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

From cppreference.com
< cpp‎ | container‎ | vector
m (fmt)
(Detached this page from the {{cpp/container/constructor}} template, and added LWG2210/N3346 DRs (the vector-exclusive notes are removed).)
Line 1: Line 1:
−
{{include page|cpp/container/constructor|vector}}
+
{{cpp/container/vector/title|vector}}
 +
{{cpp/container/vector/navbar}}
 +
{{dcl begin}}
 +
{{dcl rev begin|num=1}}
 +
{{dcl|since=c++11|until=c++17|
 +
vector() : vector(Allocator()) {}
 +
}}
 +
{{dcla|since=c++17|constexpr=c++20|
 +
vector() noexcept(noexcept(Allocator())) : vector(Allocator()) {}
 +
}}
 +
{{dcl rev end}}
 +
{{dcl rev begin|num=2}}
 +
{{dcl|until=c++11|1=
 +
explicit vector( const Allocator& alloc = Allocator() );
 +
}}
 +
{{dcla|since=c++11|noexcept=c++17|constexpr=c++20|
 +
explicit vector( const Allocator& alloc );
 +
}}
 +
{{dcl rev end}}
 +
{{dcl|num=3|since=c++11|1=
 +
explicit vector( size_type count,
 +
                const Allocator& alloc = Allocator() );
 +
}}
 +
{{dcl rev begin|num=4}}
 +
{{dcl|until=c++11|1=
 +
explicit vector( size_type count, const T& value = T(),
 +
                const Allocator& alloc = Allocator() );
 +
}}
 +
{{dcla|since=c++11|constexpr=c++20|1=
 +
vector( size_type count, const T& value,
 +
        const Allocator& alloc = Allocator() );
 +
}}
 +
{{dcl rev end}}
 +
{{dcla|num=5|constexpr=c++20|1=
 +
template< class InputIt >
 +
vector( InputIt first, InputIt last,
 +
        const Allocator& alloc = Allocator() );
 +
}}
 +
{{dcl|num=6|since=c++23|1=
 +
template< container-compatible-range<T> R >
 +
constexpr vector( std::from_range_t, R&& rg,
 +
                  const Allocator& alloc = Allocator() );
 +
}}
 +
{{dcla|num=7|constexpr=c++20|
 +
vector( const vector& other );
 +
}}
 +
{{dcla|num=8|since=c++11|noexcept=c++17|constexpr=c++20|
 +
vector( vector&& other );
 +
}}
 +
{{dcl rev begin|num=9}}
 +
{{dcla|since=c++11|constexpr=c++20|until=c++23|
 +
vector( const vector& other, const Allocator& alloc );
 +
}}
 +
{{dcl|since=c++23|
 +
constexpr vector( const vector& other,
 +
                  const std::type_identity_t<Allocator>& alloc );
 +
}}
 +
{{dcl rev end}}
 +
{{dcl rev begin|num=10}}
 +
{{dcl|since=c++11|constexpr=c++20|until=c++23|
 +
vector( vector&& other, const Allocator& alloc );
 +
}}
 +
{{dcl|since=c++23|
 +
constexpr vector( vector&& other,
 +
                  const std::type_identity_t<Allocator>& alloc );
 +
}}
 +
{{dcl rev end}}
 +
{{dcl|num=11|since=c++11|1=
 +
vector( std::initializer_list<T> init,
 +
        const Allocator& alloc = Allocator() );
 +
}}
 +
{{dcl end}}
 +
 
 +
Constructs a new {{tt|vector}} from a variety of data sources, optionally using a user supplied allocator {{c|alloc}}.
 +
 
 +
@1@ The default constructor since C++11. Constructs an empty {{tt|vector}} with a default-constructed allocator.
 +
@@ If {{tt|Allocator}} is not {{named req|DefaultConstructible}}, the behavior is undefined.
 +
 
 +
@2@ The default constructor until C++11. Constructs an empty {{tt|vector}} with the given allocator {{c|alloc}}.
 +
 
 +
@3@ Constructs a {{tt|vector}} with {{c|count}} default-inserted objects of {{tt|T}}. No copies are made.
 +
@@ If {{tt|T}} is not {{named req|DefaultInsertable}} into {{c/core|std::vector<T>}}, the behavior is undefined.
 +
 
 +
@4@ Constructs a {{tt|vector}} with {{c|count}} copies of elements with value {{c|value}}.
 +
{{rrev|since=c++11|
 +
If {{tt|T}} is not {{named req|CopyInsertable}} into {{c/core|std::vector<T>}}, the behavior is undefined.
 +
}}
 +
 
 +
@5@ Constructs a {{tt|vector}} with the contents of the range {{range|first|last}}.
 +
{{rev begin}}
 +
{{rev|until=c++11|
 +
If {{tt|InputIt}} does not satisfy the requirements of {{named req|InputIterator}}, overload {{v|4}} is called instead with arguments {{c|static_cast<size_type>(first)}}, {{c|last}} and {{c|alloc}}.
 +
}}
 +
{{rev|since=c++11|
 +
{{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.
 +
}}
 +
{{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.
 +
@@ If {{tt|T}} is not {{named req|EmplaceConstructible}} into {{c/core|std::vector<T>}} from {{c|*ranges::begin(rg)}}, the behavior is undefined.
 +
 
