an ordered pair with the first component equal to the key value and the second component equal to the data value of the element.

Insertion can occur in amortized constant time for the hint version of insert, instead of logarithmic time, if the insertion point immediately follows _Where.

The third member function inserts the sequence of element values into a hash_map corresponding to each element addressed by an iterator of in the range *[First, Last)* of a specified set.

Example

// hash_map_insert.cpp
// compile with: /EHsc
#include<hash_map>
#include<iostream>
#include <string>

int main()
{
    using namespace std;
    using namespace stdext;
    hash_map<int, int>::iterator hm1_pIter, hm2_pIter;

    hash_map<int, int> hm1, hm2;
    typedef pair<int, int> Int_Pair;

    hm1.insert(Int_Pair(1, 10));
    hm1.insert(Int_Pair(2, 20));
    hm1.insert(Int_Pair(3, 30));
    hm1.insert(Int_Pair(4, 40));

    cout << "The original elements (Key => Value) of hm1 are:";
    for (hm1_pIter = hm1.begin(); hm1_pIter != hm1.end(); hm1_pIter++)
        cout << endl << " " << hm1_pIter -> first << " => "
             << hm1_pIter->second;
    cout << endl;

    pair< hash_map<int,int>::iterator, bool > pr;
    pr = hm1.insert(Int_Pair(1, 10));

    if (pr.second == true)
    {
        cout << "The element 10 was inserted in hm1 successfully."
            << endl;
    }
    else
    {
        cout << "The element 10 already exists in hm1\n"
            << "with a key value of "
            << "((pr.first) -> first) = " << (pr.first)->first
            << "." << endl;
    }

    // The hint version of insert
    hm1.insert(--hm1.end(), Int_Pair(5, 50));

    cout << "After the insertions, the elements of hm1 are:";
    for (hm1_pIter = hm1.begin(); hm1_pIter != hm1.end(); hm1_pIter++)
        cout << endl << hm1_pIter -> first << " => "
             << hm1_pIter->second;
    cout << endl;

    hm2.insert(Int_Pair(10, 100));

    // The templatized version inserting a range
    hm2.insert( ++hm1.begin(), --hm1.end() );

    cout << "After the insertions, the elements of hm2 are:";
    for (hm2_pIter = hm2.begin(); hm2_pIter != hm2.end(); hm2_pIter++)
        cout << endl << hm2_pIter -> first << " => "
             << hm2_pIter->second;
    cout << endl;

    // The templatized versions move constructing elements
    hash_map<int, string> hm3, hm4;
    pair<int, string> is1(1, "a"), is2(2, "b");

    hm3.insert(move(is1));
    cout << "After the move insertion, hm3 contains:" << endl
      << hm3.begin()->first
      << " => " << hm3.begin()->second
      << endl;

    hm4.insert(hm4.begin(), move(is2));
    cout << "After the move insertion, hm4 contains:" << endl
      << hm4.begin()->first
      << " => " << hm4.begin()->second
      << endl;
}

The original elements (Key => Value) of hm1 are:
1 => 10
2 => 20
3 => 30
4 => 40
The element 10 already exists in hm1
with a key value of ((pr.first) -> first) = 1.
After the insertions, the elements of hm1 are:
1 => 10
2 => 20
3 => 30
4 => 40
5 => 50
After the insertions, the elements of hm2 are:
2 => 20
10 => 100
3 => 30
4 => 40
After the move insertion, hm3 contains:
1 => a
After the move insertion, hm4 contains:
2 => b

hash_map::iterator

A type that provides a bidirectional iterator that can read or modify any element in a hash_map.

typedef list<typename Traits::value_type, typename Traits::allocator_type>::iterator iterator;

Remarks

The iterator defined by hash_map points to elements that are objects of value_type, that is of type pair<const Key, Type>, whose first member is the key to the element and whose second member is the mapped datum held by the element.

To dereference an iterator named Iter pointing to an element in a multimap, use the -> operator.

To access the value of the key for the element, use Iter->first, which is equivalent to (*Iter).first. To access the value of the mapped datum for the element, use Iter->second, which is equivalent to (*Iter).second.

