vector<T, Alloc>


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.	`alloc`

Model of

Random Access Container, Back Insertion Sequence.

Type requirements

None, except for those imposed by the requirements of Random Access Container and Back Insertion Sequence.

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`	Reversible Container	Iterator used to iterate backwards through a `vector`.
`const_reverse_iterator`	Reversible Container	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()`	Reversible Container	Returns a `reverse_iterator` pointing to the beginning of the reversed vector.
`reverse_iterator rend()`	Reversible Container	Returns a `reverse_iterator` pointing to the end of the reversed vector.
`const_reverse_iterator rbegin() const`	Reversible Container	Returns a `const_reverse_iterator` pointing to the beginning of the reversed vector.
`const_reverse_iterator rend() const`	Reversible Container	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)`	Random Access Container	Returns the `n`'th element.
`const_reference operator[](size_type n) const`	Random Access Container	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()`	Back Insertion Sequence	Returns the last element.
`const_reference back() const`	Back Insertion Sequence	Returns the last element.
`void push_back(const T&)`	Back Insertion Sequence	Inserts a new element at the end.
`void pop_back()`	Back Insertion Sequence	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.
bool operator==(const vector&, const vector&)	Forward Container	Tests two vectors for equality. This is a global function, not a member function.
bool operator<(const vector&, const vector&)	Forward Container	Lexicographical comparison. This is a global function, not a member function.

New members

These members are not defined in the Random Access Container and Back Insertion Sequence 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 input iterator. 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.