[span.cons]

# 23 Containers library [[containers]](./#containers)

## 23.7 Views [[views]](views#span.cons)

### 23.7.2 Contiguous access [[views.contiguous]](views.contiguous#span.cons)

#### 23.7.2.2 Class template span [[views.span]](views.span#span.cons)

#### 23.7.2.2.2 Constructors, copy, and assignment [span.cons]

[🔗](#lib:span,constructor)

`constexpr span() noexcept;
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20344)

*Constraints*: Extent == dynamic_extent || Extent == 0 is true[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20348)

*Postconditions*: size() == 0 && data() == nullptr[.](#2.sentence-1)

[🔗](#lib:span,constructor_)

`template<class It>
  constexpr explicit(extent != dynamic_extent) span(It first, size_type count);
`

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20361)

*Constraints*: Let U be remove_reference_t<iter_reference_t<It>>[.](#3.sentence-1)

- [(3.1)](#3.1)

  It satisfies [contiguous_iterator](iterator.concept.contiguous#concept:contiguous_iterator "24.3.4.14 Concept contiguous_­iterator [iterator.concept.contiguous]")[.](#3.1.sentence-1)

- [(3.2)](#3.2)

  is_convertible_v<U(*)[], element_type(*)[]> is true[.](#3.2.sentence-1)
  [*Note [1](#note-1)*:
  The intent is to allow only qualification conversions
of the iterator reference type to element_type[.](#3.2.sentence-2)
 — *end note*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20374)

*Preconditions*: 

- [(4.1)](#4.1)

  [first, first + count) is a valid range[.](#4.1.sentence-1)

- [(4.2)](#4.2)

  It models [contiguous_iterator](iterator.concept.contiguous#concept:contiguous_iterator "24.3.4.14 Concept contiguous_­iterator [iterator.concept.contiguous]")[.](#4.2.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20381)

*Hardened preconditions*: If extent is not equal to dynamic_extent,
then count == extent is true[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20386)

*Effects*: Initializes *data_* with to_address(first) and*size_* with count[.](#6.sentence-1)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20391)

*Throws*: Nothing[.](#7.sentence-1)

[🔗](#lib:span,constructor__)

`template<class It, class End>
  constexpr explicit(extent != dynamic_extent) span(It first, End last);
`

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20403)

*Constraints*: Let U be remove_reference_t<iter_reference_t<It>>[.](#8.sentence-1)

- [(8.1)](#8.1)

  is_convertible_v<U(*)[], element_type(*)[]> is true[.](#8.1.sentence-1)
  [*Note [2](#note-2)*:
  The intent is to allow only qualification conversions
of the iterator reference type to element_type[.](#8.1.sentence-2)
 — *end note*]

- [(8.2)](#8.2)

  It satisfies [contiguous_iterator](iterator.concept.contiguous#concept:contiguous_iterator "24.3.4.14 Concept contiguous_­iterator [iterator.concept.contiguous]")[.](#8.2.sentence-1)

- [(8.3)](#8.3)

  End satisfies [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<It>[.](#8.3.sentence-1)

- [(8.4)](#8.4)

  is_convertible_v<End, size_t> is false[.](#8.4.sentence-1)

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20418)

*Preconditions*: 

- [(9.1)](#9.1)

  [first, last) is a valid range[.](#9.1.sentence-1)

- [(9.2)](#9.2)

  It models [contiguous_iterator](iterator.concept.contiguous#concept:contiguous_iterator "24.3.4.14 Concept contiguous_­iterator [iterator.concept.contiguous]")[.](#9.2.sentence-1)

- [(9.3)](#9.3)

  End models [sized_sentinel_for](iterator.concept.sizedsentinel#concept:sized_sentinel_for "24.3.4.8 Concept sized_­sentinel_­for [iterator.concept.sizedsentinel]")<It>[.](#9.3.sentence-1)

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20426)

*Hardened preconditions*: If extent is not equal to dynamic_extent,
then (last - first) == extent is true[.](#10.sentence-1)

[11](#11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20431)

*Effects*: Initializes *data_* with to_address(first) and*size_* with last - first[.](#11.sentence-1)

[12](#12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20436)

*Throws*: When and what last - first throws[.](#12.sentence-1)

[🔗](#lib:span,constructor___)

`template<size_t N> constexpr span(type_identity_t<element_type> (&arr)[N]) noexcept;
template<class T, size_t N> constexpr span(array<T, N>& arr) noexcept;
template<class T, size_t N> constexpr span(const array<T, N>& arr) noexcept;
`

[13](#13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20449)

*Constraints*: Let U be remove_pointer_t<decltype(std​::​data(arr))>[.](#13.sentence-1)

- [(13.1)](#13.1)

extent == dynamic_extent || N == extent is true, and

- [(13.2)](#13.2)

is_convertible_v<U(*)[], element_type(*)[]> is true. [*Note [3](#note-3)*:
  The intent is to allow only qualification conversions
of the array element type to element_type[.](#13.2.sentence-2)
 — *end note*]

[14](#14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20461)

