unordered_map Class
The latest version of this topic can be found at unordered_map Class.
The template class describes an object that controls a varying-length sequence of elements of type std::pair<const Key, Ty>. The sequence is weakly ordered by a hash function, which partitions the sequence into an ordered set of subsequences called buckets. Within each bucket a comparison function determines whether any pair of elements has equivalent ordering. Each element stores two objects, a sort key and a value. The sequence is represented in a way that permits lookup, insertion, and removal of an arbitrary element with a number of operations that can be independent of the number of elements in the sequence (constant time), at least when all buckets are of roughly equal length. In the worst case, when all of the elements are in one bucket, the number of operations is proportional to the number of elements in the sequence (linear time). Moreover, inserting an element invalidates no iterators, and removing an element invalidates only those iterators which point at the removed element.
Syntax
template <class Key,
class Ty,
class Hash = std::hash<Key>,
class Pred = std::equal_to<Key>,
class Alloc = std::allocator<std::pair<const Key, Ty>>>
class unordered_map;
Parameters
| Parameter | Description |
Key |
The key type. |
Ty |
The mapped type. |
Hash |
The hash function object type. |
Pred |
The equality comparison function object type. |
Alloc |
The allocator class. |
Members
| Type Definition | Description |
| unordered_map::allocator_type | The type of an allocator for managing storage. |
| unordered_map::const_iterator | The type of a constant iterator for the controlled sequence. |
| unordered_map::const_local_iterator | The type of a constant bucket iterator for the controlled sequence. |
| unordered_map::const_pointer | The type of a constant pointer to an element. |
| unordered_map::const_reference | The type of a constant reference to an element. |
| unordered_map::difference_type | The type of a signed distance between two elements. |
| unordered_map::hasher | The type of the hash function. |
| unordered_map::iterator | The type of an iterator for the controlled sequence. |
| unordered_map::key_equal | The type of the comparison function. |
| unordered_map::key_type | The type of an ordering key. |
| unordered_map::local_iterator | The type of a bucket iterator for the controlled sequence. |
| unordered_map::mapped_type | The type of a mapped value associated with each key. |
| unordered_map::pointer | The type of a pointer to an element. |
| unordered_map::reference | The type of a reference to an element. |
| unordered_map::size_type | The type of an unsigned distance between two elements. |
| unordered_map::value_type | The type of an element. |
| Member Function | Description |
| unordered_map::at | Finds an element with the specified key. |
| unordered_map::begin | Designates the beginning of the controlled sequence. |
| unordered_map::bucket | Gets the bucket number for a key value. |
| unordered_map::bucket_count | Gets the number of buckets. |
| unordered_map::bucket_size | Gets the size of a bucket. |
| unordered_map::cbegin | Designates the beginning of the controlled sequence. |
| unordered_map::cend | Designates the end of the controlled sequence. |
| unordered_map::clear | Removes all elements. |
| unordered_map::count | Finds the number of elements matching a specified key. |
| unordered_map::emplace | Adds an element constructed in place. |
| unordered_map::emplace_hint | Adds an element constructed in place, with hint. |
| unordered_map::empty | Tests whether no elements are present. |
| unordered_map::end | Designates the end of the controlled sequence. |
| unordered_map::equal_range | Finds range that matches a specified key. |
| unordered_map::erase | Removes elements at specified positions. |
| unordered_map::find | Finds an element that matches a specified key. |
| unordered_map::get_allocator | Gets the stored allocator object. |
| unordered_map::hash_function | Gets the stored hash function object. |
| unordered_map::insert | Adds elements. |
| unordered_map::key_eq | Gets the stored comparison function object. |
| unordered_map::load_factor | Counts the average elements per bucket. |
| unordered_map::max_bucket_count | Gets the maximum number of buckets. |
| unordered_map::max_load_factor | Gets or sets the maximum elements per bucket. |
| unordered_map::max_size | Gets the maximum size of the controlled sequence. |
| unordered_map::rehash | Rebuilds the hash table. |
| unordered_map::size | Counts the number of elements. |
| unordered_map::swap | Swaps the contents of two containers. |
| unordered_map::unordered_map | Constructs a container object. |
| Operator | Description |
| unordered_map::operator[] | Finds or inserts an element with the specified key. |
| unordered_map::operator= | Copies a hash table. |
Remarks
The object orders the sequence it controls by calling two stored objects, a comparison function object of type unordered_map::key_equal and a hash function object of type unordered_map::hasher. You access the first stored object by calling the member function unordered_map::key_eq(); and you access the second stored object by calling the member function unordered_map::hash_function(). Specifically, for all values X and Y of type Key, the call key_eq()(X, Y) returns true only if the two argument values have equivalent ordering; the call hash_function()(keyval) yields a distribution of values of type size_t. Unlike template class unordered_multimap Class, an object of template class unordered_map ensures that key_eq()(X, Y) is always false for any two elements of the controlled sequence. (Keys are unique.)
The object also stores a maximum load factor, which specifies the maximum desired average number of elements per bucket. If inserting an element causes unordered_map::load_factor() to exceed the maximum load factor, the container increases the number of buckets and rebuilds the hash table as needed.
The actual order of elements in the controlled sequence depends on the hash function, the comparison function, the order of insertion, the maximum load factor, and the current number of buckets. You cannot in general predict the order of elements in the controlled sequence. You can always be assured, however, that any subset of elements that have equivalent ordering are adjacent in the controlled sequence.
