22 Containers library [containers]

22.7 Views [views]

22.7.1 General [views.general]

1
#
The header <span> defines the view span.

22.7.2 Header <span> synopsis [span.syn]

namespace std {
  // constants
  inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();

  // [views.span], class template span
  template<class ElementType, size_t Extent = dynamic_extent>
    class span;

  template<class ElementType, size_t Extent>
    inline constexpr bool ranges::enable_view<span<ElementType, Extent>> =
      Extent == 0 || Extent == dynamic_extent;
  template<class ElementType, size_t Extent>
    inline constexpr bool ranges::enable_borrowed_range<span<ElementType, Extent>> = true;

  // [span.objectrep], views of object representation
  template<class ElementType, size_t Extent>
    span<const byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
      as_bytes(span<ElementType, Extent> s) noexcept;

  template<class ElementType, size_t Extent>
    span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent>
      as_writable_bytes(span<ElementType, Extent> s) noexcept;
}

22.7.3 Class template span [views.span]

22.7.3.1 Overview [span.overview]

1
#
A span is a view over a contiguous sequence of objects, the storage of which is owned by some other object.
2
#
All member functions of span have constant time complexity.
namespace std {
  template<class ElementType, size_t Extent = dynamic_extent>
  class span {
  public:
    // constants and types
    using element_type = ElementType;
    using value_type = remove_cv_t<ElementType>;
    using size_type = size_t;
    using difference_type = ptrdiff_t;
    using pointer = element_type*;
    using const_pointer = const element_type*;
    using reference = element_type&;
    using const_reference = const element_type&;
    using iterator = implementation-defined;        // see [span.iterators]
    using reverse_iterator = std::reverse_iterator<iterator>;
    static constexpr size_type extent = Extent;

    // [span.cons], constructors, copy, and assignment
    constexpr span() noexcept;
    template<class It>
      constexpr explicit(extent != dynamic_extent) span(It first, size_type count);
    template<class It, class End>
      constexpr explicit(extent != dynamic_extent) span(It first, End last);
    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;
    template<class R>
      constexpr explicit(extent != dynamic_extent) span(R&& r);
    constexpr span(const span& other) noexcept = default;
    template<class OtherElementType, size_t OtherExtent>
      constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept;

    ~span() noexcept = default;

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

    // [span.sub], subviews
    template<size_t Count>
      constexpr span<element_type, Count> first() const;
    template<size_t Count>
      constexpr span<element_type, Count> last() const;
    template<size_t Offset, size_t Count = dynamic_extent>
      constexpr span<element_type, see below> subspan() const;

    constexpr span<element_type, dynamic_extent> first(size_type count) const;
    constexpr span<element_type, dynamic_extent> last(size_type count) const;
    constexpr span<element_type, dynamic_extent> subspan(
      size_type offset, size_type count = dynamic_extent) const;

    // [span.obs], observers
    constexpr size_type size() const noexcept;
    constexpr size_type size_bytes() const noexcept;
    [[nodiscard]] constexpr bool empty() const noexcept;

    // [span.elem], element access
    constexpr reference operator[](size_type idx) const;
    constexpr reference front() const;
    constexpr reference back() const;
    constexpr pointer data() const noexcept;

    // [span.iterators], iterator support
    constexpr iterator begin() const noexcept;
    constexpr iterator end() const noexcept;
    constexpr reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator rend() const noexcept;

  private:
    pointer data_;              // exposition only
    size_type size_;            // exposition only
  };

  template<class It, class EndOrSize>
    span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>>;
  template<class T, size_t N>
    span(T (&)[N]) -> span<T, N>;
  template<class T, size_t N>
    span(array<T, N>&) -> span<T, N>;
  template<class T, size_t N>
    span(const array<T, N>&) -> span<const T, N>;
  template<class R>
    span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;
}
3
#
ElementType is required to be a complete object type that is not an abstract class type.