A type iterator can be used to modify the value of an element.

Example

See example for begin for an example of how to declare and use the iterator.

hash_map::key_comp

Retrieves a copy of the comparison object used to order keys in a hash_map.

key_compare key_comp() const;

Return Value

Returns the function object that a hash_map uses to order its elements.

Remarks

The stored object defines the member function

bool operator( const Key& left, const Key&right );

that returns true if left precedes and isn't equal to right in the sort order.

Example

// hash_map_key_comp.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;

   hash_map <int, int, hash_compare<int, less<int> > > hm1;
   hash_map <int, int, hash_compare<int, less<int> > >::key_compare
      kc1 = hm1.key_comp( ) ;

   // Operator stored in kc1 tests order & returns bool value
   bool result1 = kc1( 2, 3 ) ;
   if( result1 == true )
   {
      cout << "kc1( 2,3 ) returns value of true,"
           << "\n where kc1 is the function object of hm1"
           << " of type key_compare." << endl;
   }
   else
   {
      cout << "kc1( 2,3 ) returns value of false"
           << "\n where kc1 is the function object of hm1"
           << " of type key_compare." << endl;
   }

   hash_map <int, int, hash_compare<int, greater<int> > > hm2;
   hash_map <int, int, hash_compare<int, greater<int> > >
      ::key_compare kc2 = hm2.key_comp( );

   // Operator stored in kc2 tests order & returns bool value
   bool result2 = kc2( 2, 3 ) ;
   if( result2 == true )
   {
      cout << "kc2( 2,3 ) returns value of true,"
           << "\n where kc2 is the function object of hm2"
           << " of type key_compare." << endl;
   }
   else
   {
      cout << "kc2( 2,3 ) returns value of false,"
           << "\n where kc2 is the function object of hm2"
           << " of type key_compare." << endl;
   }
}

hash_map::key_compare

A type that provides a function object that can compare two sort keys to determine the relative order of two elements in the map.

typedef Traits key_compare;

Remarks

key_compare is a synonym for the template parameter Traits.

For more information on Traits see the hash_map Class topic.

Example

See example for key_comp for an example of how to declare and use key_compare.

hash_map::key_type

A type describes the sort key object that constitutes each element of the hash_map.

typedef Key key_type;

Remarks

key_type is a synonym for the template parameter Key.

For more information on Key, see the Remarks section of the hash_map Class topic.

Example

See example for value_type for an example of how to declare and use key_type.

hash_map::lower_bound

Returns an iterator to the first element in a hash_map with a key value that is equal to or greater than that of a specified key.

iterator lower_bound(const Key& key);

const_iterator lower_bound(const Key& key) const;

Parameters

key
The argument key value to be compared with the sort key of an element from the hash_map being searched.

Return Value

An iterator or const_iterator that addresses the location of an element in a hash_map that with a key that is equal to or greater than the argument key, or that addresses the location succeeding the last element in the hash_map if no match is found for the key.

If the return value of lower_bound is assigned to a const_iterator, the hash_map object can't be modified. If the return value of lower_bound is assigned to a iterator, the hash_map object can be modified.

Remarks

Example

// hash_map_lower_bound.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;
   hash_map <int, int> :: const_iterator hm1_AcIter, hm1_RcIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );
   hm1.insert ( Int_Pair ( 3, 30 ) );

   hm1_RcIter = hm1.lower_bound( 2 );
   cout << "The first element of hash_map hm1 with a key of 2 is: "
        << hm1_RcIter -> second << "." << endl;

   // If no match is found for the key, end( ) is returned
   hm1_RcIter = hm1. lower_bound ( 4 );

   if ( hm1_RcIter == hm1.end( ) )
      cout << "The hash_map hm1 doesn't have an element "
           << "with a key of 4." << endl;
   else
      cout << "The element of hash_map hm1 with a key of 4 is: "
           << hm1_RcIter -> second << "." << endl;