The object allocates and frees storage for the sequence it controls through a stored allocator object of type unordered_map::allocator_type. Such an allocator object must have the same external interface as an object of template class allocator. Note that the stored allocator object is not copied when the container object is assigned.
Requirements
Header: <unordered_map>
Namespace: std
unordered_map::allocator_type
The type of an allocator for managing storage.
typedef Alloc allocator_type;
Remarks
The type is a synonym for the template parameter Alloc.
Example
// std_tr1__unordered_map__unordered_map_allocator_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
typedef std::allocator<std::pair<const char, int> > Myalloc;
int main()
{
Mymap c1;
Mymap::allocator_type al = c1.get_allocator();
std::cout << "al == std::allocator() is "
<< std::boolalpha << (al == Myalloc()) << std::endl;
return (0);
}
al == std::allocator() is true
unordered_map::at
Finds an element in a unordered_map with a specified key value.
Ty& at(const Key& key);
const Ty& at(const Key& key) const;
Parameters
| Parameter | Description |
key |
The key value to find. |
Return Value
A reference to the data value of the element found.
Remarks
If the argument key value is not found, then the function throws an object of class out_of_range.
Example
// unordered_map_at.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// find and show elements
std::cout << "c1.at('a') == " << c1.at('a') << std::endl;
std::cout << "c1.at('b') == " << c1.at('b') << std::endl;
std::cout << "c1.at('c') == " << c1.at('c') << std::endl;
return (0);
}
unordered_map::begin
Designates the beginning of the controlled sequence or a bucket.
iterator begin();
const_iterator begin() const;
local_iterator begin(size_type nbucket);
const_local_iterator begin(size_type nbucket) const;
Parameters
| Parameter | Description |
nbucket |
The bucket number. |
Remarks
The first two member functions return a forward iterator that points at the first element of the sequence (or just beyond the end of an empty sequence). The last two member functions return a forward iterator that points at the first element of bucket nbucket (or just beyond the end of an empty bucket).
Example
// std_tr1__unordered_map__unordered_map_begin.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// inspect first two items " [c 3] [b 2]"
Mymap::iterator it2 = c1.begin();
std::cout << " [" << it2->first << ", " << it2->second << "]";
++it2;
std::cout << " [" << it2->first << ", " << it2->second << "]";
std::cout << std::endl;
// inspect bucket containing 'a'
Mymap::const_local_iterator lit = c1.begin(c1.bucket('a'));
std::cout << " [" << lit->first << ", " << lit->second << "]";
return (0);
}
[c, 3] [b, 2] [a, 1]
[c, 3] [b, 2]
[a, 1]
unordered_map::bucket
Gets the bucket number for a key value.
size_type bucket(const Key& keyval) const;
Parameters
keyval
The key value to map.
Remarks
The member function returns the bucket number currently corresponding to the key value keyval.
Example
// std_tr1__unordered_map__unordered_map_bucket.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// display buckets for keys
Mymap::size_type bs = c1.bucket('a');
std::cout << "bucket('a') == " << bs << std::endl;
std::cout << "bucket_size(" << bs << ") == " << c1.bucket_size(bs)
<< std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
bucket
('a') == 7
bucket_size
(7) == 1
unordered_map::bucket_count
Gets the number of buckets.
size_type bucket_count() const;
Remarks
The member function returns the current number of buckets.
Example
// std_tr1__unordered_map__unordered_map_bucket_count.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// inspect current parameters
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
// change max_load_factor and redisplay
c1.max_load_factor(0.10f);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
// rehash and redisplay
c1.rehash(100);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
bucket_count
() == 8
load_factor
() == 0.375
max_bucket_count
() == 8
max_load_factor
() == 4
bucket_count
() == 8
load_factor
() == 0.375
max_bucket_count
() == 8
max_load_factor
() == 0.1
bucket_count
() == 128
load_factor
() == 0.0234375
max_bucket_count
() == 128
max_load_factor
() == 0.1
unordered_map::bucket_size
Gets the size of a bucket
size_type bucket_size(size_type nbucket) const;
Parameters
nbucket
The bucket number.
Remarks
The member functions returns the size of bucket number nbucket.
Example
// std_tr1__unordered_map__unordered_map_bucket_size.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// display buckets for keys
Mymap::size_type bs = c1.bucket('a');
std::cout << "bucket('a') == " << bs << std::endl;
std::cout << "bucket_size(" << bs << ") == " << c1.bucket_size(bs)
<< std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
bucket
('a') == 7
bucket_size
(7) == 1
unordered_map::cbegin
Returns a const iterator that addresses the first element in the range.
const_iterator cbegin() const;
Return Value
A const forward-access iterator that points at the first element of the range, or the location just beyond the end of an empty range (for an empty range, cbegin() == cend()).
Remarks
With the return value of cbegin, the elements in the range cannot be modified.
You can use this member function in place of the begin() member function to guarantee that the return value is const_iterator. Typically, it's used in conjunction with the auto type deduction keyword, as shown in the following example. In the example, consider Container to be a modifiable (non- const) container of any kind that supports begin() and cbegin().
auto i1 = Container.begin();
// i1 is Container<T>::iterator
auto i2 = Container.cbegin();
// i2 is Container<T>::const_iterator
unordered_map::cend
Returns a const iterator that addresses the location just beyond the last element in a range.
const_iterator cend() const;
Return Value
A const forward-access iterator that points just beyond the end of the range.
