std::vector<T,Allocator>::vector

 

C++

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Named requirements

Feature test macros (C++20)

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Metaprogramming library (C++11)

Diagnostics library

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Strings library

Containers library

Iterators library

Ranges library (C++20)

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Numerics library

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Filesystem library (C++17)

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Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

vector<bool>

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

flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

Views

span (C++20)

mdspan (C++23)

Iterator invalidation

Member function table

Non-member function table

 

std::vector

Member types

Member functions

vector::vector

vector::~vector

vector::operator=

vector::assign

vector::assign_range (C++23)

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::insert_range (C++23)

vector::erase

vector::push_back

vector::emplace_back (C++11)

vector::append_range (C++23)

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)

 

(1)

vector();

(until C++17)

vector() noexcept(noexcept(Allocator()));

(since C++17) (until C++20)

constexpr vector() noexcept(noexcept(Allocator()));

(since C++20)

(2)

explicit vector( const Allocator& alloc );

(until C++17)

explicit vector( const Allocator& alloc ) noexcept;

(since C++17) (until C++20)

constexpr explicit vector( const Allocator& alloc ) noexcept;

(since C++20)

(3)

explicit vector( size_type count,                  const T& value = T(),                  const Allocator& alloc = Allocator());

(until C++11)

vector( size_type count,                  const T& value,                  const Allocator& alloc = Allocator());

(since C++11) (until C++20)

constexpr vector( size_type count,                   const T& value,                   const Allocator& alloc = Allocator());

(since C++20)

(4)

explicit vector( size_type count );

(since C++11) (until C++14)

explicit vector( size_type count, const Allocator& alloc = Allocator() );

(since C++14) (until C++20)

constexpr explicit vector( size_type count,                            const Allocator& alloc = Allocator() );

(since C++20)

(5)

template< class InputIt > vector( InputIt first, InputIt last,         const Allocator& alloc = Allocator() );

(until C++20)

template< class InputIt > constexpr vector( InputIt first, InputIt last,                   const Allocator& alloc = Allocator() );

(since C++20)

(6)

vector( const vector& other );

(until C++20)

constexpr vector( const vector& other );

(since C++20)

(7)

vector( const vector& other, const Allocator& alloc );

(since C++11) (until C++20)

constexpr vector( const vector& other, const Allocator& alloc );

(since C++20)

(8)

vector( vector&& other );

(since C++11) (until C++17)

vector( vector&& other ) noexcept;

(since C++17) (until C++20)

constexpr vector( vector&& other ) noexcept;

(since C++20)

(9)

vector( vector&& other, const Allocator& alloc );

(since C++11) (until C++20)

constexpr vector( vector&& other, const Allocator& alloc );

(since C++20)

(10)

vector( std::initializer_list<T> init,         const Allocator& alloc = Allocator() );

(since C++11) (until C++20)

constexpr vector( std::initializer_list<T> init,                   const Allocator& alloc = Allocator() );

(since C++20)

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), a) if InputIt is an integral type.	(until C++11)
This overload participates in overload resolution only if InputIt satisfies LegacyInputIterator, to avoid ambiguity with the overload (3).	(since C++11)

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

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

(since C++11)

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 vector's Allocator template parameter .

(since C++23)

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 vector's Allocator template parameter .

(since C++23)

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

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 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

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.

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.rev.reqmts]/17, and a more direct guarantee is under consideration via LWG 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> a(3); // creates a 3-element vector holding {0, 0, 0}
 
std::vector<int> d{1, 2}; // creates a 2-element vector holding {1, 2}
std::vector<int> c(1, 2); // creates a 1-element vector holding {2}

Example

Run this code

#include <vector>
#include <string>
#include <iostream>
 
template<typename T>
std::ostream& operator<<(std::ostream& s, const std::vector<T>& v) 
{
    s.put('[');
    char comma[3] = {'\0', ' ', '\0'};
    for (const auto& e : v)
    {
        s << comma << e;
        comma[0] = ',';
    }
    return s << ']';
}
 
int main() 
{
    // C++11 initializer list syntax:
    std::vector<std::string> words1 {"the", "frogurt", "is", "also", "cursed"};
    std::cout << "words1: " << words1 << '\n';
 
    // words2 == words1
    std::vector<std::string> words2(words1.begin(), words1.end());
    std::cout << "words2: " << words2 << '\n';
 
    // words3 == words1
    std::vector<std::string> words3(words1);
    std::cout << "words3: " << words3 << '\n';
 
    // words4 is {"Mo", "Mo", "Mo", "Mo", "Mo"}
    std::vector<std::string> words4(5, "Mo");
    std::cout << "words4: " << words4 << '\n';
}

Output:

words1: [the, frogurt, is, also, cursed]
words2: [the, frogurt, is, also, cursed]
words3: [the, frogurt, is, also, cursed]
words4: [Mo, Mo, Mo, Mo, Mo]

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 134	C++98	overload (5) allowed up to 2N  copy constructor calls in the input iterator case	changed to O(N) calls
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)