   // An element at a specific location in the hash_map can be
   // found using a dereferenced iterator addressing the location
   hm1_AcIter = hm1.end( );
   hm1_AcIter--;
   hm1_RcIter = hm1. lower_bound ( hm1_AcIter -> first );
   cout << "The element of hm1 with a key matching "
        << "that of the last element is: "
        << hm1_RcIter -> second << "." << endl;
}

The first element of hash_map hm1 with a key of 2 is: 20.
The hash_map hm1 doesn't have an element with a key of 4.
The element of hm1 with a key matching that of the last element is: 30.

hash_map::mapped_type

A type that represents the data type stored in a hash_map.

typedef Type mapped_type;

Remarks

The type mapped_type is a synonym for the template parameter Type.

For more information on Type see the hash_map Class topic.

Example

See example for value_type for an example of how to declare and use key_type.

hash_map::max_size

Returns the maximum length of the hash_map.

size_type max_size() const;

Return Value

The maximum possible length of the hash_map.

Remarks

Example

// hash_map_max_size.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;
   hash_map <int, int> :: size_type i;

   i = hm1.max_size( );
   cout << "The maximum possible length "
        << "of the hash_map is " << i << "."
        << endl << "(Magnitude is machine specific.)";
}

hash_map::operator[]

Inserts an element into a hash_map with a specified key value.

Type& operator[](const Key& key);

Type& operator[](Key&& key);

Parameters

key
The key value of the element that is to be inserted.

Return Value

A reference to the data value of the inserted element.

Remarks

If the argument key value isn't found, then it's inserted along with the default value of the data type.

operator[] may be used to insert elements into a hash_map m using

m[ key] = DataValue;

where DataValue is the value of the mapped_type of the element with a key value of key.

When using operator[] to insert elements, the returned reference doesn't indicate whether an insertion is changing a pre-existing element or creating a new one. The member functions find and insert can be used to determine whether an element with a specified key is already present before an insertion.

Example

// hash_map_op_ref.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>
#include <string>

int main( )
{
   using namespace std;
   using namespace stdext;
   typedef pair <const int, int> cInt2Int;
   hash_map <int, int> hm1;
   hash_map <int, int> :: iterator pIter;

   // Insert a data value of 10 with a key of 1
   // into a hash_map using the operator[] member function
   hm1[ 1 ] = 10;

   // Compare other ways to insert objects into a hash_map
   hm1.insert ( hash_map <int, int> :: value_type ( 2, 20 ) );
   hm1.insert ( cInt2Int ( 3, 30 ) );

   cout  << "The keys of the mapped elements are:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> first;
   cout << "." << endl;

   cout  << "The values of the mapped elements are:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> second;
   cout << "." << endl;

   // If the key already exists, operator[]
   // changes the value of the datum in the element
   hm1[ 2 ] = 40;

   // operator[] will also insert the value of the data
   // type's default constructor if the value is unspecified
   hm1[5];

   cout  << "The keys of the mapped elements are now:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> first;
   cout << "." << endl;

   cout  << "The values of the mapped elements are now:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> second;
   cout << "." << endl;

   // operator[] will also insert by moving a key
   hash_map <string, int> hm2;
   string str("a");
   hm2[move(str)] = 1;
   cout << "The moved key is " << hm2.begin()->first
      << ", with value " << hm2.begin()->second << endl;
}

hash_map::operator=

Replaces the elements of the hash_map with a copy of another hash_map.

hash_map& operator=(const hash_map& right);

hash_map& operator=(hash_map&& right);

Parameters

right
The right Class being copied into the hash_map.

Remarks

After erasing any existing elements in a hash_map, operator= either copies or moves the contents of right into the hash_map.

