NAME¶

std::vector - std::vector

Synopsis¶

Defined in header <vector>
template<

class T, (1)
class Allocator = std::allocator<T>

> class vector;
namespace pmr {

template< class T >
using vector = std::vector<T, (2) (since C++17)
std::pmr::polymorphic_allocator<T>>;

}

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 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()^[1].

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
(since C++11), 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.
(since C++20)
However, std::vector objects generally cannot be constexpr, because
any dynamically allocated storage must be released in the same
evaluation of constant expression.

1. ↑ In libstdc++, shrink_to_fit() is not available in C++98 mode.

Template parameters¶

The type of the elements.

T must meet the requirements of CopyAssignable and (until
CopyConstructible. C++11)
The requirements that are imposed on the elements depend on the (since
actual operations performed on the container. Generally, it is C++11)
required that element type is a complete type and meets the (until
requirements of Erasable, but many member functions impose C++17)
stricter requirements.
The requirements that are imposed on the elements depend on the
actual operations performed on the container. Generally, it is
T - 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 (since
requirements. C++17)

Feature-test macro Value Std Feature
Minimal
__cpp_lib_incomplete_container_elements 201505L (C++17) incomplete
type
support

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.
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().
If the vector changed capacity, all of them.
insert, emplace 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&
Allocator::pointer (until C++11)
pointer std::allocator_traits<Allocator>::pointer (since C++11)

Allocator::const_pointer (until C++11)
const_pointer std::allocator_traits<Allocator>::const_pointer (since C++11)

LegacyRandomAccessIterator and (until C++20)
LegacyContiguousIterator to value_type
LegacyRandomAccessIterator,
iterator contiguous_iterator, and ConstexprIterator to (since C++20)
value_type

LegacyRandomAccessIterator and (until C++20)
LegacyContiguousIterator to const value_type
LegacyRandomAccessIterator,
const_iterator contiguous_iterator, and ConstexprIterator to (since C++20)
const value_type

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)
assign_range assigns a range of values to the container
(C++23) (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 contiguous storage
(public member function)

Iterators¶

begin returns an iterator to the beginning
cbegin (public member function)
(C++11)
end returns an iterator to the end
cend (public member function)
(C++11)
rbegin returns a reverse iterator to the beginning
crbegin (public member function)
(C++11)
rend returns a reverse iterator to the end
crend (public member function)
(C++11)

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)
returns the number of elements that can be held in currently allocated
capacity storage
(public member function)
shrink_to_fit reduces memory usage by freeing unused memory
(DR*) (public member function)

Modifiers¶

clear clears the contents
(public member function)
insert inserts elements
(public member function)
insert_range inserts a range of elements
(C++23) (public member function)
emplace constructs element in-place
(C++11) (public member function)
erase erases elements
(public member function)
push_back adds an element to the end
(public member function)
emplace_back constructs an element in-place at the end
(C++11) (public member function)
append_range adds a range of elements to the end
(C++23) (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>= lexicographically compares the values of two vectors
operator<=> (function template)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(removed in C++20)
(C++20)
std::swap(std::vector) specializes the std::swap algorithm
(function template)
erase(std::vector) erases all elements satisfying specific criteria
erase_if(std::vector) (function template)
(C++20)

Deduction guides (since C++17)

Notes¶

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges construction and insertion for
containers

Example¶

// Run this code

#include <iostream>
#include <vector>

int main()
{
// Create a vector containing integers
std::vector<int> v = {8, 4, 5, 9};

// Add two more integers to vector
v.push_back(6);
v.push_back(9);

// Overwrite element at position 2
v[2] = -1;

// Print out the vector
for (int n : v)
std::cout << n << ' ';
std::cout << '\n';
}

Output:¶

8 4 -1 9 6 9

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 required
vector was not required
T was not required to be CopyConstructible T is also required to
LWG 230 C++98 (an element of type T might not be able to be CopyConstructible
be constructed)
LWG 464 C++98 access to the underlying storage of an data function provided
empty vector resulted in UB