D2908R0.bs

<pre class='metadata'>
Title: Type-erased ranges with <code>ranges::any_view</code>
Shortname: D2908
Revision: 0
Audience: LEWG, SG9
Status: D
Group: WG21
URL: http://wg21.link/P2908R0.html
Editor: Elias Kosunen, isocpp@eliaskosunen.com
Abstract:
    This paper proposes adding a new view type, <code>std::ranges::any_view</code>,
    based on range-v3, that type-erases its argument, and provides
    an interface that models the requested concept.
Date: 2023-06-05
Markup Shorthands: markdown yes
Max ToC Depth: 2
</pre>

Motivation {#motivation}
==========

In [[P1729]], there's a need for type-erasing a `forward_range`,
when passing it from `std::scan` to `std::vscan`.
In a fashion similar to `std::format`, the range used as the source
is type-erased when given to `std::vscan`, alongside the arguments to scan.

```c++
// simplified
template <typename... Args, scannable_range<char> Range>
auto scan(Range&& r, format_string fmt) {
    // ...
    auto range = map_scan_input_range(r);
    auto /* ... */ = vscan(range, /* ... */);
    // ...
}

auto vscan(string_view range, /* ... */);
auto vscan(erased_scannable_range range, /* ... */);
```

In [[RANGE-V3]], there's `any_view`, which is a more general case
of `erased_scannable_range`. Given how this facility could also be useful
outside of `std::scan`, this paper proposes adopting a view type
similar to `any_view`.

Design {#design}
======

`ranges::any_view`, as proposed in this paper, is based on `any_view` in [[RANGE-V3]].
It's a class template, with two template arguments: `Ref`, which is the reference type
of the wrapped range, and `Cat`, which is the concept modeled by the `any_view`.

```c++
namespace std::ranges {

    enum class category;

    namespace views {

        template <class Ref, category Cat = category::input>
        class any_view;

    } // namespace views
} // namespace std::ranges
```

`category` is a <i>bitmask type</i>, with elements mapping to different range concepts.

<pre highlight=cpp>
enum class category {
    <i>input-tag</i> = <i>unspecified</i>,
    <i>forward-tag</i> = <i>unspecified</i>,
    <i>bidirectional-tag</i> = <i>unspecified</i>,
    <i>random-access-tag</i> = <i>unspecified</i>,
    <i>contiguous-tag</i> = <i>unspecified</i>,
    <i>sized-tag</i> = <i>unspecified</i>,

    none = 0,

    input = <i>input-tag</i>,
    forward = input | <i>forward-tag</i>,
    bidirectional = forward | <i>bidirectional-tag</i>,
    random_access = bidirectional | <i>random-access-tag</i>,
    contiguous = random_access | <i>contiguous-tag</i>,
    <i>iterator-category-mask</i> = contiguous,

    sized = <i>sized-tag</i>
};
</pre>

`any_view` inherits from `view_interface`.

<pre highlight=cpp>
template &lt;typename Ref, category Cat&gt;
class any_view {
private:
    

    class <i>iterator</i>; // exposition only

    class <i>sentinel</i>; // exposition only
};
</pre>

<pre class=biblio>
{
  "P1729": {
    "title": "Text Parsing",
    "authors": ["Elias Kosunen", "Victor Zverovich"],
    "href": "https://wg21.link/p1729"
  },
  "RANGE-V3": {
    "title": "range-v3",
    "authors": ["Eric Niebler et al"],
    "href": "https://github.com/ericniebler/range-v3"
  }
}
</pre>