23 Containers library [containers]

Containers are objects that store other objects. They control allocation and deallocation of these objects through constructors, destructors, insert and erase operations.

All of the complexity requirements in this Clause are stated solely in terms of the number of operations on the contained objects. [ Example: the copy constructor of type vector <vector<int> > has linear complexity, even though the complexity of copying each contained vector<int> is itself linear.  — end example ]

For the components affected by this subclause that declare an allocator_type, objects stored in these components shall be constructed using the allocator_traits<allocator_type>::construct function and destroyed using the allocator_traits<allocator_type>::destroy function ([allocator.traits.members]). These functions are called only for the container's element type, not for internal types used by the container. [ Note: This means, for example, that a node-based container might need to construct nodes containing aligned buffers and call construct to place the element into the buffer.  — end note ]

In Tables [tab:containers.container.requirements], [tab:containers.reversible.requirements], and [tab:containers.optional.operations] X denotes a container class containing objects of type T, a and b denote values of type X, u denotes an identifier, r denotes a non-const value of type X, and rv denotes a non-const rvalue of type X.

Notes: the algorithm equal() is defined in Clause [algorithms]. Those entries marked “(Note A)” or “(Note B)” have linear complexity for array and have constant complexity for all other standard containers.

The member function size() returns the number of elements in the container. The number of elements is defined by the rules of constructors, inserts, and erases.

begin() returns an iterator referring to the first element in the container. end() returns an iterator which is the past-the-end value for the container. If the container is empty, then begin() == end();

In the expressions

i == j
i != j
i < j
i <= j
i >= j
i > j
i - j

where i and j denote objects of a container's iterator type, either or both may be replaced by an object of the container's const_iterator type referring to the same element with no change in semantics.

Unless otherwise specified, all containers defined in this clause obtain memory using an allocator (see [allocator.requirements]). Copy constructors for these container types obtain an allocator by calling allocator_traits<allocator_type>::select_on_container_copy_construction on the allocator belonging to the container being copied. Move constructors obtain an allocator by move construction from the allocator belonging to the container being moved. Such move construction of the allocator shall not exit via an exception. All other constructors for these container types take a const allocator_type& argument. [ Note: If an invocation of a constructor uses the default value of an optional allocator argument, then the Allocator type must support value initialization.  — end note ] A copy of this allocator is used for any memory allocation performed, by these constructors and by all member functions, during the lifetime of each container object or until the allocator is replaced. The allocator may be replaced only via assignment or swap(). Allocator replacement is performed by copy assignment, move assignment, or swapping of the allocator only if allocator_traits<allocator_type>::propagate_on_container_copy_assignment::value, allocator_traits<allocator_type>::propagate_on_container_move_assignment::value, or allocator_traits<allocator_type>::propagate_on_container_swap::value is true within the implementation of the corresponding container operation. The behavior of a call to a container's swap function is undefined unless the objects being swapped have allocators that compare equal or allocator_traits<allocator_type>::propagate_on_container_swap::value is true. In all container types defined in this Clause, the member get_allocator() returns a copy of the allocator used to construct the container or, if that allocator has been replaced, a copy of the most recent replacement.

The expression a.swap(b), for containers a and b of a standard container type other than array, shall exchange the values of a and b without invoking any move, copy, or swap operations on the individual container elements. Any Compare, Pred, or Hash objects belonging to a and b shall be swappable and shall be exchanged by unqualified calls to non-member swap. If allocator_traits<allocator_type>::propagate_on_container_swap::value is true, then the allocators of a and b shall also be exchanged using an unqualified call to non-member swap. Otherwise, they shall not be swapped, and the behavior is undefined unless a.get_allocator() == b.get_allocator(). Every iterator referring to an element in one container before the swap shall refer to the same element in the other container after the swap. It is unspecified whether an iterator with value a.end() before the swap will have value b.end() after the swap.

If the iterator type of a container belongs to the bidirectional or random access iterator categories ([iterator.requirements]), the container is called reversible and satisfies the additional requirements in Table [tab:containers.reversible.requirements].

Unless otherwise specified (see [associative.reqmts.except], [unord.req.except], [deque.modifiers], and [vector.modifiers]) all container types defined in this Clause meet the following additional requirements:

• (11.1)

  if an exception is thrown by an insert() or emplace() function while inserting a single element, that function has no effects.

• (11.2)

  if an exception is thrown by a push_back(), push_front(), emplace_back(), or emplace_front() function, that function has no effects.

• (11.3)

  no erase(), clear(), pop_back() or pop_front() function throws an exception.

