21 Containers library [containers]

21.7 Views [views]

21.7.3 Class template span [views.span]

21.7.3.1 Overview [span.overview]

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

ElementType is required to be a complete object type that is not an abstract class type.

If Extent is negative and not equal to dynamic_extent, the program is ill-formed.

The iterator types span::iterator and span::const_iterator are random access iterators ([random.access.iterators]), contiguous iterators ([iterator.requirements.general]), and constexpr iterators ([iterator.requirements.general]).

All requirements on container iterators ([container.requirements]) apply to span::iterator and span::const_iterator as well.

All member functions of span have constant time complexity.

namespace std {
  template<class ElementType, ptrdiff_t Extent = dynamic_extent>
  class span {
  public:
    // constants and types
    using element_type = ElementType;
    using value_type = remove_cv_t<ElementType>;
    using index_type = ptrdiff_t;
    using difference_type = ptrdiff_t;
    using pointer = element_type*;
    using reference = element_type&;
    using iterator = implementation-defined;
    using const_iterator = implementation-defined;
    using reverse_iterator = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    static constexpr index_type extent = Extent;

    // [span.cons], constructors, copy, and assignment
    constexpr span() noexcept;
    constexpr span(pointer ptr, index_type count);
    constexpr span(pointer first, pointer last);
    template<size_t N>
      constexpr span(element_type (&arr)[N]) noexcept;
    template<size_t N>
      constexpr span(array<value_type, N>& arr) noexcept;
    template<size_t N>
      constexpr span(const array<value_type, N>& arr) noexcept;
    template<class Container>
      constexpr span(Container& cont);
    template<class Container>
      constexpr span(const Container& cont);
    constexpr span(const span& other) noexcept = default;
    template<class OtherElementType, ptrdiff_t OtherExtent>
      constexpr span(const span<OtherElementType, OtherExtent>& s) noexcept;

    ~span() noexcept = default;

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

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

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

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

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

    // [span.iterators], iterator support
    constexpr iterator begin() const noexcept;
    constexpr iterator end() const noexcept;
    constexpr const_iterator cbegin() const noexcept;
    constexpr const_iterator cend() const noexcept;
    constexpr reverse_iterator rbegin() const noexcept;
    constexpr reverse_iterator rend() const noexcept;
    constexpr const_reverse_iterator crbegin() const noexcept;
    constexpr const_reverse_iterator crend() const noexcept;

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

  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 Container>
    span(Container&) -> span<typename Container::value_type>;
  template<class Container>
    span(const Container&) -> span<const typename Container::value_type>;
}

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

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constexpr span() noexcept;

Ensures: size() == 0 && data() == nullptr.

Remarks: This constructor shall not participate in overload resolution unless Extent <= 0 is true.

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constexpr span(pointer ptr, index_type count);

Requires: [ptr, ptr + count) shall be a valid range.

If extent is not equal to dynamic_extent, then count shall be equal to extent.

Effects: Constructs a span that is a view over the range [ptr, ptr + count).

Ensures: size() == count && data() == ptr.

Throws: Nothing.

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constexpr span(pointer first, pointer last);

Requires: [first, last) shall be a valid range.

If extent is not equal to dynamic_extent, then last - first shall be equal to extent.

Effects: Constructs a span that is a view over the range [first, last).

Ensures: size() == last - first && data() == first.

Throws: Nothing.

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template<size_t N> constexpr span(element_type (&arr)[N]) noexcept;
template<size_t N> constexpr span(array<value_type, N>& arr) noexcept;
template<size_t N> constexpr span(const array<value_type, N>& arr) noexcept;

Effects: Constructs a span that is a view over the supplied array.

Ensures: size() == N && data() == data(arr).

Remarks: These constructors shall not participate in overload resolution unless:

(13.1)
extent == dynamic_extent || N == extent is true, and
(13.2)
remove_pointer_t<decltype(data(arr))>(*)[] is convertible to ElementType(*)[].

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template<class Container> constexpr span(Container& cont);
template<class Container> constexpr span(const Container& cont);

Requires: [data(cont), data(cont) + size(cont)) shall be a valid range.

If extent is not equal to dynamic_extent, then size(cont) shall be equal to extent.

Effects: Constructs a span that is a view over the range [data(cont), data(cont) + size(cont)).

Ensures: size() == size(cont) && data() == data(cont).

Throws: What and when data(cont) and size(cont) throw.

Remarks: These constructors shall not participate in overload resolution unless:

(18.1)
Container is not a specialization of span,
(18.2)
Container is not a specialization of array,
(18.3)
is_array_v<Container> is false,
(18.4)
data(cont) and size(cont) are both well-formed, and
(18.5)
remove_pointer_t<decltype(data(cont))>(*)[] is convertible to ElementType(*)[].

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

Ensures: other.size() == size() && other.data() == data().

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template<class OtherElementType, ptrdiff_t OtherExtent>
  constexpr span(const span<OtherElementType, OtherExtent>& s) noexcept;

Effects: Constructs a span that is a view over the range [s.data(), s.data() + s.size()).

Ensures: size() == s.size() && data() == s.data().

Remarks: This constructor shall not participate in overload resolution unless:

(22.1)
Extent == dynamic_extent || Extent == OtherExtent is true, and
(22.2)
OtherElementType(*)[] is convertible to ElementType(*)[].

