Hi folks,

I've written a draft proposal for `view_interface::at` in

https://htmlpreview.github.io/?https://github.com/hewillk/view_interface-at/blob/main/view-at.html

any valuable comments are welcome, thanks.

Hewill

Hewill Kang <hewillk_at_[hidden]> 於 2023年11月13日 週一 下午6:10寫道：

> Hey C++ folks,
>
> I was surprised that C++26 actually introduced `.at()`
> <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2821r4.html>
> for `span` (which throws an exception) , even though I was sure that this
> function was initially rejected by the committee when `span` was first
> introduced.
> So, should `.at()` be also introduced for `view_interface` to ensure such
> consistency, so that all range adaptors have safe random-access methods as
> other containers in the standard?
>
> ```cpp
> template<class D>
> class view_interface {
> private:
> constexpr D& derived() noexcept {
> return static_cast<D&>(*this);
> }
>
> public:
> // [...]
> template<random_access_range R = D>
> constexpr decltype(auto) operator[](range_difference_t<R> n) {
> return ranges::begin(derived())[n];
> }
>
> template<random_access_range R = D> requires sized_range<D>
> constexpr decltype(auto) at(range_difference_t<R> n) {
> if (n < ranges::distance(derived()) || n >=
> ranges::distance(derived()))
> throw_out_of_range();
> return operator[](n);
> }
> };
> ```
>
> I believe this can be seen as an enhancement, as it enables range adaptors
> to have safe random-access operations, which were not available before.
> Additionally, `mdspan` also seems worth adding a corresponding variadic
> `.at()`:
>
> ```cpp
> template<class ElementType, class Extents, class LayoutPolicy =
> layout_right,
> class AccessorPolicy = default_accessor<ElementType>>
> class mdspan {
> // [mdspan.mdspan.members], members
> template<class... OtherIndexTypes>
> constexpr reference operator[](OtherIndexTypes... indices) const;
> template<class OtherIndexType>
> constexpr reference operator[](span<OtherIndexType, rank()> indices)
> const;
> template<class OtherIndexType>
> constexpr reference operator[](const array<OtherIndexType, rank()>&
> indices) const;
>
> template<class... OtherIndexTypes>
> constexpr reference at(OtherIndexTypes... indices) const;
> template<class OtherIndexType>
> constexpr reference at(span<OtherIndexType, rank()> indices) const;
> template<class OtherIndexType>
> constexpr reference at(const array<OtherIndexType, rank()>&
> indices) const;
> };
> ```
>
> What do you guys think? Does introducing `.at()` for the above two have
> any observable value?
>
> Hewill
>
>
>
>