Standard C++ Library
Copyright 1998, Rogue Wave Software, Inc.

 

NAME

 
map
 
 - An associative container with access to non-key values using unique keys. A map supports bidirectional iterators.
 
 
 

SYNOPSIS

 
 
#include <map>
template <class Key, class T, class Compare = less<Key>

class Allocator = allocator<pair<const Key, T>> >

class map;
 
 
 

DESCRIPTION

 
 
map_<Key,_T,_Compare,_Allocator> gives fast access to stored values of type T that are indexed by unique keys of type Key. The default operation for key comparison is the < operator. 
 
map has bidirectional iterators that point to an instance of pair<const Key x, T y> where x is the key and y is the stored value associated with that key. The definition of map includes a typedef to this pair called value_type. 
 
The types used for both the template parameters Key and T must include the following (where T is the type, t is a value of T and u is a const value of T):

Copy constructors   T(t) and T(u)

 

Destructor   t.~T()

 

Address of   &t and &u yielding T∗ and const T∗ respectively

 

Assignment   t = a where a is a (possibly const) value of T

 
 
The type used for the Compare template parameter must satisfy the requirements for binary functions. 
 
 
 

INTERFACE

 
 
template <class Key, class T, class Compare = less<Key>

class Allocator = allocator<pair<const Key, T>> >

class map {
 
public:
 
// types
 

typedef Key key_type;
typedef typename Allocator::pointer pointer;
typedef typename Allocator::const_pointer const_pointer;
typedef T mapped_type;
typedef pair<const Key, T> value_type;
typedef Compare key_compare;
typedef Allocator allocator_type;
 
typedef typename

Allocator::reference        reference;

typedef typename

Allocator::const_reference  const_reference;
 

class iterator;
class const_iterator;
 
typedef typename

Allocator::size_type        size_type;

typedef typename

Allocator::difference_type  difference_type;
 

typedef typename std::reverse_iterator<iterator>

reverse_iterator;

typedef typename std::reverse_iterator<const_iterator>

const_reverse_iterator;

 

class value_compare

: public binary_function<value_type, value_type, bool>

{

friend class map<Key, T, Compare, Allocator>;

 

protected :

Compare comp;
value_compare(Compare c): comp(c) {}

public :

bool operator() (const value_type&,

const value_type&) const;

};

 
// Construct/Copy/Destroy
 

explicit map (const Compare& = Compare(),

const Allocator& = Allocator ());

template <class InputIterator>

map (InputIterator, InputIterator,

const Compare& = Compare(),
const Allocator& = Allocator ());

map (const map<Key, T, Compare, Allocator>&);

~map();

map<Key, T, Compare, Allocator>&

operator= (const map<Key, T, Compare, Allocator>&);

allocator_type get_allocator () const;

 
// Iterators
 

iterator begin();
const_iterator begin() const;
iterator end();
const_iterator end() const;
reverse_iterator rbegin();
const_reverse_iterator rbegin() const;
reverse_iterator rend();
const_reverse_iterator rend() const;

 
// Capacity
 

bool empty() const;
size_type size() const;
size_type max_size() const;

 
// Element Access
 

mapped_type& operator[] (const key_type&);

 
// Modifiers
 

pair<iterator, bool> insert (const value_type&);
iterator insert (iterator, const value_type&);
template <class InputIterator>

void insert (InputIterator, InputIterator);

 

void erase (iterator);
size_type erase (const key_type&);
void erase (iterator, iterator);
void swap (map<Key, T, Compare, Allocator>&);
void clear();

 
// Observers
 

key_compare key_comp() const;
value_compare value_comp() const;

 
// Map operations
 

iterator find (const key_value&);
const_iterator find (const key_value&) const;
size_type count (const key_type&) const;
iterator lower_bound (const key_type&);
const_iterator lower_bound (const key_type&) const;
iterator upper_bound (const key_type&);
const_iterator upper_bound (const key_type&) const;
pair<iterator, iterator> equal_range (const key_type&);
pair<const_iterator, const_iterator>

equal_range (const key_type&) const;

};
 