22.7.3.2 Constructors, copy, and assignment [span.cons]

🔗
constexpr span() noexcept;
1
#
Constraints: Extent == dynamic_­extent || Extent == 0 is true.
2
#
Postconditions: size() == 0 && data() == nullptr.
🔗
template<class It> constexpr explicit(extent != dynamic_extent) span(It first, size_type count);
3
#
Constraints: Let U be remove_­reference_­t<iter_­reference_­t<It>>.
  • (3.1)
    It satisfies contiguous_­iterator.
  • (3.2)
    is_­convertible_­v<U(*)[], element_­type(*)[]> is true.
    [ Note
    :
    The intent is to allow only qualification conversions of the iterator reference type to element_­type.
    — end note
     ]
4
#
Preconditions:
  • (4.1)
    [first, first + count) is a valid range.
  • (4.2)
    It models contiguous_­iterator.
  • (4.3)
    If extent is not equal to dynamic_­extent, then count is equal to extent.
5
#
Effects: Initializes data_­ with to_­address(first) and size_­ with count.
6
#
Throws: Nothing.
🔗
template<class It, class End> constexpr explicit(extent != dynamic_extent) span(It first, End last);
7
#
Constraints: Let U be remove_­reference_­t<iter_­reference_­t<It>>.
  • (7.1)
    is_­convertible_­v<U(*)[], element_­type(*)[]> is true.
    [ Note
    :
    The intent is to allow only qualification conversions of the iterator reference type to element_­type.
    — end note
     ]
  • (7.2)
    It satisfies contiguous_­iterator.
  • (7.3)
    End satisfies sized_­sentinel_­for<It>.
  • (7.4)
    is_­convertible_­v<End, size_­t> is false.
8
#
Preconditions:
  • (8.1)
    If extent is not equal to dynamic_­extent, then last - first is equal to extent.
  • (8.2)
    [first, last) is a valid range.
  • (8.3)
    It models contiguous_­iterator.
  • (8.4)
    End models sized_­sentinel_­for<It>.
9
#
Effects: Initializes data_­ with to_­address(first) and size_­ with last - first.
10
#
Throws: When and what last - first throws.
🔗
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;
11
#
Constraints: Let U be remove_­pointer_­t<decltype(data(arr))>.
  • (11.1)
    extent == dynamic_­extent || N == extent is true, and
  • (11.2)
    is_­convertible_­v<U(*)[], element_­type(*)[]> is true.
    [ Note
    :
    The intent is to allow only qualification conversions of the array element type to element_­type.
    — end note
     ]
12
#
Effects: Constructs a span that is a view over the supplied array.
[ Note
:
type_­identity_­t affects class template argument deduction.
— end note
 ]
13
#
Postconditions: size() == N && data() == data(arr) is true.
🔗
template<class R> constexpr explicit(extent != dynamic_extent) span(R&& r);
14
#
Constraints: Let U be remove_­reference_­t<ranges​::​range_­reference_­t<R>>.
  • (14.1)
    R satisfies ranges​::​contiguous_­range and ranges​::​sized_­range.
  • (14.2)
    Either R satisfies ranges​::​borrowed_­range or is_­const_­v<element_­type> is true.
  • (14.3)
    remove_­cvref_­t<R> is not a specialization of span.
  • (14.4)
    remove_­cvref_­t<R> is not a specialization of array.
  • (14.5)
    is_­array_­v<remove_­cvref_­t<R>> is false.
  • (14.6)
    is_­convertible_­v<U(*)[], element_­type(*)[]> is true.
    [ Note
    :
    The intent is to allow only qualification conversions of the range reference type to element_­type.
    — end note
     ]
15
#
Preconditions:
  • (15.1)
    If extent is not equal to dynamic_­extent, then ranges​::​size(r) is equal to extent.
  • (15.2)
    R models ranges​::​contiguous_­range and ranges​::​sized_­range.
  • (15.3)
    If is_­const_­v<element_­type> is false, R models ranges​::​borrowed_­range.
16
#
Effects: Initializes data_­ with ranges​::​data(r) and size_­ with ranges​::​size(r).
17
#
Throws: What and when ranges​::​data(r) and ranges​::​size(r) throw.
🔗
constexpr span(const span& other) noexcept = default;
18
#
Postconditions: other.size() == size() && other.data() == data().
🔗
template<class OtherElementType, size_t OtherExtent> constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept;
19
#
Constraints:
  • (19.1)
    extent == dynamic_­extent || OtherExtent == dynamic_­extent || extent == OtherExtent is true, and
  • (19.2)
    is_­convertible_­v<OtherElementType(*)[], element_­type(*)[]> is true.
    [ Note
    : The intent is to allow only qualification conversions of the OtherElementType to element_­type. — end note
     ]
20
#
Preconditions: If extent is not equal to dynamic_­extent, then s.size() is equal to extent.
21
#
Effects: Constructs a span that is a view over the range [s.data(), s.data() + s.size()).
22
#
Postconditions: size() == s.size() && data() == s.data().
23
#
Remarks: The expression inside explicit is equivalent to: 
extent != dynamic_extent && OtherExtent == dynamic_extent
🔗
constexpr span& operator=(const span& other) noexcept = default;
24
#
Postconditions: size() == other.size() && data() == other.data().

22.7.3.3 Deduction guides [span.deduct]

🔗
template<class It, class EndOrSize> span(It, EndOrSize) -> span<remove_reference_t<iter_reference_t<It>>>;
1
#
Constraints: It satisfies contiguous_­iterator.
🔗
template<class R> span(R&&) -> span<remove_reference_t<ranges::range_reference_t<R>>>;
2
#
Constraints: R satisfies ranges​::​contiguous_­range.

