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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.
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&)
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.

See also

deque, list, slist


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