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.