Associative containers provide fast retrieval of data based on keys. The library provides four basic kinds of associative containers: set, multiset, map and multimap.

Each associative container is parameterized on
Key
and an ordering relation
Compare
that induces a strict weak ordering on
elements of
Key.
In addition,
map
and
multimap
associate an arbitrary *mapped type*
T
with the
Key.
The object of type
Compare
is called the
*comparison object*
of a container.

The phrase “equivalence of keys” means the equivalence relation imposed by the comparison and not the operator== on keys. That is, two keys k1 and k2 are considered to be equivalent if for the comparison object comp, comp(k1, k2) == false && comp(k2, k1) == false. For any two keys k1 and k2 in the same container, calling comp(k1, k2) shall always return the same value.

An associative container supports *unique keys* if it may contain at
most one element for each key. Otherwise, it supports *equivalent keys*.
The set and map classes support unique keys; the multiset
and multimap classes support equivalent keys.
For multiset and multimap,
insert, emplace, and erase preserve the relative ordering
of equivalent elements.

For set and multiset the value type is the same as the key type. For map and multimap it is equal to pair<const Key, T>.

iterator of an associative container is of the bidirectional iterator category. For associative containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators. It is unspecified whether or not iterator and const_iterator are the same type. [ Note: iterator and const_iterator have identical semantics in this case, and iterator is convertible to const_iterator. Users can avoid violating the one-definition rule by always using const_iterator in their function parameter lists. — end note ]

The associative containers meet all the requirements of Allocator-aware containers, except that for map and multimap, the requirements placed on value_type in Table 83 apply instead to key_type and mapped_type. [ Note: For example, in some cases key_type and mapped_type are required to be CopyAssignable even though the associated value_type, pair<const key_type, mapped_type>, is not CopyAssignable. — end note ]

In Table 90,
X denotes an associative container class,
a denotes a value of type X,
a2 denotes a value of a type with nodes compatible with type
X (Table 89),
b denotes a possibly const value of type X,
u denotes the name of a variable being declared,
a_uniq denotes a value of type X
when X supports unique keys,
a_eq denotes a value of type X
when X supports multiple keys,
a_tran denotes a possibly const value of type X
when the *qualified-id*
X::key_compare::is_transparent is valid
and denotes a type ([temp.deduct]),
i and j
satisfy input iterator requirements and refer to elements
implicitly convertible to
value_type, [i, j)
denotes a valid range,
p denotes a valid constant iterator to a,
q denotes a valid dereferenceable constant iterator to a,
r denotes a valid dereferenceable iterator to a,
[q1, q2) denotes a valid range of constant iterators in a,
il designates an object of type initializer_list<value_type>,
t denotes a value of type X::value_type,
k denotes a value of type X::key_type
and c denotes a possibly const value of type X::key_compare;
kl is a value such that a is
partitioned with respect to
c(r, kl), with r the key value of e
and e in a;
ku is a value such that a is partitioned with respect to
!c(ku, r);
ke is a value such that a is partitioned with respect to
c(r, ke) and !c(ke, r), with c(r, ke) implying
!c(ke, r).
A denotes the storage allocator used by X, if any, or allocator<X::value_type> otherwise,
m denotes an allocator of a type convertible to A,
and nh denotes a non-const rvalue of type X::node_type.

Table 90 — Associative container requirements (in addition to container)

Expression | Return type | Assertion/note | Complexity |

pre-/post-condition | |||

X::key_type | Key | compile time | |

X::mapped_type (map and multimap only) | T | compile time | |

X::value_type (set and multiset only) | Key | Requires: value_type is Erasable from X | compile time |

X::value_type (map and multimap only) | pair<const Key, T> | Requires: value_type is Erasable from X | compile time |

X::key_compare | Compare | Requires: key_compare is CopyConstructible. | compile time |

X::value_compare | a binary predicate type | is the same as key_compare for set and multiset; is an ordering relation on pairs induced by the first component (i.e., Key) for map and multimap. | compile time |

X::node_type | a specialization of a node_handle class template, such that the public nested types are the same types as the corresponding types in X. | see [container.node] | compile time |

X(c) X u(c); | Effects: Constructs an empty container. Uses a copy of c as a comparison object. | constant | |

X() X u; |
Requires: key_compare is DefaultConstructible. Effects: Constructs an empty container. Uses Compare() as a comparison object | constant | |

X(i,j,c) X u(i,j,c); |
Requires: value_type is EmplaceConstructible into X from *i. Effects: Constructs an empty container and inserts elements from the range [i, j) into it; uses c as a comparison object. | NlogN in general, where N has the value distance(i, j); linear if [i, j) is sorted with value_comp() | |

X(i,j) X u(i,j); |
Requires: key_compare is DefaultConstructible.
value_type is EmplaceConstructible into X from *i. Effects: Same as above, but uses Compare() as a comparison object. | same as above | |

