std::vector

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

Views

span (C++20)

 

std::vector

Member functions

vector::vector

vector::~vector

vector::operator=

vector::assign

vector::get_allocator

Element access

vector::at

vector::operator[]

vector::front

vector::back

vector::data

Iterators

vector::beginvector::cbegin (C++11)

vector::endvector::cend (C++11)

vector::rbeginvector::crbegin (C++11)

vector::rendvector::crend (C++11)

Capacity

vector::empty

vector::size

vector::max_size

vector::reserve

vector::capacity

vector::shrink_to_fit (C++11)

Modifiers

vector::clear

vector::insert

vector::emplace (C++11)

vector::erase

vector::push_back

vector::emplace_back (C++11)

vector::pop_back

vector::resize

vector::swap

Non-member functions

std::swap

eraseerase_if (C++20)(C++20)

operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

Deduction guides(C++17)

 

Defined in header <vector>
template<     class T,     class Allocator = std::allocator<T> > class vector;	(1)
namespace pmr {     template< class T >     using vector = std::vector<T, std::pmr::polymorphic_allocator<T>>; }	(2)	(since C++17)

1) std::vector is a sequence container that encapsulates dynamic size arrays.

2) std::pmr::vector is an alias template that uses a polymorphic allocator.

The elements are stored contiguously, which means that elements can be accessed not only through iterators, but also using offsets to regular pointers to elements. This means that a pointer to an element of a vector may be passed to any function that expects a pointer to an element of an array.

The storage of the vector is handled automatically, being expanded and contracted as needed. Vectors usually occupy more space than static arrays, because more memory is allocated to handle future growth. This way a vector does not need to reallocate each time an element is inserted, but only when the additional memory is exhausted. The total amount of allocated memory can be queried using capacity() function. Extra memory can be returned to the system via a call to shrink_to_fit(). (since C++11)

Reallocations are usually costly operations in terms of performance. The reserve() function can be used to eliminate reallocations if the number of elements is known beforehand.

The complexity (efficiency) of common operations on vectors is as follows:

• Random access - constant 𝓞(1)
• Insertion or removal of elements at the end - amortized constant 𝓞(1)
• Insertion or removal of elements - linear in the distance to the end of the vector 𝓞(n)

std::vector (for T other than bool) meets the requirements of Container, AllocatorAwareContainer, SequenceContainer , ContiguousContainer (since C++17) and ReversibleContainer.

Member functions of std::vector are constexpr: it is possible to create and use std::vector objects in the evaluation of a constant expression. However, std::vector objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.

(since C++20)

Template parameters

T	-	The type of the elements. T must meet the requirements of CopyAssignable and CopyConstructible. (until C++11) The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type is a complete type and meets the requirements of Erasable, but many member functions impose stricter requirements. (since C++11) (until C++17) The requirements that are imposed on the elements depend on the actual operations performed on the container. Generally, it is required that element type meets the requirements of Erasable, but many member functions impose stricter requirements. This container (but not its members) can be instantiated with an incomplete element type if the allocator satisfies the allocator completeness requirements. Feature-test macro: __cpp_lib_incomplete_container_elements (since C++17)
Allocator	-	An allocator that is used to acquire/release memory and to construct/destroy the elements in that memory. The type must meet the requirements of Allocator. The behavior is undefined (until C++20)The program is ill-formed (since C++20) if Allocator::value_type is not the same as T.

Specializations

The standard library provides a specialization of std::vector for the type bool, which may be optimized for space efficiency.

vector<bool>

space-efficient dynamic bitset (class template specialization)

Iterator invalidation

Operations	Invalidated
All read only operations	Never
swap, std::swap	end()
clear, operator=, assign	Always
reserve, shrink_to_fit	If the vector changed capacity, all of them. If not, none.
erase	Erased elements and all elements after them (including end())
push_back, emplace_back	If the vector changed capacity, all of them. If not, only end().
insert, emplace	If the vector changed capacity, all of them. If not, only those at or after the insertion point (including end()).
resize	If the vector changed capacity, all of them. If not, only end() and any elements erased.
pop_back	The element erased and end().

Member types

Member type	Definition
value_type	T
allocator_type	Allocator
size_type	Unsigned integer type (usually std::size_t)
difference_type	Signed integer type (usually std::ptrdiff_t)
reference	value_type&
const_reference	const value_type&
pointer	Allocator::pointer (until C++11) std::allocator_traits<Allocator>::pointer (since C++11)
const_pointer	Allocator::const_pointer (until C++11) std::allocator_traits<Allocator>::const_pointer (since C++11)
iterator	LegacyRandomAccessIterator and LegacyContiguousIterator to value_type (until C++20) LegacyRandomAccessIterator, contiguous_iterator, and ConstexprIterator to value_type (since C++20)
const_iterator	LegacyRandomAccessIterator and LegacyContiguousIterator to const value_type (until C++20) LegacyRandomAccessIterator, contiguous_iterator, and ConstexprIterator to const value_type (since C++20)
reverse_iterator	std::reverse_iterator<iterator>
const_reverse_iterator	std::reverse_iterator<const_iterator>

Member functions

(constructor)	constructs the vector (public member function)
(destructor)	destructs the vector (public member function)
operator=	assigns values to the container (public member function)
assign	assigns values to the container (public member function)
get_allocator	returns the associated allocator (public member function)
Element access
at	access specified element with bounds checking (public member function)
operator[]	access specified element (public member function)
front	access the first element (public member function)
back	access the last element (public member function)
data	direct access to the underlying array (public member function)
Iterators
begin cbegin (C++11)	returns an iterator to the beginning (public member function)
end cend (C++11)	returns an iterator to the end (public member function)
rbegincrbegin (C++11)	returns a reverse iterator to the beginning (public member function)
rendcrend (C++11)	returns a reverse iterator to the end (public member function)
Capacity
empty	checks whether the container is empty (public member function)
size	returns the number of elements (public member function)
max_size	returns the maximum possible number of elements (public member function)
reserve	reserves storage (public member function)
capacity	returns the number of elements that can be held in currently allocated storage (public member function)
shrink_to_fit (C++11)	reduces memory usage by freeing unused memory (public member function)
Modifiers
clear	clears the contents (public member function)
insert	inserts elements (public member function)
emplace (C++11)	constructs element in-place (public member function)
erase	erases elements (public member function)
push_back	adds an element to the end (public member function)
emplace_back (C++11)	constructs an element in-place at the end (public member function)
pop_back	removes the last element (public member function)
resize	changes the number of elements stored (public member function)
swap	swaps the contents (public member function)

Non-member functions

operator==operator!=operator<operator<=operator>operator>=operator<=> (removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(removed in C++20)(C++20)	lexicographically compares the values in the vector (function template)
std::swap(std::vector)	specializes the std::swap algorithm (function template)
erase(std::vector)erase_if(std::vector) (C++20)	Erases all elements satisfying specific criteria (function template)

Example

Run this code

#include <iostream>
#include <vector>
 
int main()
{
    // Create a vector containing integers
    std::vector<int> v = { 7, 5, 16, 8 };
 
    // Add two more integers to vector
    v.push_back(25);
    v.push_back(13);
 
    // Print out the vector
    std::cout << "v = { ";
    for (int n : v) {
        std::cout << n << ", ";
    }
    std::cout << "}; \n";
}

Output:

v = { 7, 5, 16, 8, 25, 13, };

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 69	C++98	contiguity of the storage for elements of vector was not required	required
LWG 464	C++98	access to the underlying storage of an empty vector resulted in UB	data function provided