Remarks
cend is used to test whether an iterator has passed the end of its range.
You can use this member function in place of the end() member function to guarantee that the return value is const_iterator. Typically, it's used in conjunction with the auto type deduction keyword, as shown in the following example. In the example, consider Container to be a modifiable (non- const) container of any kind that supports end() and cend().
auto i1 = Container.end();
// i1 is Container<T>::iterator
auto i2 = Container.cend();
// i2 is Container<T>::const_iterator
The value returned by cend should not be dereferenced.
unordered_map::clear
Removes all elements.
void clear();
Remarks
The member function calls unordered_map::erase( unordered_map::begin(), unordered_map::end()).
Example
// std_tr1__unordered_map__unordered_map_clear.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// clear the container and reinspect
c1.clear();
std::cout << "size == " << c1.size() << std::endl;
std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;
std::cout << std::endl;
c1.insert(Mymap::value_type('d', 4));
c1.insert(Mymap::value_type('e', 5));
// display contents " [e 5] [d 4]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
std::cout << "size == " << c1.size() << std::endl;
std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
size == 0
empty
() == true
[e, 5] [d, 4]
size == 2
empty
() == false
unordered_map::const_iterator
The type of a constant iterator for the controlled sequence.
typedef T1 const_iterator;
Remarks
The type describes an object that can serve as a constant forward iterator for the controlled sequence. It is described here as a synonym for the implementation-defined type T1.
Example
// std_tr1__unordered_map__unordered_map_const_iterator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
unordered_map::const_local_iterator
The type of a constant bucket iterator for the controlled sequence.
typedef T5 const_local_iterator;
Remarks
The type describes an object that can serve as a constant forward iterator for a bucket. It is described here as a synonym for the implementation-defined type T5.
Example
// std_tr1__unordered_map__unordered_map_const_local_iterator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// inspect bucket containing 'a'
Mymap::const_local_iterator lit = c1.begin(c1.bucket('a'));
std::cout << " [" << lit->first << ", " << lit->second << "]";
return (0);
}
[c, 3] [b, 2] [a, 1]
[a, 1]
unordered_map::const_pointer
The type of a constant pointer to an element.
typedef Alloc::const_pointer const_pointer;
Remarks
The type describes an object that can serve as a constant pointer to an element of the controlled sequence.
Example
// std_tr1__unordered_map__unordered_map_const_pointer.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::iterator it = c1.begin();
it != c1.end(); ++it)
{
Mymap::const_pointer p = &*it;
std::cout << " [" << p->first << ", " << p->second << "]";
}
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
unordered_map::const_reference
The type of a constant reference to an element.
typedef Alloc::const_reference const_reference;
Remarks
The type describes an object that can serve as a constant reference to an element of the controlled sequence.
Example
// std_tr1__unordered_map__unordered_map_const_reference.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::iterator it = c1.begin();
it != c1.end(); ++it)
{
Mymap::const_reference ref = *it;
std::cout << " [" << ref.first << ", " << ref.second << "]";
}
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
unordered_map::count
Finds the number of elements matching a specified key.
size_type count(const Key& keyval) const;
Parameters
keyval
Key value to search for.
Remarks
The member function returns the number of elements in the range delimited by unordered_map::equal_range(keyval).
Example
// std_tr1__unordered_map__unordered_map_count.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
std::cout << "count('A') == " << c1.count('A') << std::endl;
std::cout << "count('b') == " << c1.count('b') << std::endl;
std::cout << "count('C') == " << c1.count('C') << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
count
('A') == 0
count
('b') == 1
count
('C') == 0
unordered_map::difference_type
The type of a signed distance between two elements.
typedef T3 difference_type;
Remarks
The signed integer type describes an object that can represent the difference between the addresses of any two elements in the controlled sequence. It is described here as a synonym for the implementation-defined type T3.
Example
// std_tr1__unordered_map__unordered_map_difference_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// compute positive difference
Mymap::difference_type diff = 0;
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
++diff;
std::cout << "end()-begin() == " << diff << std::endl;
// compute negative difference
diff = 0;
for (Mymap::const_iterator it = c1.end();
it != c1.begin(); --it)
--diff;
std::cout << "begin()-end() == " << diff << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
end
()-begin
() == 3
begin
()-end
() == -3
unordered_map::emplace
Inserts an element constructed in place (no copy or move operations are performed) into an unordered_map.
template <class... Args>
pair<iterator, bool>
emplace(
Args&&... args);
Parameters
| Parameter | Description |
args |
The arguments forwarded to construct an element to be inserted into the unordered_map unless it already contains an element whose value is equivalently ordered. |
Return Value
A pair whose bool component returns true if an insertion was made and false if the unordered_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.
Remarks
No iterators or references are invalidated by this function.
During the insertion, if an exception is thrown but does not occur in the container's hash function, the container is not modified. If the exception is thrown in the hash function, the result is undefined.
For a code example, see map::emplace.
unordered_map::emplace_hint
Inserts an element constructed in place (no copy or move operations are performed), with a placement hint.
template <class... Args>
iterator emplace_hint(
const_iterator where,
Args&&... args);
Parameters
| Parameter | Description |
args |
The arguments forwarded to construct an element to be inserted into the unordered_map unless the unordered_map already contains that element or, more generally, unless it already contains an element whose key is equivalently ordered. |
where |
A hint regarding the place to start searching for the correct point of insertion. |
Return Value
An iterator to the newly inserted element.
