Difference between revisions of "cpp/container/unordered map"
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| − | Unordered map is an associative container that contains key-value pairs with unique keys. Search, insertion, and removal have average constant-time complexity. | + | Unordered map is an associative container that contains key-value pairs with unique keys. Search, insertion, and removal of elements have average constant-time complexity. |
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| + | Internally, the elements are not sorted in any particular order, but organized into buckets. Which bucket an element is placed into depends entirely on the hash of its key. This allows fast access to individual elements, since once hash is computed, it refers to the exact bucket the element is placed into. | ||
{{tt|std::unordered_map}} meets the requirements of {{concept|Container}}, {{concept|AllocatorAwareContainer}}, {{concept|UnorderedAssociativeContainer}}. | {{tt|std::unordered_map}} meets the requirements of {{concept|Container}}, {{concept|AllocatorAwareContainer}}, {{concept|UnorderedAssociativeContainer}}. | ||
Revision as of 06:29, 16 October 2013
| Defined in header <unordered_map>
|
||
| template< class Key, |
(since C++11) | |
Unordered map is an associative container that contains key-value pairs with unique keys. Search, insertion, and removal of elements have average constant-time complexity.
Internally, the elements are not sorted in any particular order, but organized into buckets. Which bucket an element is placed into depends entirely on the hash of its key. This allows fast access to individual elements, since once hash is computed, it refers to the exact bucket the element is placed into.
std::unordered_map meets the requirements of Template:concept, Template:concept, Template:concept.
Contents |
Member types
| Member type | Definition |
key_type
|
Key
|
mapped_type
|
T
|
value_type
|
std::pair<const Key, T> |
size_type
|
Unsigned integer type (usually std::size_t) |
difference_type
|
Signed integer type (usually std::ptrdiff_t) |
hasher
|
Hash
|
key_equal
|
KeyEqual
|
allocator_type
|
Allocator
|
reference
|
value_type& |
const_reference
|
const value_type& |
pointer
|
std::allocator_traits<Allocator>::pointer |
const_pointer
|
std::allocator_traits<Allocator>::const_pointer |
iterator
|
LegacyForwardIterator to value_type
|
const_iterator
|
LegacyForwardIterator to const value_type |
local_iterator
|
An iterator type whose category, value, difference, pointer and reference types are the same as iterator. This iteratorcan be used to iterate through a single bucket but not across buckets |
const_local_iterator
|
An iterator type whose category, value, difference, pointer and reference types are the same as const_iterator. This iteratorcan be used to iterate through a single bucket but not across buckets |
Member functions
constructs the unordered_map (public member function) | |
destructs the unordered_map (public member function) | |
| assigns values to the container (public member function) | |
| returns the associated allocator (public member function) | |
Iterators | |
| returns an iterator to the beginning (public member function) | |
| returns an iterator to the end (public member function) | |
Capacity | |
| checks whether the container is empty (public member function) | |
| returns the number of elements (public member function) | |
| returns the maximum possible number of elements (public member function) | |
Modifiers | |
| clears the contents (public member function) | |
| inserts elements or nodes(since C++17) (public member function) | |
| constructs element in-place (public member function) | |
| constructs elements in-place using a hint (public member function) | |
| erases elements (public member function) | |
| swaps the contents (public member function) | |
Lookup | |
| access specified element with bounds checking (public member function) | |
| access or insert specified element (public member function) | |
| returns the number of elements matching specific key (public member function) | |
| finds element with specific key (public member function) | |
| returns range of elements matching a specific key (public member function) | |
Bucket interface | |
| returns an iterator to the beginning of the specified bucket (public member function) | |
| returns an iterator to the end of the specified bucket (public member function) | |
| returns the number of buckets (public member function) | |
| returns the maximum number of buckets (public member function) | |
| returns the number of elements in specific bucket (public member function) | |
| returns the bucket for specific key (public member function) | |
Hash policy | |
| returns average number of elements per bucket (public member function) | |
| manages maximum average number of elements per bucket (public member function) | |
| reserves at least the specified number of buckets and regenerates the hash table (public member function) | |
| reserves space for at least the specified number of elements and regenerates the hash table (public member function) | |
Observers | |
| returns function used to hash the keys (public member function) | |
| returns the function used to compare keys for equality (public member function) | |
Non-member functions
| (C++11)(C++11)(removed in C++20) |
compares the values in the unordered_map (function template) |
| specializes the std::swap algorithm (function template) |
