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Subject: Re: [std-proposals] Template qualifiers
From: Gašper Ažman (gasper.azman_at_[hidden])
Date: 2019-09-29 13:15:18


struct A {
   int i;
   template <std::convertible_to<A> Other> A& operator=(Other&& other) {
       using A_cvref = copy_cvref_t<Other&&, A>;
       i = static_cast<A_cvref>(other).i;
       return *this;
   }
};

No new features required, your example is pretty nice in the current
language already.

copy_cvref_t is a metafunction that imposes the cvref qualifiers of its
first parameter to its second, and IMO should be standardized but it's
currently waiting for a proposal (from me), I've been a bit busy with other
things.

When writing http://wg21.link/P0847 we have considered what you are
proposing at length and dismissed it as insufficiently general and way too
disruptive to the language vs. the effort of introducing another token type
into the template system and the entire type deduction machinery.

I'll gladly admit I'm wrong if you can prove it's simple to do and
integrate into the current language rules in a holistic manner, but until
then I remain unconvinced that it's a better solution than P0847.

On Sun, Sep 29, 2019 at 7:01 PM Phil Bouchard <phil_at_[hidden]> wrote:

> Sorry I haven't read about recursive lambdas yet but here is a better
> example in order to have unique functionality, including move semantics:
>
>
> struct A
> {
>
> int i;
>
> A & operator = (A const & a) { i = a.i; return * this; }
> A & operator = (A && a) { i = std::move(a.i); return * this; }
> A volatile & operator = (A const & a) volatile { i = a.i; return *
> this; }
> A volatile & operator = (A && a) volatile { i = std::move(a.i); return
> * this; }
> };
>
>
> struct A
> {
>
> int i;
>
> template <qualifier Q, qualifier P>
> A Q & operator = (A P a) Q { i = std::move_or_copy(a.i); return *
> this; }
> };
>
>
> Where:
>
> template <typename T>
> typename std::remove_reference<T>::type & move_or_copy( T const & t )
> noexcept;
>
> template <typename T>
> typename std::remove_reference<T>::type && move_or_copy( T && t )
> noexcept;
>
>
> It follows the purpose of the "template" notion as well.
>
>
> --
>
> *Phil Bouchard*
> Founder
> C.: (819) 328-4743
>
> [image: Fornux Logo] <http://www.fornux.com>
>
>
> On 9/29/19 12:50 PM, Gašper Ažman wrote:
>
> This proposal introduces a new kind of entity - a qualifier - which will
> need to operate correctly with all parts of the language, and it does not
> solve the problem of recursive lambdas, nor all save the deduction of the
> qualifier that "deducing this" solves. I think there is a pretty large body
> of work required here before it's a better solution. All the other
> parameters can already be handled with existing metaprogramming, too.
>
> Gašper
>
> On Sun, Sep 29, 2019, 14:14 Phil Bouchard via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> The approach I propose is more generic as it can apply to all other
>> parameters as well; not only "this".
>>
>>
>> --
>>
>> *Phil Bouchard*
>> Founder
>> C.: (819) 328-4743
>>
>> [image: Fornux Logo] <http://www.fornux.com>
>> --
>> Std-Proposals mailing list
>> Std-Proposals_at_[hidden]
>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>
>



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