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Difference between revisions of "cpp/container/vector/vector"

From cppreference.com
< cpp‎ | container‎ | vector
(Detached this page from the {{cpp/container/constructor}} template, and added LWG2210/N3346 DRs (the vector-exclusive notes are removed).)
(Added the vector-only preconditions from the requirements of SequenceContainer.)
Line 95: Line 95:
 
{{cpp/enable if|{{tt|InputIt}} satisfies the requirements of {{named req|InputIterator}}}}.
 
{{cpp/enable if|{{tt|InputIt}} satisfies the requirements of {{named req|InputIterator}}}}.
  
−
If {{tt|T}} is not {{named req|EmplaceConstructible}} into {{c/core|std::vector<T>}} from {{c|*first}}, the behavior is undefined.
+
If any of the following conditions is satisfied, the behavior is undefined:
 +
* {{tt|T}} is not {{named req|EmplaceConstructible}} into {{c/core|std::vector<T>}} from {{c|*first}}.
 +
* {{tt|Iter}} does not satisfy the requirements of {{named req|ForwardIterator}}, and {{tt|T}} is not {{named req|MoveInsertable}} into {{c/core|std::vector<T>}}.
 
}}
 
}}
 
{{rev end}}
 
{{rev end}}
  
 
@6@ Constructs a {{tt|vector}} with the contents of the range {{c|rg}}. Each iterator in the {{c|rg}} is dereferenced exactly once.
 
@6@ Constructs a {{tt|vector}} with the contents of the range {{c|rg}}. Each iterator in the {{c|rg}} is dereferenced exactly once.
−
@@ If {{tt|T}} is not {{named req|EmplaceConstructible}} into {{c/core|std::vector<T>}} from {{c|*ranges::begin(rg)}}, the behavior is undefined.
+
@@ If any of the following conditions is satisfied, the behavior is undefined:
 +
* {{tt|T}} is not {{named req|EmplaceConstructible}} into {{c/core|std::vector<T>}} from {{c|*ranges::begin(rg)}}.
 +
* {{tt|R}} models neither {{lconcept|sized_range}} nor {{lconcept|forward_range}}, and {{tt|T}} is not {{named req|MoveInsertable}} into {{c/core|std::vector<T>}}.
  
 
@7-10@ Constructs a {{tt|vector}} with the contents of {{c|other}}.
 
@7-10@ Constructs a {{tt|vector}} with the contents of {{c|other}}.

Revision as of 19:07, 10 November 2024

 
C++
 
Containers library
 
std::vector
Member types
Member functions
Non-member functions
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides(C++17)
 
(1)
vector() : vector(Allocator()) {}
 (since C++11)
(until C++17)
vector() noexcept(noexcept(Allocator())) : vector(Allocator()) {}
 (since C++17)
(constexpr since C++20)
(2)
explicit vector( const Allocator& alloc = Allocator() );
 (until C++11)
explicit vector( const Allocator& alloc );
 (since C++11)
(noexcept since C++17)
(constexpr since C++20)
explicit vector( size_type count,
                 const Allocator& alloc = Allocator() );
 (3) (since C++11)
(4)
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)
(constexpr since C++20)
template< class InputIt >

vector( InputIt first, InputIt last,

        const Allocator& alloc = Allocator() );
 (5) (constexpr since C++20)
template< container-compatible-range<T> R >

constexpr vector( std::from_range_t, R&& rg,

                  const Allocator& alloc = Allocator() );
 (6) (since C++23)
vector( const vector& other );
 (7) (constexpr since C++20)
vector( vector&& other );
 (8) (since C++11)
(noexcept since C++17)
(constexpr since C++20)
(9)
vector( const vector& other, const Allocator& alloc );
 (since C++11)
(constexpr since C++20)
(until C++23)
constexpr vector( const vector& other,
                  const std::type_identity_t<Allocator>& alloc );
 (since C++23)
(10)
vector( vector&& other, const Allocator& alloc );
 (since C++11)
(until C++23)
constexpr vector( vector&& other,
                  const std::type_identity_t<Allocator>& alloc );
 (since C++23)
vector( std::initializer_list<T> init,
        const Allocator& alloc = Allocator() );
 (11) (since C++11)

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

1) The default constructor since C++11. Constructs an empty vector with a default-constructed allocator.
If Allocator is not DefaultConstructible, the behavior is undefined.
2) The default constructor until C++11. Constructs an empty vector with the given allocator alloc.
3) Constructs a vector with count default-inserted objects of T. No copies are made.
If T is not DefaultInsertable into std::vector<T>, the behavior is undefined.
4) Constructs a vector with count copies of elements with value value.

If T is not CopyInsertable into std::vector<T>, the behavior is undefined.

(since C++11)
5) Constructs a vector with the contents of the range [first, last).

If InputIt does not satisfy the requirements of LegacyInputIterator, overload (4) is called instead with arguments static_cast<size_type>(first), last and alloc.

(until C++11)

This overload participates in overload resolution only if InputIt satisfies the requirements of LegacyInputIterator.

If any of the following conditions is satisfied, the behavior is undefined:

(since C++11)
6) Constructs a vector with the contents of the range rg. Each iterator in the rg is dereferenced exactly once.
If any of the following conditions is satisfied, the behavior is undefined:
7-10) Constructs a vector with the contents of other.
7) The copy constructor.

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

(since C++11)
8) The move constructor. The allocator is obtained by move construction from other.get_allocator().
9) Same as the copy constructor, except that alloc is used as the allocator.
If T is not CopyInsertable into std::vector<T>, the behavior is undefined.
10) Same as the move constructor, except that alloc is used as the allocator.
If T is not MoveInsertable into std::vector<T>, the behavior is undefined.
11) Equivalent to vector(il.begin(), il.end(), alloc).

Contents

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
rg - a container compatible range

Complexity

1,2) Constant.
3,4) Linear in count.
5) Given std::distance(first, 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) 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.
7) Linear in other.size().
8) Constant.
9) Linear in other.size().
10) Linear in other.size() if alloc != other.get_allocator(), otherwise constant.
11) Linear in init.size().

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.

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion; overload (6)

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;
 
    const auto rg = {"cat", "cow", "crow"};
#ifdef __cpp_lib_containers_ranges
    std::vector<std::string> words5(std::from_range, rg); // overload (6)
#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
LWG 134 C++98 overload (5) allowed up to 2N  copy
constructor calls in the input iterator case 		changed to O(N) calls
LWG 438 C++98 overload (5) would only call overload (4)
if InputIt is an integral type 		calls overload (4) if InputIt
is not an LegacyInputIterator
LWG 2193 C++11 the default constructor was explicit made non-explicit
LWG 2210 C++11 overload (3) did not have an allocator parameter added the parameter
N3346 C++11 for overload (3), the elements in
the container were value-initialized 		they are default-inserted

See also

 assigns values to the container
(public member function) [edit]
assigns values to the container
(public member function) [edit]
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