If the insertion failed because the element already exists, returns an iterator to the existing element.
Remarks
No references are invalidated by this function.
During the insertion, if an exception is thrown but does not occur in the container's hash function, the container is not modified. If the exception is thrown in the hash function, the result is undefined.
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.
For a code example, see map::emplace_hint.
unordered_map::empty
Tests whether no elements are present.
bool empty() const;
Remarks
The member function returns true for an empty controlled sequence.
Example
// std_tr1__unordered_map__unordered_map_empty.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// clear the container and reinspect
c1.clear();
std::cout << "size == " << c1.size() << std::endl;
std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;
std::cout << std::endl;
c1.insert(Mymap::value_type('d', 4));
c1.insert(Mymap::value_type('e', 5));
// display contents " [e 5] [d 4]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
std::cout << "size == " << c1.size() << std::endl;
std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
size == 0
empty
() == true
[e, 5] [d, 4]
size == 2
empty
() == false
unordered_map::end
Designates the end of the controlled sequence.
iterator end();
const_iterator end() const;
local_iterator end(size_type nbucket);
const_local_iterator end(size_type nbucket) const;
Parameters
| Parameter | Description |
nbucket |
The bucket number. |
Remarks
The first two member functions return a forward iterator that points just beyond the end of the sequence. The last two member functions return a forward iterator that points just beyond the end of bucket nbucket.
unordered_map::equal_range
Finds range that matches a specified key.
std::pair<iterator, iterator>
equal_range(const Key& keyval);
std::pair<const_iterator, const_iterator>
equal_range(const Key& keyval) const;
Parameters
keyval
Key value to search for.
Remarks
The member function returns a pair of iterators X such that [X.first, X.second) delimits just those elements of the controlled sequence that have equivalent ordering with keyval. If no such elements exist, both iterators are end().
Example
// std_tr1__unordered_map__unordered_map_equal_range.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// display results of failed search
std::pair<Mymap::iterator, Mymap::iterator> pair1 =
c1.equal_range('x');
std::cout << "equal_range('x'):";
for (; pair1.first != pair1.second; ++pair1.first)
std::cout << " [" << pair1.first->first
<< ", " << pair1.first->second << "]";
std::cout << std::endl;
// display results of successful search
pair1 = c1.equal_range('b');
std::cout << "equal_range('b'):";
for (; pair1.first != pair1.second; ++pair1.first)
std::cout << " [" << pair1.first->first
<< ", " << pair1.first->second << "]";
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
equal_range
('x'):
equal_range
('b'): [b, 2]
unordered_map::erase
Removes an element or a range of elements in a unordered_map from specified positions or removes elements that match a specified key.
iterator erase(
const_iterator Where);
iterator erase(
const_iterator First,
const_iterator Last);
size_type erase(
const key_type& Key);
Parameters
Where
Position of the element to be removed.
First
Position of the first element to be removed.
Last
Position just beyond the last element to be removed.
Key
The key value of the elements to be removed.
Return Value
For the first two member functions, a bidirectional iterator that designates the first element remaining beyond any elements removed, or an element that is the end of the map if no such element exists.
For the third member function, returns the number of elements that have been removed from the unordered_map.
Remarks
For a code example, see map::erase.
unordered_map::find
Finds an element that matches a specified key.
const_iterator find(const Key& keyval) const;
Parameters
keyval
Key value to search for.
Remarks
The member function returns unordered_map::equal_range(keyval).first.
Example
// std_tr1__unordered_map__unordered_map_find.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// try to find and fail
std::cout << "find('A') == "
<< std::boolalpha << (c1.find('A') != c1.end()) << std::endl;
// try to find and succeed
Mymap::iterator it = c1.find('b');
std::cout << "find('b') == "
<< std::boolalpha << (it != c1.end())
<< ": [" << it->first << ", " << it->second << "]" << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
find
('A') == false
find
('b') == true: [b, 2]
unordered_map::get_allocator
Gets the stored allocator object.
Alloc get_allocator() const;
Remarks
The member function returns the stored allocator object.
Example
// std_tr1__unordered_map__unordered_map_get_allocator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
typedef std::allocator<std::pair<const char, int> > Myalloc;
int main()
{
Mymap c1;
Mymap::allocator_type al = c1.get_allocator();
std::cout << "al == std::allocator() is "
<< std::boolalpha << (al == Myalloc()) << std::endl;
return (0);
}
al == std::allocator() is true
unordered_map::hash_function
Gets the stored hash function object.
Hash hash_function() const;
Remarks
The member function returns the stored hash function object.
Example
// std_tr1__unordered_map__unordered_map_hash_function.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
Mymap::hasher hfn = c1.hash_function();
std::cout << "hfn('a') == " << hfn('a') << std::endl;
std::cout << "hfn('b') == " << hfn('b') << std::endl;
return (0);
}
hfn
('a') == 1630279
hfn
('b') == 1647086
unordered_map::hasher
The type of the hash function.
typedef Hash hasher;
Remarks
The type is a synonym for the template parameter Hash.
Example
// std_tr1__unordered_map__unordered_map_hasher.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
Mymap::hasher hfn = c1.hash_function();
std::cout << "hfn('a') == " << hfn('a') << std::endl;
std::cout << "hfn('b') == " << hfn('b') << std::endl;
return (0);
}
hfn
('a') == 1630279
hfn
('b') == 1647086
unordered_map::insert
Inserts an element or a range of elements into an unordered_map.
