Unordered associative containers are Container
s 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 Key
s.
std::unordered_map and std::unordered_multimap also have a mapped type T
associated with the Key
.
If two Key
s 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 rearranged 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
).
Requirements
Legend

X

Container type

a

Object of type X

b

const Object of type X

a_uniq

Object in X when X supports unique keys

a_eq

Object in X when X supports multiple equivalent keys

i , j

InputIterator s denoting a valid range

p , q2

valid const_iterator to a

q , q1

dereferenceable const_iterator to a denoting a valid range

il

Object of std::initializer_list<X::value_type>

t

Object of type X::value_type

k

Object of type X::key_type

hf

const Object of type X::hasher

eq

const Object of type X::key_equal

n

Value of type X::size_type

z

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) 
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 )

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() )

a = b 
X& 

Copy assignment, also copies the hash function, predicate and maximum load factor 
average linear, worst quadratic (in b.size() )

a = il 
X& 
value_type CopyAssignable and CopyInsertable into X 
a = X(il) 
see above

UnorderedAssociativeContainers in the standard library

collection of unique keys, hashed by keys (class template) 

collection of keys, hashed by keys (class template) 

collection of keyvalue pairs, hashed by keys, keys are unique (class template) 

collection of keyvalue pairs, hashed by keys (class template) 