22.7.3.4 Subviews [span.sub]

🔗
template<size_t Count> constexpr span<element_type, Count> first() const;
1
#
Mandates: Count <= Extent is true.
2
#
Preconditions: Count <= size() is true.
3
#
Effects: Equivalent to: return R{data(), Count}; where R is the return type.
🔗
template<size_t Count> constexpr span<element_type, Count> last() const;
4
#
Mandates: Count <= Extent is true.
5
#
Preconditions: Count <= size() is true.
6
#
Effects: Equivalent to: return R{data() + (size() - Count), Count}; where R is the return type.
🔗
template<size_t Offset, size_t Count = dynamic_extent> constexpr span<element_type, see below> subspan() const;
7
#
Mandates: 
Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset)
is true.
8
#
Preconditions: 
Offset <= size() && (Count == dynamic_extent || Count <= size() - Offset)
is true.
9
#
Effects: Equivalent to: 
return span<ElementType, see below>(
  data() + Offset, Count != dynamic_extent ? Count : size() - Offset);
10
#
Remarks: The second template argument of the returned span type is: 
Count != dynamic_extent ? Count
                        : (Extent != dynamic_extent ? Extent - Offset
                                                    : dynamic_extent)
🔗
constexpr span<element_type, dynamic_extent> first(size_type count) const;
11
#
Preconditions: count <= size() is true.
12
#
Effects: Equivalent to: return {data(), count};
🔗
constexpr span<element_type, dynamic_extent> last(size_type count) const;
13
#
Preconditions: count <= size() is true.
14
#
Effects: Equivalent to: return {data() + (size() - count), count};
🔗
constexpr span<element_type, dynamic_extent> subspan( size_type offset, size_type count = dynamic_extent) const;
15
#
Preconditions: 
offset <= size() && (count == dynamic_extent || count <= size() - offset)
is true.
16
#
Effects: Equivalent to: 
return {data() + offset, count == dynamic_extent ? size() - offset : count};

22.7.3.5 Observers [span.obs]

🔗
constexpr size_type size() const noexcept;
1
#
Effects: Equivalent to: return size_­;
🔗
constexpr size_type size_bytes() const noexcept;
2
#
Effects: Equivalent to: return size() * sizeof(element_­type);
🔗
[[nodiscard]] constexpr bool empty() const noexcept;
3
#
Effects: Equivalent to: return size() == 0;

22.7.3.6 Element access [span.elem]

🔗
constexpr reference operator[](size_type idx) const;
1
#
Preconditions: idx < size() is true.
2
#
Effects: Equivalent to: return *(data() + idx);
🔗
constexpr reference front() const;
3
#
Preconditions: empty() is false.
4
#
Effects: Equivalent to: return *data();
🔗
constexpr reference back() const;
5
#
Preconditions: empty() is false.
6
#
Effects: Equivalent to: return *(data() + (size() - 1));
🔗
constexpr pointer data() const noexcept;
7
#
Effects: Equivalent to: return data_­;

22.7.3.7 Iterator support [span.iterators]

🔗
using iterator = implementation-defined;
1
#
The type models contiguous_­iterator ([iterator.concept.contiguous]), meets the Cpp17RandomAccessIterator requirements ([random.access.iterators]), and meets the requirements for constexpr iterators ([iterator.requirements.general]).
All requirements on container iterators ([container.requirements]) apply to span​::​iterator as well.
🔗
constexpr iterator begin() const noexcept;
2
#
Returns: An iterator referring to the first element in the span.
If empty() is true, then it returns the same value as end().
🔗
constexpr iterator end() const noexcept;
3
#
Returns: An iterator which is the past-the-end value.
🔗
constexpr reverse_iterator rbegin() const noexcept;
4
#
Effects: Equivalent to: return reverse_­iterator(end());
🔗
constexpr reverse_iterator rend() const noexcept;
5
#
Effects: Equivalent to: return reverse_­iterator(begin());

22.7.3.8 Views of object representation [span.objectrep]

🔗
template<class ElementType, size_t Extent> span<const byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent> as_bytes(span<ElementType, Extent> s) noexcept;
1
#
Effects: Equivalent to: return R{reinterpret_­cast<const byte*>(s.data()), s.size_­bytes()}; where R is the return type.
🔗
template<class ElementType, size_t Extent> span<byte, Extent == dynamic_extent ? dynamic_extent : sizeof(ElementType) * Extent> as_writable_bytes(span<ElementType, Extent> s) noexcept;
2
#
Constraints: is_­const_­v<ElementType> is false.
3
#
Effects: Equivalent to: return R{reinterpret_­cast<byte*>(s.data()), s.size_­bytes()}; where R is the return type.