// Non-member Map Operators
 
template <class Key, class T, class Compare, class Allocator>

bool operator== (const map<Key, T, Compare, Allocator>&,

const map<Key, T, Compare, Allocator>&);

 
template <class Key, class T, class Compare, class Allocator>

bool operator!= (const map<Key, T, Compare, Allocator>&,

const map<Key, T, Compare, Allocator>&);

 
template <class Key, class T, class Compare, class Allocator>
bool operator< (const map<Key, T, Compare, Allocator>&,

const map<Key, T, Compare, Allocator>&);

 
template <class Key, class T, class Compare, class Allocator>
bool operator> (const map<Key, T, Compare, Allocator>&,

const map<Key, T, Compare, Allocator>&);

 
template <class Key, class T, class Compare, class Allocator>
bool operator<= (const map<Key, T, Compare, Allocator>&,

const map<Key, T, Compare, Allocator>&);

 
template <class Key, class T, class Compare, class Allocator>
bool operator>= (const map<Key, T, Compare, Allocator>&,

const map<Key, T, Compare, Allocator>&);

 
 
// Specialized Algorithms
 
template <class Key, class T, class Compare, class Allocator>
void swap (map<∗Key,T,Compare,Allocator>&,

map<Key,T,Compare,Allocator>&);
 
 
 

CONSTRUCTORS

 
 
 

explicit map(const Compare& comp = Compare(),

const Allocator& alloc = Allocator());

 
 
Constructs an empty map that uses the relation comp to order keys, if it is supplied. The map uses the allocator alloc for all storage management. 
 

 
 

template <class InputIterator>
map(InputIterator first, InputIterator last,

const Compare& comp = Compare(),
const Allocator& alloc = Allocator());

 
 
Constructs a map containing values in the range [first, last). Creation of the new map is only guaranteed to succeed if the iterators first and last return values of type pair<class Key,       class Value> and all values of Key in the range[first, last) are unique. The map uses the relation comp to order keys, and the allocator alloc for all storage management. 
 

 
 

map(const map<Key,T,Compare,Allocator>& x);

 
 
Creates a new map by copying all pairs of key and value from x. 
 

 
 
 

DESTRUCTORS

 
 
 
~map();

 
 
Releases any allocated memory for this map.
 

 
 
 

ALLOCATORS

 
 
 
allocator_type get_allocator() const;

 
 
Returns a copy of the allocator used by self for storage management.
 

 
 
 

ITERATORS

 
 
 
iterator
begin();

 
 
Returns an iterator pointing to the first element stored in the map. "First" is defined by the map’s comparison operator, Compare. 
 

 
 
const_iterator
begin() const;

 
 
Returns a const_iterator pointing to the first element stored in the map. 
 

 
 
iterator
end();

 
 
Returns an iterator pointing to the last element stored in the map (in other words, the off-the-end value). 
 

 
 
const_iterator
end() const;

 
 
Returns a const_iterator pointing to the last element stored in the map. 
 

 
 
reverse_iterator
rbegin();

 
 
Returns a reverse_iterator pointing to the first element stored in the map. "First" is defined by the map’s comparison operator, Compare. 
 

 
 
const_reverse_iterator
rbegin() const;

 
 
Returns a const_reverse_iterator pointing to the first element stored in the map. 
 

 
 
reverse_iterator
rend();

 
 
Returns a reverse_iterator pointing to the last element stored in the map (in other words, the off-the-end value). 
 

 
 
const_reverse_iterator
rend() const;

 
 
Returns a const_reverse_iterator pointing to the last element stored in the map. 
 

 
 
 

MEMBER OPERATORS

 
 
 
map<Key, T, Compare, Allocator>&
operator=(const map<Key, T, Compare, Allocator>& x);

 
 
Replaces the contents of ∗this with a copy of the map x. 
 

 
 
mapped_type&
operator[](const key_type& x);

 
 
If an element with the key x exists in the map, then a reference to its associated value is returned. Otherwise the pair x,T() is inserted into the map and a reference to the default object T() is returned. 
 

 
 
 

MEMBER FUNCTIONS

 
 
 
void
clear();

 
 
Erases all elements from the self.
 

 
 
size_type
count(const key_type& x) const;

 
 
Returns a 1 if a value with the key x exists in the map. Otherwise returns a 0. 
 

