24 Ranges library [ranges]

24.7 Range adaptors [range.adaptors]

24.7.3 All view [range.all]

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views​::​all returns a view that includes all elements of its range argument.
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The name views​::​all denotes a range adaptor object ([range.adaptor.object]).
Given a subexpression E, the expression views​::​all(E) is expression-equivalent to:
  • (2.1)
    decay-copy(E) if the decayed type of E models view.
  • (2.2)
    Otherwise, ref_­view{E} if that expression is well-formed.
  • (2.3)
    Otherwise, subrange{E}.

24.7.3.1 Class template ref_­view [range.ref.view]

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ref_­view is a view of the elements of some other range.
namespace std::ranges {
  template<range R>
    requires is_object_v<R>
  class ref_view : public view_interface<ref_view<R>> {
  private:
    R* r_ = nullptr;            // exposition only
  public:
    constexpr ref_view() noexcept = default;

    template<not-same-as<ref_view> T>
      requires see below
    constexpr ref_view(T&& t);

    constexpr R& base() const { return *r_; }

    constexpr iterator_t<R> begin() const { return ranges::begin(*r_); }
    constexpr sentinel_t<R> end() const { return ranges::end(*r_); }

    constexpr bool empty() const
      requires requires { ranges::empty(*r_); }
    { return ranges::empty(*r_); }

    constexpr auto size() const requires sized_range<R>
    { return ranges::size(*r_); }

    constexpr auto data() const requires contiguous_range<R>
    { return ranges::data(*r_); }
  };
  template<class R>
    ref_view(R&) -> ref_view<R>;
}
🔗
template<not-same-as<ref_view> T> requires see below constexpr ref_view(T&& t);
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Remarks: Let FUN denote the exposition-only functions 
void FUN(R&);
void FUN(R&&) = delete;
The expression in the requires-clause is equivalent to 
convertible_to<T, R&> && requires { FUN(declval<T>()); }
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Effects: Initializes r_­ with addressof(static_­cast<R&>(std​::​forward<T>(t))).