C++ concepts: UnorderedAssociativeContainer
Unordered associative containers are Containers that provide fast lookup of objects based on keys.
Worst case complexity is linear but on average much faster for most of the operations.
Unordered associative containers are parametrized by Key; Hash, a Hash function object which acts as hash function on Key; and Pred, a BinaryPredicate evaluating equivalence between Keys.
std::unordered_map and std::unordered_multimap also have a mapped type T associated with the Key.
If two Keys are equal according to Pred, Hash must return the same value for both keys.
std::unordered_map and std::unordered_set can contain at most one element with a given key, std::unordered_multiset and std::unordered_multimap instead can have multiple elements with the same key (which must always be adjacent on iterations).
For std::unordered_set and std::unordered_multiset the value type is the same as the key type and both iterator and const_iterator are constant iterators. For std::unordered_map and std::unordered_multimap the value type is std::pair<const Key, T>.
Elements in an unordered associative container are organized into buckets, keys with the same hash will end up in the same bucket. The number of buckets is increased when the size of the container increases to keep the average number of elements in each bucket under a certain value.
Rehashing invalidates iterator and might cause the elements to be re-arranged in different buckets but it doesn't invalidate references to the elements.
Unordered associative containers meet the requirements of AllocatorAwareContainer.
For std::unordered_map and std::unordered_multimap the requirements of value_type in AllocatorAwareContainer apply to key_type and
mapped_type (not to value_type).
[edit] Requirements
Legend |
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X
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Container type |
a
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Object of type X
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b
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const Object of type X
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a_uniq
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Object in X when X supports unique keys
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a_eq
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Object in X when X supports multiple equivalent keys
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i, j
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InputIterators denoting a valid range
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p, q2
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valid const_iterator to a
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q, q1
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dereferenceable const_iterator to a denoting a valid range
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il
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Object of std::initializer_list<X::value_type> |
t
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Object of type X::value_type
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k
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Object of type X::key_type
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hf
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const Object of type X::hasher
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eq
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const Object of type X::key_equal
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n
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Value of type X::size_type
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z
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Value of type float |
| expression | return type | pre/requirements | post/effects | complexity |
|---|---|---|---|---|
| X::key_type | Key |
compile time | ||
| X::mapped_type | T |
std::unordered_map and std::unordered_multimap only | compile time | |
| X::value_type | Key |
std::unordered_set and std::unordered_multiset only. Erasable in X |
compile time | |
| X::value_type | std::pair<const Key, T> |
std::unordered_map and std::unordered_multimap only. Erasable in X |
compile time | |
| X::hasher | Hash |
Hash |
compile time | |
| X::key_equal | Pred |
BinaryPredicate taking two arguments of type Key and expressing an equivalence relation. |
compile time | |
| X::local_iterator | An Iterator whose category and types are the same as X::iterator |
Can be used to iterate through a single bucket | compile time | |
| X::const_local_iterator | An Iterator whose category and types are the same as X::const_iterator |
Can be used to iterate through a single bucket | compile time | |
| X(n,hf,eq) | X |
hasher and key_equal CopyConstructible |
Constructs an empty container with at least n buckets, using the given hash function and equality predicate |
linear in n
|
| X(n,hf) | X |
hasher CopyConstructible, key_equal DefaultConstructible |
Constructs an empty container with at least n buckets, using the given hash function and key_equal() as equality predicate |
linear in n
|
| X(n,hf) | X |
hasher and key_equal DefaultConstructible |
Constructs an empty container with at least n buckets, using hasher() as hash function and key_equal() as equality predicate |
linear in n
|
| X() | X |
hasher and key_equal DefaultConstructible |
Constructs an empty container with an unspecified number of buckets, using hasher() as hash function and key_equal() as equality predicate |
constant |
| X(i,j,n,hf,eq) | X |
hasher and key_equal CopyConstructible, value_type EmplaceConstructible into X from *i |
Constructs an empty container with at least n buckets, using the given hash function and equality predicate, and inserts elements from [i,j) into it. |
average linear, worst quadratin (on the distance between i and j)
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| X(i,j,n,hf) | X |
key_equal DefaultConstructible
|
As above, with eq=key_equal() | see above |
| X(i,j,n) | X |
hasher DefaultConstructible
|
As above, with hf=hasher() | see above |
| X(i,j) | X |
As above, with an unspecified number of buckets | see above | |
| X(il) | X |
X(il.begin(),il.end() | see above | |
| X(il,n) | X |
X(il.begin(),il.end(),n | see above | |
| X(il,n,hf) | X |
X(il.begin(),il.end(),n,hf | see above | |
| X(il,n,hf,eq) | X |
X(il.begin(),il.end(),n,hf,eq | see above | |
| X(b) | X |
Copy constructors, also copies the hash function, predicate and maximum load factor | average linear, worst quadratic (in b.size())
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| a = b | X& |
Copy assignment, also copies the hash function, predicate and maximum load factor | average linear, worst quadratic (in b.size())
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| a = il | X& |
value_type CopyAssignable and CopyInsertable into X
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a = X(il) | see above |
| This section is incomplete Reason: requirements regarfing member functions |
[edit] UnorderedAssociativeContainers in the standard library
| (since C++11)
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collection of unique keys, hashed by keys (class template) |
| (since C++11)
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collection of keys, hashed by keys (class template) |
| (since C++11)
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collection of key-value pairs, hashed by keys, keys are unique (class template) |
| (since C++11)
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collection of key-value pairs, hashed by keys (class template) |