Vector

Category: containers		Component type: type

Description

A vector is a Sequence that supports random access to elements, constant time insertion and removal of elements at the end, and linear time insertion and removal of elements at the beginning or in the middle. The number of elements in a vector may vary dynamically; memory management is automatic. Vector is the simplest of the STL container classes, and in many cases the most efficient.

Example

vector<int> V;
V.insert(V.begin(), 3);
assert(V.size() == 1 && V.capacity() >= 1 && V[0] == 3);

Definition

Defined in the standard header vector, and in the nonstandard backward-compatibility header vector.h.

Template parameters

Parameter	Description	Default
T	The vector's value type: the type of object that is stored in the vector.
Alloc	The vector's allocator, used for all internal memory management.	Allocators

Model of

RandomAccessContainer, BackInsertionSequence.

Type requirements

None, except for those imposed by the requirements of RandomAccessContainer and BackInsertionSequence.

Public base classes

None.

Members

Member	Where defined	Description
value_type	Container	The type of object, T, stored in the vector.
pointer	Container	Pointer to T.
reference	Container	Reference to T
const_reference	Container	Const reference to T
size_type	Container	An unsigned integral type.
difference_type	Container	A signed integral type.
iterator	Container	Iterator used to iterate through a vector.
const_iterator	Container	Const iterator used to iterate through a vector.
reverse_iterator	ReversibleContainer	Iterator used to iterate backwards through a vector.
const_reverse_iterator	ReversibleContainer	Const iterator used to iterate backwards through a vector.
iterator begin()	Container	Returns an iterator pointing to the beginning of the vector.
iterator end()	Container	Returns an iterator pointing to the end of the vector.
const_iterator begin() const	Container	Returns a const_iterator pointing to the beginning of the vector.
const_iterator end() const	Container	Returns a const_iterator pointing to the end of the vector.
reverse_iterator rbegin()	ReversibleContainer	Returns a reverse_iterator pointing to the beginning of the reversed vector.
reverse_iterator rend()	ReversibleContainer	Returns a reverse_iterator pointing to the end of the reversed vector.
const_reverse_iterator rbegin() const	ReversibleContainer	Returns a const_reverse_iterator pointing to the beginning of the reversed vector.
const_reverse_iterator rend() const	ReversibleContainer	Returns a const_reverse_iterator pointing to the end of the reversed vector.
size_type size() const	Container	Returns the size of the vector.
size_type max_size() const	Container	Returns the largest possible size of the vector.
size_type capacity() const	vector	See below.
bool empty() const	Container	true if the vector's size is 0.
reference operator[](size_type n)	RandomAccessContainer	Returns the n'th element.
const_reference operator[](size_type n) const	RandomAccessContainer	Returns the n'th element.
vector()	Container	Creates an empty vector.
vector(size_type n)	Sequence	Creates a vector with n elements.
vector(size_type n, const T& t)	Sequence	Creates a vector with n copies of t.
vector(const vector&)	Container	The copy constructor.
template <class InputIterator> vector(InputIterator, InputIterator) [1]	Sequence	Creates a vector with a copy of a range.
~vector()	Container	The destructor.
vector& operator=(const vector&)	Container	The assignment operator
void reserve(size_t)	vector	See below.
reference front()	Sequence	Returns the first element.
const_reference front() const	Sequence	Returns the first element.
reference back()	BackInsertionSequence	Returns the last element.
const_reference back() const	BackInsertionSequence	Returns the last element.
void push_back(const T&)	BackInsertionSequence	Inserts a new element at the end.
void pop_back()	BackInsertionSequence	Removes the last element.
void swap(vector&)	Container	Swaps the contents of two vectors.
iterator insert(iterator pos, const T& x)	Sequence	Inserts x before pos.
template <class InputIterator> void insert(iterator pos, InputIterator f, InputIterator l) [1]	Sequence	Inserts the range [first, last) before pos.
void insert(iterator pos, size_type n, const T& x)	Sequence	Inserts n copies of x before pos.
iterator erase(iterator pos)	Sequence	Erases the element at position pos.
iterator erase(iterator first, iterator last)	Sequence	Erases the range [first, last)
void clear()	Sequence	Erases all of the elements.
void resize(n, t = T())	Sequence	Inserts or erases elements at the end such that the size becomes n.
bool operator==(const vector&, const vector&)	ForwardContainer	Tests two vectors for equality. This is a global function, not a member function.
bool operator<(const vector&, const vector&)	ForwardContainer	Lexicographical comparison. This is a global function, not a member function.

New members

These members are not defined in the RandomAccessContainer and BackInsertionSequence requirements, but are specific to vector.

Member	Description
size_type capacity() const	Number of elements for which memory has been allocated. capacity() is always greater than or equal to size(). [2] [3]
void reserve(size_type n)	If n is less than or equal to capacity(), this call has no effect. Otherwise, it is a request for allocation of additional memory. If the request is successful, then capacity() is greater than or equal to n; otherwise, capacity() is unchanged. In either case, size() is unchanged. [2] [4]

Notes

[1] This member function relies on member template functions, which at present (early 1998) are not supported by all compilers. If your compiler supports member templates, you can call this function with any type of InputIterator. If your compiler does not yet support member templates, though, then the arguments must be of type const value_type*.

[2] Memory will be reallocated automatically if more than capacity() - size() elements are inserted into the vector. Reallocation does not change size(), nor does it change the values of any elements of the vector. It does, however, increase capacity(), and it invalidates [5] any iterators that point into the vector.

[3] When it is necessary to increase capacity(), vector usually increases it by a factor of two. It is crucial that the amount of growth is proportional to the current capacity(), rather than a fixed constant: in the former case inserting a series of elements into a vector is a linear time operation, and in the latter case it is quadratic.

[4] Reserve() causes a reallocation manually. The main reason for using reserve() is efficiency: if you know the capacity to which your vector must eventually grow, then it is usually more efficient to allocate that memory all at once rather than relying on the automatic reallocation scheme. The other reason for using reserve() is so that you can control the invalidation of iterators. [5]

[5] A vector's iterators are invalidated when its memory is reallocated. Additionally, inserting or deleting an element in the middle of a vector invalidates all iterators that point to elements following the insertion or deletion point. It follows that you can prevent a vector's iterators from being invalidated if you use reserve() to preallocate as much memory as the vector will ever use, and if all insertions and deletions are at the vector's end.

See also

Deque, List, Slist