24 Ranges library [ranges]

24.6 Range factories [range.factories]

24.6.4 Istream view [range.istream]

24.6.4.2 Class template basic_­istream_­view [range.istream.view]

namespace std::ranges {
  template<class Val, class CharT, class Traits>
    concept stream-extractable =                // exposition only
      requires(basic_istream<CharT, Traits>& is, Val& t) {
         is >> t;
      };

  template<movable Val, class CharT, class Traits>
    requires default_initializable<Val> &&
             stream-extractable<Val, CharT, Traits>
  class basic_istream_view : public view_interface<basic_istream_view<Val, CharT, Traits>> {
  public:
    basic_istream_view() = default;
    constexpr explicit basic_istream_view(basic_istream<CharT, Traits>& stream);

    constexpr auto begin()
    {
      if (stream_) {
        *stream_ >> object_;
      }
      return iterator{*this};
    }

    constexpr default_sentinel_t end() const noexcept;

  private:
    struct iterator;                                    // exposition only
    basic_istream<CharT, Traits>* stream_ = nullptr;    // exposition only
    Val object_ = Val();                                // exposition only
  };
}