X(il) | same as X(il.begin(), il.end()) | same as X(il.begin(), il.end()) | |

X(il,c) | same as X(il.begin(), il.end(), c) | same as X(il.begin(), il.end(), c) | |

a = il | X& |
Requires: value_type is
CopyInsertable into X
and CopyAssignable. Effects: Assigns the range [il.begin(), il.end()) into a. All existing elements of a are either assigned to or destroyed. | NlogN in general, where N has the value il.size() + a.size(); linear if [il.begin(), il.end()) is sorted with value_comp() |

b.key_comp() | X::key_compare | returns the comparison object out of which b was constructed. | constant |

b.value_comp() | X::value_compare | returns an object of value_compare constructed out of the comparison object | constant |

a_uniq.emplace(args) | pair<iterator, bool> |
Requires: value_type shall be EmplaceConstructible into X from args. Effects: Inserts a value_type object t constructed with std::forward<Args>(args)... if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t. | logarithmic |

a_eq.emplace(args) | iterator |
Requires: value_type shall be EmplaceConstructible into X from args. Effects: Inserts a value_type object t constructed with std::forward<Args>(args)... and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent to t exists in a_eq, t is inserted at the end of that range. | logarithmic |

a.emplace_hint(p, args) | iterator | equivalent to a.emplace( std::forward<Args>(args)...). Return value is an iterator pointing to the element with the key equivalent to the newly inserted element. The element is inserted as close as possible to the position just prior to p. | logarithmic in general, but amortized constant if the element is inserted right before p |

a_uniq.insert(t) | pair<iterator, bool> |
Requires: If t is a non-const rvalue expression, value_type shall be
MoveInsertable into X; otherwise, value_type shall be
CopyInsertable into X. Effects: Inserts t if and only if there is no element in the container with key equivalent to the key of t. The bool component of the returned pair is true if and only if the insertion takes place, and the iterator component of the pair points to the element with key equivalent to the key of t. | logarithmic |

a_eq.insert(t) | iterator |
Requires: If t is a non-const rvalue expression, value_type shall be
MoveInsertable into X; otherwise, value_type shall be
CopyInsertable into X. Effects: Inserts t and returns the iterator pointing to the newly inserted element. If a range containing elements equivalent to t exists in a_eq, t is inserted at the end of that range. | logarithmic |

a.insert(p, t) | iterator |
Requires: If t is a non-const rvalue expression, value_type shall be
MoveInsertable into X; otherwise, value_type shall be
CopyInsertable into X. Effects: Inserts t if and only if there is no element with key equivalent to the key of t in containers with unique keys; always inserts t in containers with equivalent keys. Always returns the iterator pointing to the element with key equivalent to the key of t. t is inserted as close as possible to the position just prior to p. | logarithmic in general, but amortized constant if t is inserted right before p. |

a.insert(i, j) | void |
Requires: value_type shall be EmplaceConstructible into X from *i. Requires: i, j are not iterators into a. inserts each element from the range [i, j) if and only if there is no element with key equivalent to the key of that element in containers with unique keys; always inserts that element in containers with equivalent keys. | Nlog(a.size()+N), where N has the value distance(i, j) |

a.insert(il) | void | equivalent to a.insert(il.begin(), il.end()) | |

a_uniq.insert(nh) | insert_return_type |
Requires: nh is empty or
a_uniq.get_allocator() == nh.get_allocator(). Effects: If nh is empty, has no effect. Otherwise, inserts the element owned by nh if and only if there is no element in the container with a key equivalent to nh.key(). Postconditions: If nh is empty, inserted is false, position is end(), and node is empty. Otherwise if the insertion took place, inserted is true, position points to the inserted element, and node is empty; if the insertion failed, inserted is false, node has the previous value of nh, and position points to an element with a key equivalent to nh.key(). | logarithmic |

a_eq.insert(nh) | iterator |
Requires: nh is empty or
a_eq.get_allocator() == nh.get_allocator(). Effects: If nh is empty, has no effect and returns a_eq.end(). Otherwise, inserts the element owned by nh and returns an iterator pointing to the newly inserted element. If a range containing elements with keys equivalent to nh.key() exists in a_eq, the element is inserted at the end of that range. Postconditions: nh is empty. | logarithmic |

a.insert(p, nh) | iterator |
Requires: nh is empty or
a.get_allocator() == nh.get_allocator(). Effects: If nh is empty, has no effect and returns a.end(). Otherwise, inserts the element owned by nh if and only if there is no element with key equivalent to nh.key() in containers with unique keys; always inserts the element owned by nh in containers with equivalent keys. Always returns the iterator pointing to the element with key equivalent to nh.key(). The element is inserted as close as possible to the position just prior to p. Postconditions: nh is empty if insertion succeeds, unchanged if insertion fails. | logarithmic in general, but amortized constant if the element is inserted right before p. |

a.extract(k) | node_type | removes the first element in the container with key equivalent to k. Returns a node_type owning the element if found, otherwise an empty node_type. | log(a.size()) |

a.extract(q) | node_type | removes the element pointed to by q. Returns a node_type owning that element. | amortized constant |