 
 
bool
empty() const;

 
 
Returns true if the map is empty, false otherwise. 
 

 
 
pair<iterator, iterator>
equal_range (const key_type& x);

 
 
Returns the pair (lower_bound(x), upper_bound(x)). 
 

 
 
pair<const_iterator,const_iterator>
equal_range (const key_type& x) const;

 
 
Returns the pair (lower_bound(x), upper_bound(x)). 
 

 
 
void
erase(iterator position);

 
 
Deletes the map element pointed to by the iterator position. Returns an iterator pointing to the element following the deleted element, or end() if the deleted item was the last one in this list. 
 

 
 
void
erase(iterator first, iterator last);

 
 
If the iterators first and last point to the same map and last is reachable from first, all elements in the range (first, last) are deleted from the map. Returns an iterator pointing to the element following the last deleted element, or end() if there were no elements after the deleted range. 
 

 
 
size_type
erase(const key_type& x);

 
 
Deletes the element with the key value x from the map, if one exists. Returns 1 if x existed in the map, 0 otherwise. 
 

 
 
iterator
find(const key_type& x);

 
 
Searches the map for a pair with the key value x and returns an iterator to that pair if it is found. If such a pair is not found the value end() is returned. 
 

 
 
const_iterator find(const key_type& x) const;

 
 
Same as find above but returns a const_iterator. 
 

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

 
 
If a value_type with the same key as x is not present in the map, then x is inserted into the map. Otherwise, the pair is not inserted. A position may be supplied as a hint regarding where to do the insertion. If the insertion is done right after position, then it takes amortized constant time. Otherwise it takes O(log N) time. 
 

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

 
 
Copies of each element in the range [first, last) that possess a unique key (one not already in the map) are inserted into the map. The iterators first and last must return values of type pair<T1,T2>. This operation takes approximately O(N∗log(size()+N)) time. 
 

 
 
key_compare
key_comp() const;

 
 
Returns a function object capable of comparing key values using the comparison operation, Compare, of the current map. 
 

 
 
iterator
lower_bound(const key_type& x);

 
 
Returns a reference to the first entry with a key greater than or equal to x. 
 

 
 
const_iterator
lower_bound(const key_type& x) const;

 
 
Same as lower_bound above but returns a const_iterator. 
 

 
 
size_type
max_size() const;

 
 
Returns the maximum possible size of the map.   This size is only constrained by the number of unique keys that can be represented by the type Key. 
 

 
 
size_type
size() const;

 
 
Returns the number of elements in the map.
 

 
 
void
swap(map<Key, T, Compare, Allocator>& x);

 
 
Swaps the contents of the map x with the current map, ∗this. 
 

 
 
iterator
upper_bound(const key_type& x);

 
 
Returns a reference to the first entry with a key less than or equal to x. 
 

 
 
const_iterator
upper_bound(const key_type& x) const;

 
 
Same as upper_bound above but returns a const_iterator.
 

 
 
value_compare
value_comp() const;

 
 
Returns a function object capable of comparing pair<const Key, T> values using the comparison operation, Compare, of the current map. This function is identical to key_comp for sets. 
 

 
 
 

NON-MEMBER OPERATORS

 
 
 
template <class Key, class T, class Compare, class Allocator>
bool operator==(const map<Key, T, Compare, Allocator>& x,

const map<Key, T, Compare, Allocator>& y);

 
 
Returns true if all elements in x are element-wise equal to all elements in y, using (T::operator==). Otherwise it returns false. 
 

 
 

template <class Key, class T, class Compare, class Allocator>
bool operator!=(const map<Key, T, Compare, Allocator>& x,

const map<Key, T, Compare, Allocator>& y);

 
 
Returns !(x==y). 
 

 
 

template <class Key, class T, class Compare, class Allocator>
bool operator<(const map<Key, T, Compare, Allocator>& x,

const map<Key, T, Compare, Allocator>& y);

 
 
Returns true if x is lexicographically less than y. Otherwise, it returns false. 
 

 
 

template <class Key, class T, class Compare, class Allocator>
bool operator>(const map<Key, T, Compare, Allocator>& x,

const map<Key, T, Compare, Allocator>& y);

 
 
Returns y < x. 
 