Example

// hash_map_operator_as.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map<int, int> v1, v2, v3;
   hash_map<int, int>::iterator iter;

   v1.insert(pair<int, int>(1, 10));

   cout << "v1 = " ;
   for (iter = v1.begin(); iter != v1.end(); iter++)
      cout << iter->second << " ";
   cout << endl;

   v2 = v1;
   cout << "v2 = ";
   for (iter = v2.begin(); iter != v2.end(); iter++)
      cout << iter->second << " ";
   cout << endl;

// move v1 into v2
   v2.clear();
   v2 = move(v1);
   cout << "v2 = ";
   for (iter = v2.begin(); iter != v2.end(); iter++)
      cout << iter->second << " ";
   cout << endl;
}

hash_map::pointer

A type that provides a pointer to an element in a hash_map.

typedef list<typename _Traits::value_type, typename _Traits::allocator_type>::pointer pointer;

Remarks

A type pointer can be used to modify the value of an element.

In most cases, an iterator should be used to access the elements in a hash_map object.

hash_map::rbegin

Returns an iterator addressing the first element in a reversed hash_map.

const_reverse_iterator rbegin() const;

reverse_iterator rbegin();

Return Value

A reverse bidirectional iterator addressing the first element in a reversed hash_map or addressing what had been the last element in the unreversed hash_map.

Remarks

rbegin is used with a reversed hash_map just as begin is used with a hash_map.

If the return value of rbegin is assigned to a const_reverse_iterator, then the hash_map object can't be modified. If the return value of rbegin is assigned to a reverse_iterator, then the hash_map object can be modified.

rbegin can be used to iterate through a hash_map backwards.

Example

// hash_map_rbegin.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;

   hash_map <int, int> :: iterator hm1_Iter;
   hash_map <int, int> :: reverse_iterator hm1_rIter;
   hash_map <int, int> :: const_reverse_iterator hm1_crIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );
   hm1.insert ( Int_Pair ( 3, 30 ) );

   hm1_rIter = hm1.rbegin( );
   cout << "The first element of the reversed hash_map hm1 is "
        << hm1_rIter -> first << "." << endl;

   // begin can be used to start an iteration
   // through a hash_map in a forward order
   cout << "The hash_map is: ";
   for ( hm1_Iter = hm1.begin( ) ; hm1_Iter != hm1.end( ); hm1_Iter++)
      cout << hm1_Iter -> first << " ";
      cout << "." << endl;

   // rbegin can be used to start an iteration
   // through a hash_map in a reverse order
   cout << "The reversed hash_map is: ";
   for ( hm1_rIter = hm1.rbegin( ) ; hm1_rIter != hm1.rend( ); hm1_rIter++)
      cout << hm1_rIter -> first << " ";
      cout << "." << endl;

   // A hash_map element can be erased by dereferencing to its key
   hm1_rIter = hm1.rbegin( );
   hm1.erase ( hm1_rIter -> first );

   hm1_rIter = hm1.rbegin( );
   cout << "After the erasure, the first element "
        << "in the reversed hash_map is "
        << hm1_rIter -> first << "." << endl;
}

The first element of the reversed hash_map hm1 is 3.
The hash_map is: 1 2 3 .
The reversed hash_map is: 3 2 1 .
After the erasure, the first element in the reversed hash_map is 2.

hash_map::reference

A type that provides a reference to an element stored in a hash_map.

typedef list<typename _Traits::value_type, typename _Traits::allocator_type>::reference reference;

Remarks

Example

// hash_map_reference.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );

   // Declare and initialize a const_reference &Ref1
   // to the key of the first element
   const int &Ref1 = ( hm1.begin( ) -> first );

   // The following line would cause an error as the
   // non-const_reference cannot be used to access the key
   // int &Ref1 = ( hm1.begin( ) -> first );

   cout << "The key of first element in the hash_map is "
        << Ref1 << "." << endl;

   // Declare and initialize a reference &Ref2
   // to the data value of the first element
   int &Ref2 = ( hm1.begin( ) -> second );

   cout << "The data value of first element in the hash_map is "
        << Ref2 << "." << endl;

   // The non-const_reference can be used to modify the
   // data value of the first element
   Ref2 = Ref2 + 5;
   cout << "The modified data value of first element is "
        << Ref2 << "." << endl;
}

The key of first element in the hash_map is 1.
The data value of first element in the hash_map is 10.
The modified data value of first element is 15.

hash_map::rend

Returns an iterator that addresses the location succeeding the last element in a reversed hash_map.

const_reverse_iterator rend() const;

reverse_iterator rend();

Return Value

A reverse bidirectional iterator that addresses the location succeeding the last element in a reversed hash_map (the location that had preceded the first element in the unreversed hash_map).

