22 Containers library [containers]

22.7 Views [views]

22.7.3 Class template span [views.span]

22.7.3.1 Overview [span.overview]

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A span is a view over a contiguous sequence of objects, the storage of which is owned by some other object.
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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>>>; }
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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]

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constexpr span() noexcept;
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Constraints: Extent == dynamic_­extent || Extent == 0 is true.
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Postconditions: size() == 0 && data() == nullptr.
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template<class It> constexpr explicit(extent != dynamic_extent) span(It first, size_type count);
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Constraints: Let U be remove_­reference_­t<iter_­reference_­t<It>>.
  • (3.1)
  • (3.2)
    is_­convertible_­v<U(*)[], element_­type(*)[]> is true.
    [Note 1:
    The intent is to allow only qualification conversions of the iterator reference type to element_­type.
    — end note]
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Preconditions:
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Effects: Initializes data_­ with to_­address(first) and size_­ with count.
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Throws: Nothing.
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template<class It, class End> constexpr explicit(extent != dynamic_extent) span(It first, End last);
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Constraints: Let U be remove_­reference_­t<iter_­reference_­t<It>>.
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Preconditions:
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Effects: Initializes data_­ with to_­address(first) and size_­ with last - first.
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Throws: When and what last - first throws.
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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;
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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 3:
    The intent is to allow only qualification conversions of the array element type to element_­type.
    — end note]
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Effects: Constructs a span that is a view over the supplied array.
[Note 4:
type_­identity_­t affects class template argument deduction.
— end note]
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Postconditions: size() == N && data() == data(arr) is true.
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template<class R> constexpr explicit(extent != dynamic_extent) span(R&& r);
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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 5:
    The intent is to allow only qualification conversions of the range reference type to element_­type.
    — end note]
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Preconditions:
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Effects: Initializes data_­ with ranges​::​data(r) and size_­ with ranges​::​size(r).
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Throws: What and when ranges​::​data(r) and ranges​::​size(r) throw.
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constexpr span(const span& other) noexcept = default;
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Postconditions: other.size() == size() && other.data() == data().
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template<class OtherElementType, size_t OtherExtent> constexpr explicit(see below) span(const span<OtherElementType, OtherExtent>& s) noexcept;
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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 6:
    The intent is to allow only qualification conversions of the OtherElementType to element_­type.
    — end note]
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Preconditions: If extent is not equal to dynamic_­extent, then s.size() is equal to extent.
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Effects: Constructs a span that is a view over the range [s.data(), s.data() + s.size()).
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Postconditions: size() == s.size() && data() == s.data().
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Remarks: The expression inside explicit is equivalent to: extent != dynamic_extent && OtherExtent == dynamic_extent
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constexpr span& operator=(const span& other) noexcept = default;
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Postconditions: size() == other.size() && data() == other.data().

22.7.3.3 Deduction guides [span.deduct]

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

22.7.3.4 Subviews [span.sub]

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template<size_t Count> constexpr span<element_type, Count> first() const;
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Mandates: Count <= Extent is true.
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Preconditions: Count <= size() is true.
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Effects: Equivalent to: return R{data(), Count}; where R is the return type.
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template<size_t Count> constexpr span<element_type, Count> last() const;
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Mandates: Count <= Extent is true.
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Preconditions: Count <= size() is true.
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Effects: Equivalent to: return R{data() + (size() - Count), Count}; where R is the return type.
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template<size_t Offset, size_t Count = dynamic_extent> constexpr span<element_type, see below> subspan() const;
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Mandates: Offset <= Extent && (Count == dynamic_extent || Count <= Extent - Offset) is true.
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Preconditions: Offset <= size() && (Count == dynamic_extent || Count <= size() - Offset) is true.
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Effects: Equivalent to: return span<ElementType, see below>( data() + Offset, Count != dynamic_extent ? Count : size() - Offset);
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Remarks: The second template argument of the returned span type is: Count != dynamic_extent ? Count : (Extent != dynamic_extent ? Extent - Offset : dynamic_extent)
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constexpr span<element_type, dynamic_extent> first(size_type count) const;
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Preconditions: count <= size() is true.
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Effects: Equivalent to: return {data(), count};
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constexpr span<element_type, dynamic_extent> last(size_type count) const;
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Preconditions: count <= size() is true.
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Effects: Equivalent to: return {data() + (size() - count), count};
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constexpr span<element_type, dynamic_extent> subspan( size_type offset, size_type count = dynamic_extent) const;
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Preconditions: offset <= size() && (count == dynamic_extent || count <= size() - offset) is true.
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Effects: Equivalent to: return {data() + offset, count == dynamic_extent ? size() - offset : count};

22.7.3.5 Observers [span.obs]

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

22.7.3.6 Element access [span.elem]

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

22.7.3.7 Iterator support [span.iterators]

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using iterator = implementation-defined;
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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]), whose value type is value_­type and whose reference type is reference.
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All requirements on container iterators ([container.requirements]) apply to span​::​iterator as well.
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constexpr iterator begin() const noexcept;
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Returns: An iterator referring to the first element in the span.
If empty() is true, then it returns the same value as end().
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constexpr iterator end() const noexcept;
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Returns: An iterator which is the past-the-end value.
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constexpr reverse_iterator rbegin() const noexcept;
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Effects: Equivalent to: return reverse_­iterator(end());
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constexpr reverse_iterator rend() const noexcept;
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Effects: Equivalent to: return reverse_­iterator(begin());

22.7.3.8 Views of object representation [span.objectrep]

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