• (11.4)

  no copy constructor or assignment operator of a returned iterator throws an exception.

• (11.5)

  no swap() function throws an exception.

• (11.6)

  no swap() function invalidates any references, pointers, or iterators referring to the elements of the containers being swapped. [ Note: The end() iterator does not refer to any element, so it may be invalidated.  — end note ]

Unless otherwise specified (either explicitly or by defining a function in terms of other functions), invoking a container member function or passing a container as an argument to a library function shall not invalidate iterators to, or change the values of, objects within that container.

Table [tab:containers.optional.operations] lists operations that are provided for some types of containers but not others. Those containers for which the listed operations are provided shall implement the semantics described in Table [tab:containers.optional.operations] unless otherwise stated.

Note: the algorithm lexicographical_compare() is defined in Clause [algorithms].

All of the containers defined in this Clause and in ([basic.string]) except array meet the additional requirements of an allocator-aware container, as described in Table [tab:containers.allocatoraware].

Given a container type X having an allocator_type identical to A and a value_type identical to T and given an lvalue m of type A, a pointer p of type T*, an expression v of type (possibly const) T, and an rvalue rv of type T, the following terms are defined. If X is not allocator-aware, the terms below are defined as if A were std::allocator<T> — no allocator object needs to be created and user specializations of std::allocator<T> are not instantiated:

• (14.1)

  T is DefaultInsertable into X means that the following expression is well-formed:

  allocator_traits<A>::construct(m, p)
  

• (14.2)

  An element of X is default-inserted if it is initialized by evaluation of the expression

  allocator_traits<A>::construct(m, p)
  

  where p is the address of the uninitialized storage for the element allocated within X.

• (14.3)

  T is MoveInsertable into X means that the following expression is well-formed:

  allocator_traits<A>::construct(m, p, rv)
  

  and its evaluation causes the following postcondition to hold: The value of *p is equivalent to the value of rv before the evaluation. [ Note: rv remains a valid object. Its state is unspecified  — end note ]

• (14.4)

  T is CopyInsertable into X means that, in addition to T being MoveInsertable into X, the following expression is well-formed:

  allocator_traits<A>::construct(m, p, v)
  

  and its evaluation causes the following postcondition to hold: The value of v is unchanged and is equivalent to *p.

• (14.5)

  T is EmplaceConstructible into X from args, for zero or more arguments args, means that the following expression is well-formed:

  allocator_traits<A>::construct(m, p, args)
  

• (14.6)

  T is Erasable from X means that the following expression is well-formed:

  allocator_traits<A>::destroy(m, p)
  

[ Note: A container calls allocator_traits<A>::construct(m, p, args) to construct an element at p using args. The default construct in std::allocator will call ::new((void*)p) T(args), but specialized allocators may choose a different definition.  — end note ]

In Table [tab:containers.allocatoraware], X denotes an allocator-aware container class with a value_type of T using allocator of type A, u denotes a variable, a and b denote non-const lvalues of type X, t denotes an lvalue or a const rvalue of type X, rv denotes a non-const rvalue of type X, and m is a value of type A.

For purposes of avoiding data races ([res.on.data.races]), implementations shall consider the following functions to be const: begin, end, rbegin, rend, front, back, data, find, lower_bound, upper_bound, equal_range, at and, except in associative or unordered associative containers, operator[].

Notwithstanding ([res.on.data.races]), implementations are required to avoid data races when the contents of the contained object in different elements in the same container, excepting vector<bool>, are modified concurrently.

[ Note: For a vector<int> x with a size greater than one, x[1] = 5 and *x.begin() = 10 can be executed concurrently without a data race, but x[0] = 5 and *x.begin() = 10 executed concurrently may result in a data race. As an exception to the general rule, for a vector<bool> y, y[0] = true may race with y[1] = true.  — end note ]

A sequence container organizes a finite set of objects, all of the same type, into a strictly linear arrangement. The library provides four basic kinds of sequence containers: vector, forward_list, list, and deque. In addition, array is provided as a sequence container which provides limited sequence operations because it has a fixed number of elements. The library also provides container adaptors that make it easy to construct abstract data types, such as stacks or queues, out of the basic sequence container kinds (or out of other kinds of sequence containers that the user might define).

The sequence containers offer the programmer different complexity trade-offs and should be used accordingly. vector or array is the type of sequence container that should be used by default. list or forward_list should be used when there are frequent insertions and deletions from the middle of the sequence. deque is the data structure of choice when most insertions and deletions take place at the beginning or at the end of the sequence.