Remarks

rend is used with a reversed hash_map just as end is used with a hash_map.

If the return value of rend is assigned to a const_reverse_iterator, then the hash_map object can't be modified. If the return value of rend is assigned to a reverse_iterator, then the hash_map object can be modified.

rend can be used to test to whether a reverse iterator has reached the end of its hash_map.

The value returned by rend shouldn't be dereferenced.

Example

// hash_map_rend.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;

   hash_map <int, int> :: iterator hm1_Iter;
   hash_map <int, int> :: reverse_iterator hm1_rIter;
   hash_map <int, int> :: const_reverse_iterator hm1_crIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );
   hm1.insert ( Int_Pair ( 3, 30 ) );

   hm1_rIter = hm1.rend( );
   hm1_rIter--;
   cout << "The last element of the reversed hash_map hm1 is "
        << hm1_rIter -> first << "." << endl;

   // begin can be used to start an iteration
   // through a hash_map in a forward order
   cout << "The hash_map is: ";
   for ( hm1_Iter = hm1.begin( ) ; hm1_Iter != hm1.end( );
   hm1_Iter++)
      cout << hm1_Iter -> first << " ";
      cout << "." << endl;

   // rbegin can be used to start an iteration
   // through a hash_map in a reverse order
   cout << "The reversed hash_map is: ";
   for ( hm1_rIter = hm1.rbegin( ) ; hm1_rIter != hm1.rend( );
      hm1_rIter++)
      cout << hm1_rIter -> first << " ";
      cout << "." << endl;

   // A hash_map element can be erased by dereferencing to its key
   hm1_rIter = --hm1.rend( );
   hm1.erase ( hm1_rIter -> first );

   hm1_rIter = hm1.rend( );
   hm1_rIter--;
   cout << "After the erasure, the last element "
        << "in the reversed hash_map is "
        << hm1_rIter -> first << "." << endl;
}

The last element of the reversed hash_map hm1 is 1.
The hash_map is: 1 2 3 .
The reversed hash_map is: 3 2 1 .
After the erasure, the last element in the reversed hash_map is 2.

hash_map::reverse_iterator

A type that provides a bidirectional iterator that can read or modify an element in a reversed hash_map.

typedef list<typename Traits::value_type, typename Traits::allocator_type>::reverse_iterator reverse_iterator;

Remarks

A type reverse_iterator can't modify the value of an element and is used to iterate through the hash_map in reverse.

The reverse_iterator defined by hash_map points to elements that are objects of value_type, that is of type pair<const Key, Type>, whose first member is the key to the element and whose second member is the mapped datum held by the element.

To dereference a reverse_iterator rIter pointing to an element in a hash_map, use the -> operator.

To access the value of the key for the element, use rIter->first, which is equivalent to (*rIter).first. To access the value of the mapped datum for the element, use rIter->second, which is equivalent to (*rIter).first.

Example

See example for rbegin for an example of how to declare and use reverse_iterator.

hash_map::size

Returns the number of elements in the hash_map.

size_type size() const;

Return Value

The current length of the hash_map.

Remarks

Example

The following example demonstrates the use of the hash_map::size member function.

// hash_map_size.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
    using namespace std;
    using namespace stdext;
    hash_map<int, int> hm1, hm2;
    hash_map<int, int>::size_type i;
    typedef pair<int, int> Int_Pair;

    hm1.insert(Int_Pair(1, 1));
    i = hm1.size();
    cout << "The hash_map length is " << i << "." << endl;

    hm1.insert(Int_Pair(2, 4));
    i = hm1.size();
    cout << "The hash_map length is now " << i << "." << endl;
}

The hash_map length is 1.
The hash_map length is now 2.

hash_map::size_type

An unsigned integer type that can represent the number of elements in a hash_map.

typedef list<typename _Traits::value_type, typename _Traits::allocator_type>::size_type size_type;

Remarks

Example

See example for size for an example of how to declare and use size_type

hash_map::swap

Exchanges the elements of two hash_maps.

void swap(hash_map& right);

Parameters

right
The argument hash_map providing the elements to be swapped with the target hash_map.