// (1) single element
pair<iterator, bool> insert(
const value_type& Val);
// (2) single element, perfect forwarded
template <class ValTy>
pair<iterator, bool>
insert(
ValTy&& Val);
// (3) single element with hint
iterator insert(
const_iterator Where,
const value_type& Val);
// (4) single element, perfect forwarded, with hint
template <class ValTy>
iterator insert(
const_iterator Where,
ValTy&& Val);
// (5) range
template <class InputIterator>
void insert(
InputIterator First,
InputIterator Last);
// (6) initializer list
void insert(
initializer_list<value_type>
IList);
Parameters
| Parameter | Description |
Val |
The value of an element to be inserted into the unordered_map unless it already contains an element whose key is equivalently ordered. |
Where |
The place to start searching for the correct point of insertion. |
ValTy |
Template parameter that specifies the argument type that the unordered_map can use to construct an element of value_type, and perfect-forwards Val as an argument. |
First |
The position of the first element to be copied. |
Last |
The position just beyond the last element to be copied. |
InputIterator |
Template function argument that meets the requirements of an input iterator that points to elements of a type that can be used to construct value_type objects. |
IList |
The initializer_list from which to copy the elements. |
Return Value
The single-element member functions, (1) and (2), return a pair whose bool component is true if an insertion was made, and false if the unordered_map already contained an element whose key had an equivalent value in the ordering. The iterator component of the return-value pair points to the newly inserted element if the bool component is true, or to the existing element if the bool component is false.
The single-element-with-hint member functions, (3) and (4), return an iterator that points to the position where the new element was inserted into the unordered_map or, if an element with an equivalent key already exists, to the existing element.
Remarks
No iterators, pointers, or references are invalidated by this function.
During the insertion of just one element, if an exception is thrown but does not occur in the container's hash function, the container's state is not modified. If the exception is thrown in the hash function, the result is undefined. During the insertion of multiple elements, if an exception is thrown, the container is left in an unspecified but valid state.
To access the iterator component of a pair``pr that's returned by the single-element member functions, use pr.first; to dereference the iterator within the returned pair, use *pr.first, giving you an element. To access the bool component, use pr.second. For an example, see the sample code later in this article.
The value_type of a container is a typedef that belongs to the container, and for map, map<K, V>::value_type is pair<const K, V>. The value of an element is an ordered pair in which the first component is equal to the key value and the second component is equal to the data value of the element.
The range member function (5) inserts the sequence of element values into an unordered_map that corresponds to each element addressed by an iterator in the range [First, Last); therefore, Last does not get inserted. The container member function end() refers to the position just after the last element in the container—for example, the statement m.insert(v.begin(), v.end()); attempts to insert all elements of v into m. Only elements that have unique values in the range are inserted; duplicates are ignored. To observe which elements are rejected, use the single-element versions of insert.
The initializer list member function (6) uses an initializer_list to copy elements into the unordered_map.
For insertion of an element constructed in place—that is, no copy or move operations are performed—see unordered_map::emplace and unordered_map::emplace_hint.
For a code example, see map::insert.
unordered_map::iterator
The type of an iterator for the controlled sequence.
typedef T0 iterator;
Remarks
The type describes an object that can serve as a forward iterator for the controlled sequence. It is described here as a synonym for the implementation-defined type T0.
Example
// std_tr1__unordered_map__unordered_map_iterator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
unordered_map::key_eq
Gets the stored comparison function object.
Pred key_eq() const;
Remarks
The member function returns the stored comparison function object.
Example
// std_tr1__unordered_map__unordered_map_key_eq.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
Mymap::key_equal cmpfn = c1.key_eq();
std::cout << "cmpfn('a', 'a') == "
<< std::boolalpha << cmpfn('a', 'a') << std::endl;
std::cout << "cmpfn('a', 'b') == "
<< std::boolalpha << cmpfn('a', 'b') << std::endl;
return (0);
}
cmpfn
('a', 'a') == true
cmpfn
('a', 'b') == false
unordered_map::key_equal
The type of the comparison function.
typedef Pred key_equal;
Remarks
The type is a synonym for the template parameter Pred.
Example
// std_tr1__unordered_map__unordered_map_key_equal.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
Mymap::key_equal cmpfn = c1.key_eq();
std::cout << "cmpfn('a', 'a') == "
<< std::boolalpha << cmpfn('a', 'a') << std::endl;
std::cout << "cmpfn('a', 'b') == "
<< std::boolalpha << cmpfn('a', 'b') << std::endl;
return (0);
}
cmpfn
('a', 'a') == true
cmpfn
('a', 'b') == false
unordered_map::key_type
The type of an ordering key.
typedef Key key_type;
Remarks
The type is a synonym for the template parameter Key.
Example
// std_tr1__unordered_map__unordered_map_key_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// add a value and reinspect
Mymap::key_type key = 'd';
Mymap::mapped_type mapped = 4;
Mymap::value_type val = Mymap::value_type(key, mapped);
c1.insert(val);
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
[d, 4] [c, 3] [b, 2] [a, 1]
unordered_map::load_factor
Counts the average elements per bucket.
float load_factor() const;
Remarks
The member function returns (float)unordered_map::size() / (float)unordered_map::bucket_count(), the average number of elements per bucket.
