basic_istream_view is a simple input_range in C 20 and has only one begin() and end() function:

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:
  constexpr iterator begin();
  constexpr default_sentinel_t end() const noexcept;
};

It inherits view_interface which is defined in [view.interface] as:

template<class D>
class view_interface {
 public:
  constexpr bool empty() requires forward_­range<D>;

  constexpr explicit operator bool()
    requires requires { ranges::empty(derived()); };

  constexpr auto data() requires contiguous_­iterator<iterator_t<D>>;

  constexpr auto size() requires forward_­range<D>;

  constexpr decltype(auto) front() requires forward_­range<D>;
  constexpr decltype(auto) back() requires bidirectional_­range<D>;

  template<random_­access_­range R = D>
  constexpr decltype(auto) operator[](range_difference_t<R> n);
};

You can see that almost all member functions of view_interface require D to be forward_range, while operator bool() requires ranges::empty(derived()) to be well-formed, which also requires D to be forward_range.

It seems that the only purpose of basic_istream_view inheriting view_interface is to model a view since view requires view_base or view_interface to be inherited.

But if you want to make basic_istream_view a view, why not just inherit it directly from view_base? I can't see any benefit from inheriting view_interface except for the cost of template instantiation.

So, why does basic_istream_view inherit the view_interface instead of view_base? Does it have historical reasons?

CodePudding user response：

Just because the member functions of view_interface today all need forward ranges doesn't mean that we will never add member functions in the future that can work on input ranges too.

Also, why not?