 +
@7-10@ Constructs a {{tt|vector}} with the contents of {{c|other}}.
 +
 
 +
:@7@ The copy constructor.
 +
{{rrev|since=c++11|
 +
The allocator is obtained as if by calling {{c multi|std::allocator_traits<Allocator>::|    select_on_container_copy_construction|        (other.get_allocator())}}.
 +
}}
 +
 
 +
:@8@ The move constructor. The allocator is obtained by move construction from {{c|other.get_allocator()}}.
 +
 
 +
:@9@ Same as the copy constructor, except that {{c|alloc}} is used as the allocator.
 +
:@@ If {{tt|T}} is not {{named req|CopyInsertable}} into {{c/core|std::vector<T>}}, the behavior is undefined.
 +
 
 +
:@10@ Same as the move constructor, except that {{c|alloc}} is used as the allocator.
 +
:@@ If {{tt|T}} is not {{named req|MoveInsertable}} into {{c/core|std::vector<T>}}, the behavior is undefined.
 +
 
 +
@11@ Equivalent to {{c|vector(il.begin(), il.end(), alloc)}}.
 +
 
 +
===Parameters===
 +
{{par begin}}
 +
{{par|alloc|allocator to use for all memory allocations of this container}}
 +
{{par|count|the size of the container}}
 +
{{par|value|the value to initialize elements of the container with}}
 +
{{par|first, last|the range to copy the elements from}}
 +
{{par|other|another container to be used as source to initialize the elements of the container with}}
 +
{{par|init|initializer list to initialize the elements of the container with}}
 +
{{par|rg|a container compatible range}}
 +
{{par end}}
 +
 
 +
===Complexity===
 +
@1,2@ Constant.
 +
 
 +
@3,4@ Linear in {{c|count}}.
 +
 
 +
@5@ Given {{c|std::distance(first, last)}} as {{math|N}}:
 +
* If {{c|first}} and {{c|last}} are both forward, bidirectional or random-access iterators,
 +
:* The copy constructor of {{tt|T}} is only called {{math|N}}{{nbsp}} times, and
 +
:* No reallocation occurs.
 +
* Otherwise ({{c|first}} and {{c|last}} are just input iterators),
 +
:* The copy constructor of {{tt|T}} is called {{math|O(N)}} times, and
 +
:* Reallocation occurs {{math|O(log N)}} times.
 +
 
 +
@6@ Given {{c|ranges::distance(rg)}} as {{math|N}}:
 +
* If {{tt|R}} models {{lc|ranges::forward_range}} or {{lc|ranges::sized_range}},
 +
:* Initializes exactly {{math|N}} elements from the result of dereferencing successive iterators of {{c|rg}}, and
 +
:* No reallocation occurs.
 +
* Otherwise ({{tt|R}} models input range),
 +
:* The copy or move constructor of {{tt|T}} is called {{math|O(N)}} times, and
 +
:* Reallocation occurs {{math|O(log N)}} times.
 +
 
 +
@7@ Linear in {{c|other.size()}}.
 +
 
 +
@8@ Constant.
 +
 
 +
@9@ Linear in {{c|other.size()}}.
 +
 
 +
@10@ Linear in {{c|other.size()}} if {{c|1=alloc != other.get_allocator()}}, otherwise constant.
 +
 
 +
@11@ Linear in {{c|init.size()}}.
 +
 
 +
===Exceptions===
 +
Calls to {{c|Allocator::allocate}} may throw.
 +
 
 +
===Notes===
 +
{{cpp/container/lwg2321-note|8}}
 +
 
 +
{{feature test macro|__cpp_lib_containers_ranges|[[cpp/ranges/to#container compatible range|Ranges-aware]] construction and insertion; overload {{v|6}}|value=202202L|std=C++23}}
 +
 
 +
===Example===
 +
{{example
 +
|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===
 +
{{dr list begin}}
 +
{{dr list item|wg=lwg|dr=134|std=C++98|before=overload {{v|5}} allowed up to {{math|2N}}{{sep}} copy<br>constructor calls in the input iterator case|after=changed to {{math|O(N)}} calls}}
 +
{{dr list item|wg=lwg|dr=438|std=C++98|before=overload {{v|5}} would only call overload {{v|4}}<br>if {{tt|InputIt}} is an integral type|after=calls overload {{v|4}} if {{tt|InputIt}}<br>is not an {{named req|InputIterator}}}}
 +
{{dr list item|wg=lwg|dr=2193|std=C++11|before=the default constructor was explicit|after=made non-explicit}}
 +
{{dr list item|wg=lwg|dr=2210|std=C++11|before=overload {{v|3}} did not have an allocator parameter|after=added the parameter}}
 +
{{dr list item|paper=N3346|std=C++11|before=for overload {{v|3}}, the elements in<br>the container were value-initialized|after=they are default-inserted}}
 +
{{dr list end}}
 +
 
 +
===See also===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/container/dsc assign|vector}}
 +
{{dsc inc|cpp/container/dsc operator{{=}}|vector}}
 +
{{dsc end}}
  
 
{{langlinks|de|es|fr|it|ja|pl|pt|ru|zh}}
 
{{langlinks|de|es|fr|it|ja|pl|pt|ru|zh}}

Revision as of 01:21, 7 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 T is not EmplaceConstructible into std::vector<T> from *first, 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 T is not EmplaceConstructible into std::vector<T> from *ranges::begin(rg), 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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