a.merge(a2) | void |
Requires: a.get_allocator() == a2.get_allocator(). Attempts to extract each element in a2 and insert it into a using the comparison object of a. In containers with unique keys, if there is an element in a with key equivalent to the key of an element from a2, then that element is not extracted from a2. Postconditions: Pointers and references to the transferred elements of a2 refer to those same elements but as members of a. Iterators referring to the transferred elements will continue to refer to their elements, but they now behave as iterators into a, not into a2. Throws: Nothing unless the comparison object throws. | Nlog(a.size()+N), where N has the value a2.size(). |

a.erase(k) | size_type | erases all elements in the container with key equivalent to k. returns the number of erased elements. | log(a.size())+a.count(k) |

a.erase(q) | iterator | erases the element pointed to by q. Returns an iterator pointing to the element immediately following q prior to the element being erased. If no such element exists, returns a.end(). | amortized constant |

a.erase(r) | iterator | erases the element pointed to by r. Returns an iterator pointing to the element immediately following r prior to the element being erased. If no such element exists, returns a.end(). | amortized constant |

a.erase( q1, q2) | iterator | erases all the elements in the range [q1, q2). Returns an iterator pointing to the element pointed to by q2 prior to any elements being erased. If no such element exists, a.end() is returned. | log(a.size())+N, where N has the value distance(q1, q2). |

a.clear() | void |
a.erase(a.begin(),a.end()) Postconditions: a.empty() returns true. | linear in a.size(). |

b.find(k) | iterator; const_iterator for constant b. | returns an iterator pointing to an element with the key equivalent to k, or b.end() if such an element is not found | logarithmic |

a_tran. find(ke) | iterator; const_iterator for constant a_tran. | returns an iterator pointing to an element with key r such that !c(r, ke) && !c(ke, r), or a_tran.end() if such an element is not found | logarithmic |

b.count(k) | size_type | returns the number of elements with key equivalent to k | log(b.size())+b.count(k) |

a_tran. count(ke) | size_type | returns the number of elements with key r such that !c(r, ke) && !c(ke, r) | log(a_tran.size())+a_tran.count(ke) |

b.lower_bound(k) | iterator; const_iterator for constant b. | returns an iterator pointing to the first element with key not less than k, or b.end() if such an element is not found. | logarithmic |

a_tran. lower_bound(kl) | iterator; const_iterator for constant a_tran. | returns an iterator pointing to the first element with key r such that !c(r, kl), or a_tran.end() if such an element is not found. | logarithmic |

b.upper_bound(k) | iterator; const_iterator for constant b. | returns an iterator pointing to the first element with key greater than k, or b.end() if such an element is not found. | logarithmic |

a_tran. upper_bound(ku) | iterator; const_iterator for constant a_tran. | returns an iterator pointing to the first element with key r such that c(ku, r), or a_tran.end() if such an element is not found. | logarithmic |

b.equal_range(k) | pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant b. | equivalent to make_pair(b.lower_bound(k), b.upper_bound(k)). | logarithmic |

a_tran. equal_range(ke) | pair<iterator, iterator>; pair<const_iterator, const_iterator> for constant a_tran. |
equivalent to make_pair( a_tran.lower_bound(ke), a_tran.upper_bound(ke)). | logarithmic |

The insert and emplace members shall not affect the validity of iterators and references to the container, and the erase members shall invalidate only iterators and references to the erased elements.

The extract members invalidate only iterators to the removed element; pointers and references to the removed element remain valid. However, accessing the element through such pointers and references while the element is owned by a node_type is undefined behavior. References and pointers to an element obtained while it is owned by a node_type are invalidated if the element is successfully inserted.

The fundamental property of iterators of associative containers is that they iterate through the containers in the non-descending order of keys where non-descending is defined by the comparison that was used to construct them. For any two dereferenceable iterators i and j such that distance from i to j is positive, the following condition holds:

value_comp(*j, *i) == false

For associative containers with unique keys the stronger condition holds:

value_comp(*i, *j) != false

When an associative container is constructed by passing a comparison object the container shall not store a pointer or reference to the passed object, even if that object is passed by reference. When an associative container is copied, either through a copy constructor or an assignment operator, the target container shall then use the comparison object from the container being copied, as if that comparison object had been passed to the target container in its constructor.

The member function templates find, count, lower_bound,
upper_bound, and equal_range shall not participate in overload
resolution unless the *qualified-id* Compare::is_transparent is valid
and denotes a type ([temp.deduct]).

A deduction guide for an associative container shall not participate in overload resolution if any of the following are true:

It has an InputIterator template parameter and a type that does not qualify as an input iterator is deduced for that parameter.

It has an Allocator template parameter and a type that does not qualify as an allocator is deduced for that parameter.

It has a Compare template parameter and a type that qualifies as an allocator is deduced for that parameter.

For associative containers, no clear() function throws an exception. erase(k) does not throw an exception unless that exception is thrown by the container's Compare object (if any).

For associative containers, if an exception is thrown by any operation from within an insert or emplace function inserting a single element, the insertion has no effect.