Example
// std_tr1__unordered_map__unordered_map_load_factor.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// inspect current parameters
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
// change max_load_factor and redisplay
c1.max_load_factor(0.10f);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
// rehash and redisplay
c1.rehash(100);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
bucket_count
() == 8
load_factor
() == 0.375
max_bucket_count
() == 8
max_load_factor
() == 4
bucket_count
() == 8
load_factor
() == 0.375
max_bucket_count
() == 8
max_load_factor
() == 0.1
bucket_count
() == 128
load_factor
() == 0.0234375
max_bucket_count
() == 128
max_load_factor
() == 0.1
unordered_map::local_iterator
The type of a bucket iterator.
typedef T4 local_iterator;
Remarks
The type describes an object that can serve as a forward iterator for a bucket. It is described here as a synonym for the implementation-defined type T4.
Example
// std_tr1__unordered_map__unordered_map_local_iterator.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// inspect bucket containing 'a'
Mymap::local_iterator lit = c1.begin(c1.bucket('a'));
std::cout << " [" << lit->first << ", " << lit->second << "]";
return (0);
}
[c, 3] [b, 2] [a, 1]
[a, 1]
unordered_map::mapped_type
The type of a mapped value associated with each key.
typedef Ty mapped_type;
Remarks
The type is a synonym for the template parameter Ty.
Example
// std_tr1__unordered_map__unordered_map_mapped_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// add a value and reinspect
Mymap::key_type key = 'd';
Mymap::mapped_type mapped = 4;
Mymap::value_type val = Mymap::value_type(key, mapped);
c1.insert(val);
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
[d, 4] [c, 3] [b, 2] [a, 1]
unordered_map::max_bucket_count
Gets the maximum number of buckets.
size_type max_bucket_count() const;
Remarks
The member function returns the maximum number of buckets currently permitted.
Example
// std_tr1__unordered_map__unordered_map_max_bucket_count.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// inspect current parameters
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
// change max_load_factor and redisplay
c1.max_load_factor(0.10f);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
// rehash and redisplay
c1.rehash(100);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
bucket_count
() == 8
load_factor
() == 0.375
max_bucket_count
() == 8
max_load_factor
() == 4
bucket_count
() == 8
load_factor
() == 0.375
max_bucket_count
() == 8
max_load_factor
() == 0.1
bucket_count
() == 128
load_factor
() == 0.0234375
max_bucket_count
() == 128
max_load_factor
() == 0.1
unordered_map::max_load_factor
Gets or sets the maximum elements per bucket.
float max_load_factor() const;
void max_load_factor(float factor);
Parameters
factor
The new maximum load factor.
Remarks
The first member function returns the stored maximum load factor. The second member function replaces the stored maximum load factor with factor.
Example
// std_tr1__unordered_map__unordered_map_max_load_factor.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// inspect current parameters
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
// change max_load_factor and redisplay
c1.max_load_factor(0.10f);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
// rehash and redisplay
c1.rehash(100);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_bucket_count() == "
<< c1.max_bucket_count() << std::endl;
std::cout << "max_load_factor() == "
<< c1.max_load_factor() << std::endl;
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
bucket_count
() == 8
load_factor
() == 0.375
max_bucket_count
() == 8
max_load_factor
() == 4
bucket_count
() == 8
load_factor
() == 0.375
max_bucket_count
() == 8
max_load_factor
() == 0.1
bucket_count
() == 128
load_factor
() == 0.0234375
max_bucket_count
() == 128
max_load_factor
() == 0.1
unordered_map::max_size
Gets the maximum size of the controlled sequence.
size_type max_size() const;
Remarks
The member function returns the length of the longest sequence that the object can control.
Example
// std_tr1__unordered_map__unordered_map_max_size.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
std::cout << "max_size() == " << c1.max_size() << std::endl;
return (0);
}
max_size
() == 536870911
unordered_map::operator[]
Finds or inserts an element with the specified key.
Ty& operator[](const Key& keyval);
Ty& operator[](Key&& keyval);
Parameters
| Parameter | Description |
Keyval |
The key value to find or insert. |
Return Value
A reference to the data value of the inserted element.
Remarks
If the argument key value is not found, then it is inserted along with the default value of the data type.
operator[] may be used to insert elements into a 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 does not 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
// std__unordered_map__unordered_map_operator_sub.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
#include <string>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// try to find and fail
std::cout << "c1['A'] == " << c1['A'] << std::endl;
// try to find and succeed
std::cout << "c1['a'] == " << c1['a'] << std::endl;
// redisplay contents
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// insert by moving key
std::tr1::unordered_map<string, int> c2;
std::string str("abc");
std::cout << "c2[std::move(str)] == " << c2[std::move(str)] << std::endl;
std::cout << "c2["abc"] == " << c2["abc"] << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
c1['A'] == 0
c1['a'] == 1
[c, 3] [b, 2] [A, 0] [a, 1]
c2[move(str)] == 0
c2["abc"] == 1
Remarks
The member function determines the iterator where as the return value of unordered_map::insert( unordered_map::value_type(keyval, Ty()). (It inserts an element with the specified key if no such element exists.) It then returns a reference to (*where).second.
unordered_map::operator=
Replaces the elements of this unordered_map using the elements from another unordered_map.
unordered_map& operator=(const unordered_map& right);
unordered_map& operator=(unordered_map&& right);
Parameters
| Parameter | Description |
right |
The unordered_map that the operator function assigns content from. |
Remarks
The first version copies all of the elements from right to this unordered_map.