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

Ensures: size() == other.size() && data() == other.data().

21.7.3.3 Subviews [span.sub]

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template<ptrdiff_t Count> constexpr span<element_type, Count> first() const;

Requires: 0 <= Count && Count <= size().

Effects: Equivalent to: return {data(), Count};

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template<ptrdiff_t Count> constexpr span<element_type, Count> last() const;

Requires: 0 <= Count && Count <= size().

Effects: Equivalent to: return {data() + (size() - Count), Count};

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template<ptrdiff_t Offset, ptrdiff_t Count = dynamic_extent>
  constexpr span<element_type, see below> subspan() const;

Requires:

(0 <= Offset && Offset <= size())
&& (Count == dynamic_extent || Count >= 0 && Offset + Count <= size())

Effects: Equivalent to:

return span<ElementType, see below>(
  data() + Offset, Count != dynamic_extent ? Count : size() - Offset);

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(index_type count) const;

Requires: 0 <= count && count <= size().

Effects: Equivalent to: return {data(), count};

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constexpr span<element_type, dynamic_extent> last(index_type count) const;

Requires: 0 <= count 0 && count <= size().

Effects: Equivalent to: return {data() + (size() - count), count};

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constexpr span<element_type, dynamic_extent> subspan(
  index_type offset, index_type count = dynamic_extent) const;

Requires:

(0 <= offset && offset <= size())
&& (count == dynamic_extent || count >= 0 && offset + count <= size())

Effects: Equivalent to:

return {data() + offset, count == dynamic_extent ? size() - offset : count};

21.7.3.4 Observers [span.obs]

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constexpr index_type size() const noexcept;

Effects: Equivalent to: return size_;

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constexpr index_type size_bytes() const noexcept;

Effects: Equivalent to: return size() * sizeof(element_type);

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constexpr bool empty() const noexcept;

Effects: Equivalent to: return size() == 0;

21.7.3.5 Element access [span.elem]

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constexpr reference operator[](index_type idx) const;
constexpr reference operator()(index_type idx) const;

Requires: 0 <= idx && idx < size().

Effects: Equivalent to: return *(data() + idx);

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constexpr pointer data() const noexcept;

Effects: Equivalent to: return data_;

21.7.3.6 Iterator support [span.iterators]

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constexpr iterator begin() const noexcept;

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;

Returns: An iterator which is the past-the-end value.

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constexpr reverse_iterator rbegin() const noexcept;

Effects: Equivalent to: return reverse_iterator(end());

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constexpr reverse_iterator rend() const noexcept;

Returns: Equivalent to: return reverse_iterator(begin());

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constexpr const_iterator cbegin() const noexcept;

Returns: A constant iterator referring to the first element in the span.

If empty() is true, then it returns the same value as cend().

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constexpr const_iterator cend() const noexcept;

Returns: A constant iterator which is the past-the-end value.

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constexpr const_reverse_iterator crbegin() const noexcept;

Effects: Equivalent to: return const_reverse_iterator(cend());

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constexpr const_reverse_iterator crend() const noexcept;

Effects: Equivalent to: return const_reverse_iterator(cbegin());

21.7.3.7 Comparison operators [span.comparison]

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template<class T, ptrdiff_t X, class U, ptrdiff_t Y>
  constexpr bool operator==(span<T, X> l, span<U, Y> r);

Effects: Equivalent to: return equal(l.begin(), l.end(), r.begin(), r.end());

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template<class T, ptrdiff_t X, class U, ptrdiff_t Y>
  constexpr bool operator!=(span<T, X> l, span<U, Y> r);

Effects: Equivalent to: return !(l == r);

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template<class T, ptrdiff_t X, class U, ptrdiff_t Y>
  constexpr bool operator<(span<T, X> l, span<U, Y> r);

Effects: Equivalent to:

return lexicographical_compare(l.begin(), l.end(), r.begin(), r.end());

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template<class T, ptrdiff_t X, class U, ptrdiff_t Y>
  constexpr bool operator>(span<T, X> l, span<U, Y> r);

Effects: Equivalent to: return (r < l);

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template<class T, ptrdiff_t X, class U, ptrdiff_t Y>
  constexpr bool operator<=(span<T, X> l, span<U, Y> r);

Effects: Equivalent to: return !(r < l);

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template<class T, ptrdiff_t X, class U, ptrdiff_t Y>
  constexpr bool operator>=(span<T, X> l, span<U, Y> r);

Effects: Equivalent to: return !(l < r);

21.7.3.8 Views of object representation [span.objectrep]

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template <class ElementType, ptrdiff_t Extent>
  span<const byte,
       Extent == dynamic_extent ? dynamic_extent
                                : static_cast<ptrdiff_t>(sizeof(ElementType)) * Extent>
    as_bytes(span<ElementType, Extent> s) noexcept;

Effects: Equivalent to: return {reinterpret_cast<const byte*>(s.data()), s.size_bytes()};

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

Effects: Equivalent to: return {reinterpret_cast<byte*>(s.data()), s.size_bytes()};

Remarks: This function shall not participate in overload resolution unless is_const_v<ElementType> is false.