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