NAME¶

std::vector::vector - std::vector::vector

Synopsis¶

vector(); (until
C++17)
(since
vector() C++17)
noexcept(noexcept(Allocator())); (constexpr
since
C++20)
explicit vector( const (until
Allocator& alloc ); C++17)
(since
explicit vector( const C++17)
Allocator& alloc ) noexcept; (constexpr
since
C++20)
explicit vector( size_type
count,

const T& value (until
= T(), C++11)

const
Allocator& alloc = Allocator()
);
vector( size_type count, (since
C++11)
const T& value, (constexpr
since
const Allocator& alloc = C++20)
Allocator() );
(since
explicit vector( size_type count C++11)
); (until
C++14)
explicit vector( size_type
count, (since
const C++14)
Allocator& alloc = Allocator()
);
template< class InputIt >

vector( InputIt first, InputIt (1) (constexpr
last, (5) since
C++20)
const Allocator& alloc = (2)
Allocator() );
(constexpr
vector( const vector& other ); (6) since
(3) C++20)
(since
vector( const vector& other, C++11)
const Allocator& alloc ); (7) (constexpr
since
C++20)
(since
(4) C++11)
(noexcept
vector( vector&& other ); (8) since
C++17)
(constexpr
since
C++20)
(since
vector( vector&& other, const C++11)
Allocator& alloc ); (9) (constexpr
since
C++20)
vector( std::initializer_list<T> (since
init, C++11)
const Allocator& alloc = (10) (constexpr
Allocator() ); since
C++20)
template<
container-compatible-range<T> R
>

constexpr vector( (11) (since
std::from_range_t, R&& rg, C++23)

const
Allocator& alloc = Allocator()
);

Constructs a new container from a variety of data sources, optionally using a user
supplied allocator alloc.

1) Default constructor. Constructs an empty container with a default-constructed
allocator.
2) Constructs an empty container with the given allocator alloc.
3) Constructs the container with count copies of elements with value value.
4) Constructs the container with count default-inserted instances of T. No copies
are made.
5) Constructs the container with the contents of the range [first, last).

This constructor has the same effect as
vector(static_cast<size_type>(first), static_cast<value_type>(last), (until C++11)
a) if InputIt is an integral type.
This overload participates in overload resolution only if InputIt
satisfies LegacyInputIterator, to avoid ambiguity with the overload (since C++11)
(3).

6) Copy constructor. Constructs the container with the copy of the contents of
other.

The allocator is obtained as if by calling (since
std::allocator_traits<allocator_type>::select_on_container_copy_construction( C++11)
other.get_allocator()).

7) Constructs the container with the copy of the contents of other, using alloc as
the allocator.

During class template argument deduction, only the first argument
contributes to the deduction of the container's Allocator template (since C++23)
parameter.

8) Move constructor. Constructs the container with the contents of other using move
semantics. Allocator is obtained by move-construction from the allocator belonging
to other. After the move, other is guaranteed to be empty().
9) Allocator-extended move constructor. Using alloc as the allocator for the new
container, moving the contents from other; if alloc != other.get_allocator(), this
results in an element-wise move. (In that case, other is not guaranteed to be empty
after the move.)

During class template argument deduction, only the first argument
contributes to the deduction of the container's Allocator template (since C++23)
parameter.

10) Constructs the container with the contents of the initializer list init.
11) Constructs the container with the contents of the range rg.

Parameters¶

alloc - allocator to use for all memory allocations of this container
count - the size of the container
value - the value to initialize elements of the container with
first, last - the range [first, last) to copy the elements from
other - another container to be used as source to initialize the elements of
the container with
init - initializer list to initialize the elements of the container with
rg - a container compatible range, that is, an input_range whose elements
are convertible to T

Complexity¶

1,2) Constant.
3,4) Linear in count.
5) Given the distance between first and last as N,
* If first and last are both forward, bidirectional or random-access iterators,

* The copy constructor of T is only called N times, and
* No reallocation occurs.
* Otherwise (first and last are just input iterators),

* The copy constructor of T is called O(N) times, and
* Reallocation occurs O(log N) times.
6,7) Linear in size of other.
8) Constant.
9) Linear if alloc != other.get_allocator(), otherwise constant.
10) Linear in size of init.
11) Given ranges::distance(rg) as N,
* If R models ranges::forward_range or ranges::sized_range,

* Initializes exactly N elements from the result of dereferencing successive
iterators of rg, and
* No reallocation occurs.
* Otherwise (R models input range),

* The copy or move constructor of T is called O(N) times, and
* Reallocation occurs O(log N) times.

Exceptions¶

Calls to Allocator::allocate may throw.

Notes¶

After container move construction (overload (8)), references, pointers, and
iterators (other than the end iterator) to other remain valid, but refer to elements
that are now in *this. The current standard makes this guarantee via the blanket
statement in [container.reqmts]/67, and a more direct guarantee is under
consideration via LWG issue 2321.

The overload (4) zeroes out elements of non-class types such as int, which is
different from the behavior of new[], which leaves them uninitialized. To match the
behavior of new[], a custom Allocator::construct can be provided which leaves such
elements uninitialized.

Note that the presence of list-initializing constructor (10) means list
initialization and direct initialization do different things:

std::vector<int> b{3}; // creates a 1-element vector holding {3}
std::vector<int> d(3); // creates a 3-element vector holding {0, 0, 0}

std::vector<int> p{1, 2}; // creates a 2-element vector holding {1, 2}
std::vector<int> q(1, 2); // creates a 1-element vector holding {2}

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion;
overload (11)

Example¶

// Run this code

#include <iostream>
#include <string>
#include <vector>

template<typename T>
std::ostream& operator<<(std::ostream& s, const std::vector<T>& v)
{
s.put('{');
for (char comma[]{'\0', ' ', '\0'}; const auto& e : v)
s << comma << e, comma[0] = ',';
return s << "}\n";
}

int main()
{
// C++11 initializer list syntax:
std::vector<std::string> words1{"the", "frogurt", "is", "also", "cursed"};
std::cout << "1: " << words1;

// words2 == words1
std::vector<std::string> words2(words1.begin(), words1.end());
std::cout << "2: " << words2;

// words3 == words1
std::vector<std::string> words3(words1);
std::cout << "3: " << words3;

// words4 is {"Mo", "Mo", "Mo", "Mo", "Mo"}
std::vector<std::string> words4(5, "Mo");
std::cout << "4: " << words4;

auto const rg = {"cat", "cow", "crow"};
#ifdef __cpp_lib_containers_ranges
std::vector<std::string> words5(std::from_range, rg); // overload (11)
#else
std::vector<std::string> words5(rg.begin(), rg.end()); // overload (5)
#endif
std::cout << "5: " << words5;
}

Output:¶

1: {the, frogurt, is, also, cursed}
2: {the, frogurt, is, also, cursed}
3: {the, frogurt, is, also, cursed}
4: {Mo, Mo, Mo, Mo, Mo}
5: {cat, cow, crow}

Defect reports

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

DR Applied to Behavior as published Correct behavior
overload (5) allowed up to 2N  copy
LWG 134 C++98 constructor calls in the input changed to O(N) calls
iterator case
LWG 868 C++98 for overload (4), the elements in the they are value-initialized
container were default constructed
LWG 2193 C++11 the default constructor is explicit made non-explicit

See also¶

assign assigns values to the container
(public member function)
operator= assigns values to the container
(public member function)