std::vector::vector(3)

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

Synopsis
vector(); (until
                        C++17)
                        (since vector() C++17) noexcept(noexcept(Allocator())); (until
                        C++20) constexpr vector() (since noexcept(noexcept(Allocator())); C++20) explicit vector( const (until Allocator& alloc ); C++17)
                        (since explicit vector( const C++17) Allocator& alloc ) noexcept; (until
                        C++20) constexpr explicit vector( const (since Allocator& alloc ) noexcept; C++20) explicit vector( size_type count,
                        (until const T& value = T(), C++11) const Allocator& alloc = Allocator()); vector( size_type count,
                        (since const T& value, C++11)
                        (until const Allocator& alloc = C++20) Allocator()); constexpr vector( size_type count,
                        (since const T& value, C++20) const Allocator& alloc = Allocator());
                        (since explicit vector( size_type count C++11) ); (until
                        C++14) explicit vector( size_type (since count, const Allocator& alloc = C++14) Allocator() ); (until
                        C++20) constexpr explicit vector( size_type count, (since const Allocator& alloc = C++20) Allocator() ); (1) template< class InputIt > vector( InputIt first, InputIt (until last, (2) C++20) const Allocator& alloc = Allocator() ); template< class InputIt > (3) constexpr vector( InputIt first, (since InputIt last, C++20) const Allocator& alloc = Allocator() ); vector( const vector& other ); (until
                        C++20) constexpr vector( const vector& (since other ); (4) C++20)
                        (since vector( const vector& other, C++11) const Allocator& alloc ); (until
                        C++20) constexpr vector( const vector& (since other, const Allocator& alloc ); (5) C++20)
                        (since vector( vector&& other ); C++11)
                        (until
                        C++17)
                        (since vector( vector&& other ) C++17) noexcept; (6) (until
                        C++20) constexpr vector( vector&& other (7) (since ) noexcept; C++20)
                        (since vector( vector&& other, const (8) C++11) Allocator& alloc ); (until
                        C++20) constexpr vector( vector&& (since other, const Allocator& alloc ); C++20) vector( std::initializer_list<T> (9) (since init, C++11) const Allocator& alloc = (until Allocator() ); (10) C++20) constexpr vector( std::initializer_list<T> init, (since const Allocator& alloc = C++20) 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.
   The template parameter Allocator is only deduced from the first (since C++23)
   argument while used in class template argument deduction.
   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.)
   The template parameter Allocator is only deduced from the first (since C++23)
   argument while used in class template argument deduction.
   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) Linear in distance between first and last
   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.requirements.general]/12, 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 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)