*Effects*: Constructs a span that is a view over the supplied array[.](#14.sentence-1)

[*Note [4](#note-4)*:

type_identity_t affects class template argument deduction[.](#14.sentence-2)

— *end note*]

[15](#15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20468)

*Postconditions*: size() == N && data() == std​::​data(arr) is true[.](#15.sentence-1)

[🔗](#lib:span,constructor____)

`template<class R> constexpr explicit(extent != dynamic_extent) span(R&& r);
`

[16](#16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20479)

*Constraints*: Let U be remove_reference_t<ranges​::​range_reference_t<R>>[.](#16.sentence-1)

- [(16.1)](#16.1)

  R satisfies ranges​::​[contiguous_range](range.refinements#concept:contiguous_range "25.4.6 Other range refinements [range.refinements]") and ranges​::​[sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")[.](#16.1.sentence-1)

- [(16.2)](#16.2)

  Either R satisfies ranges​::​[borrowed_range](range.range#concept:borrowed_range "25.4.2 Ranges [range.range]") oris_const_v<element_type> is true[.](#16.2.sentence-1)

- [(16.3)](#16.3)

  remove_cvref_t<R> is not a specialization of span[.](#16.3.sentence-1)

- [(16.4)](#16.4)

  remove_cvref_t<R> is not a specialization of array[.](#16.4.sentence-1)

- [(16.5)](#16.5)

  is_array_v<remove_cvref_t<R>> is false[.](#16.5.sentence-1)

- [(16.6)](#16.6)

  is_convertible_v<U(*)[], element_type(*)[]> is true[.](#16.6.sentence-1)
  [*Note [5](#note-5)*:
  The intent is to allow only qualification conversions
of the range reference type to element_type[.](#16.6.sentence-2)
 — *end note*]

[17](#17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20498)

*Preconditions*: 

- [(17.1)](#17.1)

  R models ranges​::​[contiguous_range](range.refinements#concept:contiguous_range "25.4.6 Other range refinements [range.refinements]") andranges​::​[sized_range](range.sized#concept:sized_range "25.4.4 Sized ranges [range.sized]")[.](#17.1.sentence-1)

- [(17.2)](#17.2)

  If is_const_v<element_type> is false,R models ranges​::​[borrowed_range](range.range#concept:borrowed_range "25.4.2 Ranges [range.range]")[.](#17.2.sentence-1)

[18](#18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20507)

*Hardened preconditions*: If extent is not equal to dynamic_extent,
then ranges​::​size(r) == extent is true[.](#18.sentence-1)

[19](#19)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20512)

*Effects*: Initializes *data_* with ranges​::​data(r) and*size_* with ranges​::​size(r)[.](#19.sentence-1)

[20](#20)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20517)

*Throws*: What and when ranges​::​data(r) and ranges​::​size(r) throw[.](#20.sentence-1)

[🔗](#lib:span,constructor_____)

`constexpr explicit(extent != dynamic_extent) span(std::initializer_list<value_type> il);
`

[21](#21)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20528)

*Constraints*: is_const_v<element_type> is true[.](#21.sentence-1)

[22](#22)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20532)

*Hardened preconditions*: If extent is not equal to dynamic_extent,
then il.size() == extent is true[.](#22.sentence-1)

[23](#23)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20537)

*Effects*: Initializes *data_* with il.begin() and*size_* with il.size()[.](#23.sentence-1)

[🔗](#lib:span,constructor______)

`constexpr span(const span& other) noexcept = default;
`

[24](#24)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20549)

*Postconditions*: other.size() == size() && other.data() == data()[.](#24.sentence-1)

[🔗](#lib:span,constructor_______)

`template<class OtherElementType, size_t OtherExtent>
  constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept;
`

[25](#25)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20561)

*Constraints*: 

- [(25.1)](#25.1)

extent == dynamic_extent || OtherExtent == dynamic_extent || extent == OtherExtent is true, and

- [(25.2)](#25.2)

is_convertible_v<OtherElementType(*)[], element_type(*)[]> is true[.](#25.sentence-1)
  [*Note [6](#note-6)*:
  The intent is to allow only qualification conversions
of the OtherElementType to element_type[.](#25.2.sentence-2)
 — *end note*]

[26](#26)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20572)

*Hardened preconditions*: If extent is not equal to dynamic_extent,
then s.size() == extent is true[.](#26.sentence-1)

[27](#27)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20577)

*Effects*: Constructs a span that is a view over the range
[s.data(), s.data() + s.size())[.](#27.sentence-1)

[28](#28)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20582)

*Postconditions*: size() == s.size() && data() == s.data()[.](#28.sentence-1)

[29](#29)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20586)

*Remarks*: The expression inside explicit is equivalent to:extent != dynamic_extent && OtherExtent == dynamic_extent

[🔗](#lib:operator=,span)

`constexpr span& operator=(const span& other) noexcept = default;
`

[30](#30)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/containers.tex#L20600)

*Postconditions*: size() == other.size() && data() == other.data()[.](#30.sentence-1)