In Tables [tab:containers.sequence.requirements] and [tab:containers.sequence.optional], X denotes a sequence container class, a denotes a value of X containing elements of type T, A denotes X::allocator_type if it exists and std::allocator<T> if it doesn't, i and j denote iterators satisfying input iterator requirements and refer to elements implicitly convertible to value_type, [i, j) denotes a valid range, il designates an object of type initializer_list<value_type>, n denotes a value of X::size_type, p denotes a valid const iterator to a, q denotes a valid dereferenceable const iterator to a, [q1, q2) denotes a valid range of const iterators in a, t denotes an lvalue or a const rvalue of X::value_type, and rv denotes a non-const rvalue of X::value_type. Args denotes a template parameter pack; args denotes a function parameter pack with the pattern Args&&.

The complexities of the expressions are sequence dependent.

iterator and const_iterator types for sequence containers shall be at least of the forward iterator category.

The iterator returned from a.insert(p, t) points to the copy of t inserted into a.

The iterator returned from a.insert(p, rv) points to the copy of rv inserted into a.

The iterator returned from a.insert(p, n, t) points to the copy of the first element inserted into a, or p if n == 0.

The iterator returned from a.insert(p, i, j) points to the copy of the first element inserted into a, or p if i == j.

The iterator returned from a.insert(p, il) points to the copy of the first element inserted into a, or p if il is empty.

The iterator returned from a.emplace(p, args) points to the new element constructed from args into a.

The iterator returned from a.erase(q) points to the element immediately following q prior to the element being erased. If no such element exists, a.end() is returned.

The iterator returned by a.erase(q1,q2) points to the element pointed to by q2 prior to any elements being erased. If no such element exists, a.end() is returned.

For every sequence container defined in this Clause and in Clause [strings]:

• (14.1)

  If the constructor

  template <class InputIterator>
  X(InputIterator first, InputIterator last,
    const allocator_type& alloc = allocator_type())
  

  is called with a type InputIterator that does not qualify as an input iterator, then the constructor shall not participate in overload resolution.

• (14.2)

  If the member functions of the forms:

  template <class InputIterator>          // such as insert()
  rt fx1(const_iterator p, InputIterator first, InputIterator last);
  
  template <class InputIterator>          // such as append(), assign()
  rt fx2(InputIterator first, InputIterator last);
  
  template <class InputIterator>          // such as replace()
  rt fx3(const_iterator i1, const_iterator i2, InputIterator first, InputIterator last);
  

  are called with a type InputIterator that does not qualify as an input iterator, then these functions shall not participate in overload resolution.

The extent to which an implementation determines that a type cannot be an input iterator is unspecified, except that as a minimum integral types shall not qualify as input iterators.

Table [tab:containers.sequence.optional] lists operations that are provided for some types of sequence containers but not others. An implementation shall provide these operations for all container types shown in the “container” column, and shall implement them so as to take amortized constant time.

The member function at() provides bounds-checked access to container elements. at() throws out_of_range if n >= a.size().

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 ([alg.sorting]) 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 ([container.requirements.general]), except that for map and multimap, the requirements placed on value_type in Table [tab:containers.container.requirements] 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 [tab:containers.associative.requirements], X denotes an associative container class, a denotes a value of X, a_uniq denotes a value of X when X supports unique keys, a_eq denotes a value of X when X supports multiple keys, a_tran denotes a value of 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 const iterator to a, q denotes a valid dereferenceable const iterator to a, [q1, q2) denotes a valid range of const iterators in a, il designates an object of type initializer_list<value_type>, t denotes a value of X::value_type, k denotes a value of X::key_type and c denotes a value of type X::key_compare; kl is a value such that a is partitioned ([alg.sorting]) 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 std::allocator<X::value_type> otherwise, and m denotes an allocator of a type convertible to A.

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 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,

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]).

Unordered associative containers provide an ability for fast retrieval of data based on keys. The worst-case complexity for most operations is linear, but the average case is much faster. The library provides four unordered associative containers: unordered_set, unordered_map, unordered_multiset, and unordered_multimap.

Unordered associative containers conform to the requirements for Containers ([container.requirements]), except that the expressions a == b and a != b have different semantics than for the other container types.

Each unordered associative container is parameterized by Key, by a function object type Hash that meets the Hash requirements ([hash.requirements]) and acts as a hash function for argument values of type Key, and by a binary predicate Pred that induces an equivalence relation on values of type Key. Additionally, unordered_map and unordered_multimap associate an arbitrary mapped type T with the Key.

The container's object of type Hash — denoted by hash — is called the hash function of the container. The container's object of type Pred — denoted by pred — is called the key equality predicate of the container.