Remarks

The member function invalidates no references, pointers, or iterators that designate elements in the two hash_maps whose elements are being exchanged.

Example

// hash_map_swap.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1, hm2, hm3;
   hash_map <int, int>::iterator hm1_Iter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );
   hm1.insert ( Int_Pair ( 3, 30 ) );
   hm2.insert ( Int_Pair ( 10, 100 ) );
   hm2.insert ( Int_Pair ( 20, 200 ) );
   hm3.insert ( Int_Pair ( 30, 300 ) );

   cout << "The original hash_map hm1 is:";
   for ( hm1_Iter = hm1.begin( ); hm1_Iter != hm1.end( ); hm1_Iter++ )
      cout << " " << hm1_Iter -> second;
   cout   << "." << endl;

   // This is the member function version of swap
   // hm2 is said to be the argument hash_map;
   // hm1 is said to be the target hash_map
   hm1.swap( hm2 );

   cout << "After swapping with hm2, hash_map hm1 is:";
   for ( hm1_Iter = hm1.begin( ); hm1_Iter != hm1.end( ); hm1_Iter++ )
      cout << " " << hm1_Iter -> second;
   cout  << "." << endl;

   // This is the specialized template version of swap
   swap( hm1, hm3 );

   cout << "After swapping with hm3, hash_map hm1 is:";
   for ( hm1_Iter = hm1.begin( ); hm1_Iter != hm1.end( ); hm1_Iter++ )
      cout << " " << hm1_Iter -> second;
   cout   << "." << endl;
}

The original hash_map hm1 is: 10 20 30.
After swapping with hm2, hash_map hm1 is: 100 200.
After swapping with hm3, hash_map hm1 is: 300.

hash_map::upper_bound

Returns an iterator to the first element in a hash_map that with a key having a value that is greater than that of a specified key.

iterator upper_bound(const Key& key);

const_iterator upper_bound(const Key& key) const;

Parameters

key
The argument key value to be compared with the sort key value of an element from the hash_map being searched.

Return Value

An iterator or const_iterator that addresses the location of an element in a hash_map that with a key that is greater than the argument key, or that addresses the location succeeding the last element in the hash_map if no match is found for the key.

If the return value is assigned to a const_iterator, the hash_map object can't be modified. If the return value is assigned to an iterator, the hash_map object can be modified.

Remarks

Example

// hash_map_upper_bound.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   hash_map <int, int> hm1;
   hash_map <int, int> :: const_iterator hm1_AcIter, hm1_RcIter;
   typedef pair <int, int> Int_Pair;

   hm1.insert ( Int_Pair ( 1, 10 ) );
   hm1.insert ( Int_Pair ( 2, 20 ) );
   hm1.insert ( Int_Pair ( 3, 30 ) );

   hm1_RcIter = hm1.upper_bound( 2 );
   cout << "The first element of hash_map hm1 with a key "
        << "greater than 2 is: "
        << hm1_RcIter -> second << "." << endl;

   // If no match is found for the key, end is returned
   hm1_RcIter = hm1. upper_bound ( 4 );

   if ( hm1_RcIter == hm1.end( ) )
      cout << "The hash_map hm1 doesn't have an element "
           << "with a key greater than 4." << endl;
   else
      cout << "The element of hash_map hm1 with a key > 4 is: "
           << hm1_RcIter -> second << "." << endl;

   // The element at a specific location in the hash_map can be found
   // using a dereferenced iterator addressing the location
   hm1_AcIter = hm1.begin( );
   hm1_RcIter = hm1. upper_bound ( hm1_AcIter -> first );
   cout << "The 1st element of hm1 with a key greater than that\n"
        << "of the initial element of hm1 is: "
        << hm1_RcIter -> second << "." << endl;
}

The first element of hash_map hm1 with a key greater than 2 is: 30.
The hash_map hm1 doesn't have an element with a key greater than 4.
The 1st element of hm1 with a key greater than that
of the initial element of hm1 is: 20.

hash_map::value_comp

Returns a function object that determines the order of elements in a hash_map by comparing their key values.

value_compare value_comp() const;

Return Value

Returns the comparison function object that a hash_map uses to order its elements.