 
 

template <class Key, class T, class Compare, class Allocator>
bool operator<=(const map<Key, T, Compare, Allocator>& x,

const map<Key, T, Compare, Allocator>& y);

 
 
Returns !(y < x). 
 

 
 

template <class Key, class T, class Compare, class Allocator>
bool operator>=(const map<Key, T, Compare, Allocator>& x,

const map<Key, T, Compare, Allocator>& y);

 
 
Returns !(x < y). 
 

 
 
 

SPECIALIZED ALGORITHMS

 
 
 

template <class Key, class T, class Compare, class Allocator>
void swap(map<Key, T, Compare, Allocator>& a,

map<Key, T, Compare, Allocator>& b);

 
 
Swaps the contents of a and b. 
 

 
 
 

EXAMPLE

 
 
 

//
// map.cpp
//

#include <string>
#include <map>
#include <iostream>

using namespace std;
 
typedef map<string, int, less<string> > months_type;
 

// Print out a pair

template <class First, class Second>
ostream& operator<<(ostream& out,

const pair<First,Second> & p)

{

cout << p.first << " has " << p.second << " days";
return out;

}

 

// Print out a map

ostream& operator<<(ostream& out, const months_type & l)

{

copy(l.begin(),l.end(), ostream_iterator

<months_type::value_type,char>(cout,"\n"));

return out;

}

 
 
int main(void)

{

// create a map of months and the number of days
// in the month

months_type months;

 

typedef months_type::value_type value_type;

 

// Put the months in the multimap

months.insert(value_type(string("January"),   31));
months.insert(value_type(string("February"),   28));
months.insert(value_type(string("February"),   29));
months.insert(value_type(string("March"),     31));
months.insert(value_type(string("April"),     30));
months.insert(value_type(string("May"),       31));
months.insert(value_type(string("June"),      30));
months.insert(value_type(string("July"),      31));
months.insert(value_type(string("August"),    31));
months.insert(value_type(string("September"), 30));
months.insert(value_type(string("October"),   31));
months.insert(value_type(string("November"),  30));
months.insert(value_type(string("December"),  31));

 

// print out the months
// Second February is not present

cout << months << endl;

 

// Find the Number of days in June

months_type::iterator p = months.find(string("June"));

 

// print out the number of days in June

if (p != months.end())

cout << endl << ∗p << endl;

 

return 0;

}
 

Program Output
 
 
 

 
April has 30 days
August has 31 days
December has 31 days
February has 28 days
January has 31 days
July has 31 days
June has 30 days
March has 31 days
May has 31 days
November has 30 days
October has 31 days
September has 30 days
 
 
 

WARNINGS

 
 
Member function templates are used in all containers included in the Standard Template Library. An example of this feature is the constructor for map<Key,T,Compare,Allocator> that takes two templatized iterators:
 
 
template <class InputIterator>
map (InputIterator, InputIterator,

const Compare& = Compare(),
const Allocator& = Allocator());

 
map also has an insert function of this type. These functions, when not restricted by compiler limitations, allow you to use any type of input iterator as arguments. For compilers that do not support this feature, substitute functions allow you to use an iterator obtained from the same type of container as the one you are constructing (or calling a member function on), or you can use a pointer to the type of element you have in the container.
 
For example, if your compiler does not support member function templates, you can construct a map in the following two ways:
 
 

map<int, int, less<int> >::value_type intarray[10];
map<int, int, less<int> > first_map(intarray,

intarray + 10);

map<int, int, less<int> > second_map(first_map.begin(),

first_map.end());

 
But not this way:
 
 

map<long, long, less<long> > long_map(first_map.begin(),

first_map.end());

 
Since the long_map and first_map are not the same type. 
 
Also, many compilers do not support default template arguments. If your compiler is one of these, you always need to supply the Compare template argument and the Allocator template argument. For instance, you have to write:
 
map<int, int, less<int>, allocator<int> >
 
instead of:
 
map<int, int>
 
If your compiler does not support namespaces, then you do not need the using declaration for std. 
 
 
 

SEE ALSO

 
 
allocator, Containers, Iterators, multimap
 

Rogue Wave Software  —  Last change: 02 Apr 1998