C++ named requirements: 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 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.
If both Hash::is_transparent and Pred::is_transparent exist and each names a type, member functions 
(since C++20) 
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
).
[edit] 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

LegacyInputIterators 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 LegacyIterator 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 LegacyIterator 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 
This section is incomplete Reason: requirements regarding member functions 
[edit] Unordered associative containers in the standard library
(C++11) 
collection of unique keys, hashed by keys (class template) 
(C++11) 
collection of keys, hashed by keys (class template) 
(C++11) 
collection of keyvalue pairs, hashed by keys, keys are unique (class template) 
(C++11) 
collection of keyvalue pairs, hashed by keys (class template) 