The second version moves all of the elements from right to this unordered_map.
Any elements that are in this unordered_map before operator= executes are discarded.
Example
// unordered_map_operator_as.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
int main( )
{
using namespace std;
unordered_map<int, int> v1, v2, v3;
unordered_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;
}
unordered_map::pointer
The type of a pointer to an element.
typedef Alloc::pointer pointer;
Remarks
The type describes an object that can serve as a pointer to an element of the controlled sequence.
Example
// std_tr1__unordered_map__unordered_map_pointer.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::iterator it = c1.begin();
it != c1.end(); ++it)
{
Mymap::pointer p = &*it;
std::cout << " [" << p->first << ", " << p->second << "]";
}
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
unordered_map::reference
The type of a reference to an element.
typedef Alloc::reference reference;
Remarks
The type describes an object that can serve as a reference to an element of the controlled sequence.
Example
// std_tr1__unordered_map__unordered_map_reference.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::iterator it = c1.begin();
it != c1.end(); ++it)
{
Mymap::reference ref = *it;
std::cout << " [" << ref.first << ", " << ref.second << "]";
}
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
unordered_map::rehash
Rebuilds the hash table.
void rehash(size_type nbuckets);
Parameters
nbuckets
The requested number of buckets.
Remarks
The member function alters the number of buckets to be at least nbuckets and rebuilds the hash table as needed.
Example
// std_tr1__unordered_map__unordered_map_rehash.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// inspect current parameters
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_load_factor() == " << c1.max_load_factor() << std::endl;
std::cout << std::endl;
// change max_load_factor and redisplay
c1.max_load_factor(0.10f);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_load_factor() == " << c1.max_load_factor() << std::endl;
std::cout << std::endl;
// rehash and redisplay
c1.rehash(100);
std::cout << "bucket_count() == " << c1.bucket_count() << std::endl;
std::cout << "load_factor() == " << c1.load_factor() << std::endl;
std::cout << "max_load_factor() == " << c1.max_load_factor() << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
bucket_count
() == 8
load_factor
() == 0.375
max_load_factor
() == 4
bucket_count
() == 8
load_factor
() == 0.375
max_load_factor
() == 0.1
bucket_count
() == 128
load_factor
() == 0.0234375
max_load_factor
() == 0.1
unordered_map::size
Counts the number of elements.
size_type size() const;
Remarks
The member function returns the length of the controlled sequence.
Example
// std_tr1__unordered_map__unordered_map_size.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// clear the container and reinspect
c1.clear();
std::cout << "size == " << c1.size() << std::endl;
std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;
std::cout << std::endl;
c1.insert(Mymap::value_type('d', 4));
c1.insert(Mymap::value_type('e', 5));
// display contents " [e 5] [d 4]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
std::cout << "size == " << c1.size() << std::endl;
std::cout << "empty() == " << std::boolalpha << c1.empty() << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
size == 0
empty
() == true
[e, 5] [d, 4]
size == 2
empty
() == false
unordered_map::size_type
The type of an unsigned distance between two elements.
typedef T2 size_type;
Remarks
The unsigned integer type describes an object that can represent the length of any controlled sequence. It is described here as a synonym for the implementation-defined type T2.
Example
// std_tr1__unordered_map__unordered_map_size_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
Mymap::size_type sz = c1.size();
std::cout << "size == " << sz << std::endl;
return (0);
}
size == 0
unordered_map::swap
Swaps the contents of two containers.
void swap(unordered_map& right);
Parameters
right
The container to swap with.
Remarks
The member function swaps the controlled sequences between *this and right. If unordered_map::get_allocator() == right.get_allocator(), it does so in constant time, it throws an exception only as a result of copying the stored traits object of type Tr, and it invalidates no references, pointers, or iterators that designate elements in the two controlled sequences. Otherwise, it performs a number of element assignments and constructor calls proportional to the number of elements in the two controlled sequences.
Example
// std_tr1__unordered_map__unordered_map_swap.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
Mymap c2;
c2.insert(Mymap::value_type('d', 4));
c2.insert(Mymap::value_type('e', 5));
c2.insert(Mymap::value_type('f', 6));
c1.swap(c2);
// display contents " [f 6] [e 5] [d 4]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
swap(c1, c2);
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
[f, 6] [e, 5] [d, 4]
[c, 3] [b, 2] [a, 1]
unordered_map::unordered_map
Constructs a container object.
unordered_map(
const unordered_map& Right);
explicit unordered_map(
size_type Bucket_count = N0,
const Hash& Hash = Hash(),
const Comp& Comp = Comp(),
const Allocator& Al = Allocator());
unordered_map(
unordered_map&& Right);
unordered_map(
initializer_list<Type> IList);
unordered_map(
initializer_list<Type> IList,
size_type Bucket_count);
unordered_map(
initializer_list<Type> IList,
size_type Bucket_count,
const Hash& Hash);
unordered_map(
initializer_list<Type> IList,
size_type Bucket_count,
const Hash& Hash,
KeyEqual& equal);
unordered_map(
initializer_list<Type> IList,
size_type Bucket_count,
const Hash& Hash,
KeyEqual& Equal
const Allocator& Al);
template <class InIt>
unordered_map(
InputIterator First,
InputIterator Last,
size_type Bucket_count = N0,
const Hash& Hash = Hash(),
const Comp& Comp = Comp(),
const Allocator& Al = Alloc());
Parameters
| Parameter | Description |
Al |
The allocator object to store. |
Comp |
The comparison function object to store. |
Hash |
The hash function object to store. |
Bucket_count |
The minimum number of buckets. |
Right |
The container to copy. |
First |
|
Last |
|
IList |
The initializer_list that contains the elements to be copied. |
Remarks
The first constructor specifies a copy of the sequence controlled by right. The second constructor specifies an empty controlled sequence. The third constructor inserts the sequence of element values [first, last). The fourth constructor specifies a copy of the sequence by moving right.