Remarks

For a hash_map m, if two elements e1 (k1, d1) and e2 (k2, d2) are objects of type value_type, where k1 and k2 are their keys of type key_type and d1 and d2 are their data of type mapped_type, then m.value_comp()(e1, e2) is equivalent to m.key_comp()(k1, k2). A stored object defines the member function

bool operator(value_type& left, value_type& right);

which returns true if the key value of left precedes and isn't equal to the key value of right in the sort order.

Example

// hash_map_value_comp.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;

   hash_map <int, int, hash_compare<int, less<int> > > hm1;
   hash_map <int, int, hash_compare<int, less<int> > >
   ::value_compare vc1 = hm1.value_comp( );
   pair< hash_map<int,int>::iterator, bool > pr1, pr2;

   pr1= hm1.insert ( hash_map <int, int> :: value_type ( 1, 10 ) );
   pr2= hm1.insert ( hash_map <int, int> :: value_type ( 2, 5 ) );

   if( vc1( *pr1.first, *pr2.first ) == true )
   {
      cout << "The element ( 1,10 ) precedes the element ( 2,5 )."
           << endl;
   }
   else
   {
      cout << "The element ( 1,10 ) does not precede the element ( 2,5 )."
           << endl;
   }

   if( vc1 ( *pr2.first, *pr1.first ) == true )
   {
      cout << "The element ( 2,5 ) precedes the element ( 1,10 )."
           << endl;
   }
   else
   {
      cout << "The element ( 2,5 ) does not precede the element ( 1,10 )."
           << endl;
   }
}

hash_map::value_type

A type that represents the type of object stored in a hash_map.

typedef pair<const Key, Type> value_type;

Remarks

value_type is declared to be pair<const key_type, mapped_type> and not pair<key_type, mapped_type> because the keys of an associative container may not be changed using a nonconstant iterator or reference.

Example

// hash_map_value_type.cpp
// compile with: /EHsc
#include <hash_map>
#include <iostream>

int main( )
{
   using namespace std;
   using namespace stdext;
   typedef pair <const int, int> cInt2Int;
   hash_map <int, int> hm1;
   hash_map <int, int> :: key_type key1;
   hash_map <int, int> :: mapped_type mapped1;
   hash_map <int, int> :: value_type value1;
   hash_map <int, int> :: iterator pIter;

   // value_type can be used to pass the correct type
   // explicitly to avoid implicit type conversion
   hm1.insert ( hash_map <int, int> :: value_type ( 1, 10 ) );

   // Compare other ways to insert objects into a hash_map
   hm1.insert ( cInt2Int ( 2, 20 ) );
   hm1[ 3 ] = 30;

   // Initializing key1 and mapped1
   key1 = ( hm1.begin( ) -> first );
   mapped1 = ( hm1.begin( ) -> second );

   cout << "The key of first element in the hash_map is "
        << key1 << "." << endl;

   cout << "The data value of first element in the hash_map is "
        << mapped1 << "." << endl;

   // The following line would cause an error because
   // the value_type is not assignable
   // value1 = cInt2Int ( 4, 40 );

   cout  << "The keys of the mapped elements are:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> first;
   cout << "." << endl;

   cout  << "The values of the mapped elements are:";
   for ( pIter = hm1.begin( ) ; pIter != hm1.end( ) ; pIter++ )
      cout << " " << pIter -> second;
   cout << "." << endl;
}

The key of first element in the hash_map is 1.
The data value of first element in the hash_map is 10.
The keys of the mapped elements are: 1 2 3.
The values of the mapped elements are: 10 20 30.

See also

Thread Safety in the C++ Standard Library
C++ Standard Library Reference