Two values k1 and k2 of type Key are considered equivalent if the container's key equality predicate returns true when passed those values. If k1 and k2 are equivalent, the container's hash function shall return the same value for both. [ Note: Thus, when an unordered associative container is instantiated with a non-default Pred parameter it usually needs a non-default Hash parameter as well.  — end note ] For any two keys k1 and k2 in the same container, calling pred(k1, k2) shall always return the same value. For any key k in a container, calling hash(k) shall always return the same value.

An unordered associative container supports unique keys if it may contain at most one element for each key. Otherwise, it supports equivalent keys. unordered_set and unordered_map support unique keys. unordered_multiset and unordered_multimap support equivalent keys. In containers that support equivalent keys, elements with equivalent keys are adjacent to each other in the iteration order of the container. Thus, although the absolute order of elements in an unordered container is not specified, its elements are grouped into equivalent-key groups such that all elements of each group have equivalent keys. Mutating operations on unordered containers shall preserve the relative order of elements within each equivalent-key group unless otherwise specified.

For unordered_set and unordered_multiset the value type is the same as the key type. For unordered_map and unordered_multimap it is std::pair<const Key, T>.

For unordered 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 elements of an unordered associative container are organized into buckets. Keys with the same hash code appear in the same bucket. The number of buckets is automatically increased as elements are added to an unordered associative container, so that the average number of elements per bucket is kept below a bound. Rehashing invalidates iterators, changes ordering between elements, and changes which buckets elements appear in, but does not invalidate pointers or references to elements. For unordered_multiset and unordered_multimap, rehashing preserves the relative ordering of equivalent elements.

The unordered associative containers meet all the requirements of Allocator-aware containers ([container.requirements.general]), except that for unordered_map and unordered_multimap, the requirements placed on value_type in Table [tab:containers.container.requirements] apply instead to key_type and mapped_type. [ Note: For example, key_type and mapped_type are sometimes required to be CopyAssignable even though the associated value_type, pair<const key_type, mapped_type>, is not CopyAssignable.  — end note ]

In table [tab:HashRequirements]: X is an unordered associative container class, a is an object of type X, b is a possibly const object of type X, a_uniq is an object of type X when X supports unique keys, a_eq is an object of type X when X supports equivalent keys, i and j are input iterators that refer to value_type, [i, j) is a valid range, p and q2 are valid const iterators to a, q and q1 are valid dereferenceable const iterators to a, [q1, q2) is a valid range in a, il designates an object of type initializer_list<value_type>, t is a value of type X::value_type, k is a value of type key_type, hf is a possibly const value of type hasher, eq is a possibly const value of type key_equal, n is a value of type size_type, and z is a value of type float.

Two unordered containers a and b compare equal if a.size() == b.size() and, for every equivalent-key group [Ea1,Ea2) obtained from a.equal_range(Ea1), there exists an equivalent-key group [Eb1,Eb2) obtained from b.equal_range(Ea1), such that is_permutation(Ea1, Ea2, Eb1, Eb2) returns true. For unordered_set and unordered_map, the complexity of operator== (i.e., the number of calls to the == operator of the value_type, to the predicate returned by key_equal(), and to the hasher returned by hash_function()) is proportional to N in the average case and to N² in the worst case, where N is a.size(). For unordered_multiset and unordered_multimap, the complexity of operator== is proportional to in the average case and to N² in the worst case, where N is a.size(), and E_i is the size of the i^th equivalent-key group in a. However, if the respective elements of each corresponding pair of equivalent-key groups Ea_i and Eb_i are arranged in the same order (as is commonly the case, e.g., if a and b are unmodified copies of the same container), then the average-case complexity for unordered_multiset and unordered_multimap becomes proportional to N (but worst-case complexity remains Ο(N²), e.g., for a pathologically bad hash function). The behavior of a program that uses operator== or operator!= on unordered containers is undefined unless the Hash and Pred function objects respectively have the same behavior for both containers and the equality comparison operator for Key is a refinement264 of the partition into equivalent-key groups produced by Pred.

The iterator types iterator and const_iterator of an unordered associative container are of at least the forward iterator category. For unordered associative containers where the key type and value type are the same, both iterator and const_iterator are const iterators.

The insert and emplace members shall not affect the validity of references to container elements, but may invalidate all iterators to the container. The erase members shall invalidate only iterators and references to the erased elements, and preserve the relative order of the elements that are not erased.

The insert and emplace members shall not affect the validity of iterators if (N+n) < z * B, where N is the number of elements in the container prior to the insert operation, n is the number of elements inserted, B is the container's bucket count, and z is the container's maximum load factor.