Re: [scope] Scope review starts on November 26th

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2023-11-30 21:47:11

On 11/30/23 22:37, Peter Dimov via Boost wrote:
> Andrey Semashev wrote:
>>> That's
>>>
>>> template<class R, auto Def, auto Del> class resource;
>>>
>>>> This would also prohibit resource types with non-constexpr constructors,
>>>> as well as non-default-constructible resources. I don't like this tradeoff.
>>>
>>> It doesn't. The two operations required from R are construction from Def
>>> and comparing to Def.
>>>
>>> https://godbolt.org/z/h7z3qGzvs
>>>
>>> Note that the type of Def doesn't have to be R.
>>
>> What I meant is something like this won't work:
>>
>> https://godbolt.org/z/hczoP4WMx
> 
> That's a common objection but the fix is simple:
> 
> https://godbolt.org/z/KTM9eb5r8
> 
>> And if you're suggesting to define some magic placeholder type that can
>> be used as Def and automatically converts to R then that looks like a
>> more contrived usage to me than just defining resource traits.
> 
> That's typically not that hard either, although for std::string specifically
> it doesn't work very well because operator!= is templated. So we're hitting
> worst case:
> 
> constexpr struct invalid_t
> {
>     operator std::string() const noexcept { return "invalid"; }

Not noexcept, std::string constructor may throw.

>     friend bool operator!=( std::string const& s, invalid_t )
>     {
>         return s != "invalid";
>     }
> }
> invalid;
> 
> Most of the time it's much simpler, though.

Just for reference, here's how the current implementation would look:

  struct string_resource_traits
  {
    static std::string const& make_default() noexcept
    {
      return g_invalid;
    }

    static bool is_allocated(std::string const& res) noexcept
    {
      return res != g_invalid;
    }

    static const std::string g_invalid;
  };

  const std::string string_resource_traits::g_invalid{ "invalid" };

  struct string_deleter
  {
    void operator()(std::string const& res) noexcept
    {
      close(res);
    }
  };

  using fd = unique_resource<
    std::string,
    string_deleter,
    string_resource_traits
  >;

https://godbolt.org/z/s4YTnbc5v