C++ Standard Library |
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Standard Template Library |
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C standard library |
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In computing, unordered associative containers refer to a group of class templates in the standard library of the C++ programming language that implement variations of hash table. Being templates, they can be used to store arbitrary elements, such as integers or custom classes. The following containers are defined in the current revision of the C++ standard: unordered_set
, unordered_map
, unordered_multiset
, unordered_multimap
. Each of these containers differ only on constraints placed on their elements.
The unordered associative containers are similar to the associative containers in C++ standard library but has different constraints. As their name implies, the elements in the unordered associative containers are not ordered. This is due to the use of hashing to store objects. The containers can still be iterated through like a regular associative containers.
Contents |
The first widely used implementation of hash tables in the C++ language was hash_map
, hash_set
, hash_multimap
, hash_multiset
class templates of the SGI STL.[1]. Due to its usefulness it was later included in several other implementations of the C++ standard library, e.g. in the GCC's libstdc++[2] or in MSVC standard library.
The hash_*
class templates were proposed into C++ TR1 and were accepted under names unordered_*
.[3] Later, they were incorporated into the C++11 revision of the C++ standard.[4]. An implementation is also available in the Boost C++ Libraries as <boost/unordered_map.hpp>
[5].
The containers are defined in headers named after the names of the containers, e.g. unordored_set
is defined in header <unordered_set>
. All containers satisfy the requirements of the Container concept, which means they have begin()
, end()
, size()
, max_size()
, empty()
, and swap()
methods.
unordered_set (C++11) |
unordered_map (C++11) |
unordered_multiset (C++11) |
unordered_multimap (C++11) |
Description | |
---|---|---|---|---|---|
(constructor) | (constructor) | (constructor) | (constructor) | Constructs the container from variety of sources | |
(destructor) | (destructor) | (destructor) | (destructor) | Destructs the set and the contained elements | |
operator= |
operator= |
operator= |
operator= |
Assigns values to the container | |
get_allocator |
get_allocator |
get_allocator |
get_allocator |
Returns the allocator used to allocate memory for the elements | |
Element access | N/A | at |
N/A | N/A | Accesses specified element with bounds checking. |
N/A | operator[] |
N/A | N/A | Accesses specified element without bounds checking. | |
Iterators | begin |
begin |
begin |
begin |
Returns an iterator to the beginning of the container |
end |
end |
end |
end |
Returns an iterator to the end of the container | |
Capacity | empty |
empty |
empty |
empty |
Checks whether the container is empty |
size |
size |
size |
size |
Returns number of elements in the container. | |
max_size |
max_size |
max_size |
max_size |
Returns the maximum possible number of elements in the container | |
Modifiers | clear |
clear |
clear |
clear |
Clears the contents. |
insert |
insert |
insert |
insert |
Inserts elements. | |
emplace |
emplace |
emplace |
emplace |
Constructs elements in-place (C++11) | |
emplace_hint |
emplace_hint |
emplace_hint |
emplace_hint |
Constructs elements in-place using a hint (C++11) | |
erase |
erase |
erase |
erase |
Erases elements. | |
swap |
swap |
swap |
swap |
Swaps the contents with another container. | |
Lookup | count |
count |
count |
count |
Returns the number of elements matching specific key. |
find |
find |
find |
find |
Finds an element with specific key. | |
equal_range |
equal_range |
equal_range |
equal_range |
Returns a range of elements matching specific key. | |
Bucket interface | ... | ||||
Hash policy | ... | ||||
Observers | hash_function |
hash_function |
hash_function |
hash_function |
Returns the function used to create hash of a key |
key_eq |
key_eq |
key_eq |
key_eq |
Returns key comparison function. |
#include <iostream> #include <string> #include <unordered_map> int main() { std::unordered_map<std::string, int> months; months["january"] = 31; months["february"] = 28; months["march"] = 31; months["april"] = 30; months["may"] = 31; months["june"] = 30; months["july"] = 31; months["august"] = 31; months["september"] = 30; months["october"] = 31; months["november"] = 30; months["december"] = 31; std::cout << "september -> " << months["september"] << std::endl; std::cout << "april -> " << months["april"] << std::endl; std::cout << "december -> " << months["december"] << std::endl; std::cout << "february -> " << months["february"] << std::endl; return 0; }