All constructors also initialize several stored values. For the copy constructor, the values are obtained from Right. Otherwise:
the minimum number of buckets is the argument Bucket_count, if present; otherwise it is a default value described here as the implementation-defined value N0.
the hash function object is the argument Hash, if present; otherwise it is Hash().
The comparison function object is the argument Comp, if present; otherwise it is Pred().
The allocator object is the argument Al, if present; otherwise, it is Alloc().
Example
// std__unordered_map__unordered_map_construct.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
#include <initializer_list>
using namespace std;
using Mymap = unordered_map<char, int>;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (const auto& c : c1) {
cout << " [" << c.first << ", " << c.second << "]";
}
cout << endl;
Mymap c2(8,
tr1::hash<char>(),
equal_to<char>(),
allocator<pair<const char, int> >());
c2.insert(Mymap::value_type('d', 4));
c2.insert(Mymap::value_type('e', 5));
c2.insert(Mymap::value_type('f', 6));
// display contents " [f 6] [e 5] [d 4]"
for (const auto& c : c2) {
cout << " [" << c.first << ", " << c.second << "]";
}
cout << endl;
Mymap c3(c1.begin(),
c1.end(),
8,
tr1::hash<char>(),
equal_to<char>(),
allocator<pair<const char, int> >());
// display contents " [c 3] [b 2] [a 1]"
for (const auto& c : c3) {
cout << " [" << c.first << ", " << c.second << "]";
}
cout << endl;
Mymap c4(move(c3));
// display contents " [c 3] [b 2] [a 1]"
for (const auto& c : c4) {
cout << " [" << c.first << ", " << c.second << "]";
}
cout << endl;
cout << endl;
// Construct with an initializer_list
unordered_map<int, char> c5({ { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } });
for (const auto& c : c5) {
cout << " [" << c.first << ", " << c.second << "]";
}
cout << endl;
// Initializer_list plus size
unordered_map<int, char> c6({ { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } }, 4);
for (const auto& c : c1) {
cout << " [" << c.first << ", " << c.second << "]";
}
cout << endl;
cout << endl;
// Initializer_list plus size and hash
unordered_map<int, char, tr1::hash<char>> c7(
{ { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } },
4,
tr1::hash<char>()
);
for (const auto& c : c1) {
cout << " [" << c.first << ", " << c.second << "]";
}
cout << endl;
// Initializer_list plus size, hash, and key_equal
unordered_map<int, char, tr1::hash<char>, equal_to<char>> c8(
{ { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } },
4,
tr1::hash<char>(),
equal_to<char>()
);
for (const auto& c : c1) {
cout << " [" << c.first << ", " << c.second << "]";
}
cout << endl;
// Initializer_list plus size, hash, key_equal, and allocator
unordered_map<int, char, tr1::hash<char>, equal_to<char>> c9(
{ { 5, 'g' }, { 6, 'h' }, { 7, 'i' }, { 8, 'j' } },
4,
tr1::hash<char>(),
equal_to<char>(),
allocator<pair<const char, int> >()
);
for (const auto& c : c1) {
cout << " [" << c.first << ", " << c.second << "]";
}
cout << endl;
}
[a, 1] [b, 2] [c, 3] [d, 4] [e, 5] [f, 6] [a, 1] [b, 2] [c, 3] [a, 1] [b, 2] [c, 3] [5, g] [6, h] [7, i] [8, j] [5, g] [6, h] [7, i] [8, j] [5, g] [6, h] [7, i] [8, j] [5, g] [6, h] [7, i] [8, j] [5, g] [6, h] [7, i] [8, j]
unordered_map::value_type
The type of an element.
typedef std::pair<const Key, Ty> value_type;
Remarks
The type describes an element of the controlled sequence.
Example
// std_tr1__unordered_map__unordered_map_value_type.cpp
// compile with: /EHsc
#include <unordered_map>
#include <iostream>
typedef std::unordered_map<char, int> Mymap;
int main()
{
Mymap c1;
c1.insert(Mymap::value_type('a', 1));
c1.insert(Mymap::value_type('b', 2));
c1.insert(Mymap::value_type('c', 3));
// display contents " [c 3] [b 2] [a 1]"
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
// add a value and reinspect
Mymap::key_type key = 'd';
Mymap::mapped_type mapped = 4;
Mymap::value_type val = Mymap::value_type(key, mapped);
c1.insert(val);
for (Mymap::const_iterator it = c1.begin();
it != c1.end(); ++it)
std::cout << " [" << it->first << ", " << it->second << "]";
std::cout << std::endl;
return (0);
}
[c, 3] [b, 2] [a, 1]
[d, 4] [c, 3] [b, 2] [a, 1]
See Also
<unordered_map>
Containers
Thread Safety in the C++ Standard Library
Standard Template Library