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Subject: Re: [std-proposals] Generic template 'this'
From: Phil Bouchard (phil_at_[hidden])
Date: 2020-03-08 16:55:42


Well I believe a proposal should be open for debate, just like a thesis.

We both agree these overloads should be simplified but we disagree on
the syntax.

I saw the recursive lambda thing and I think I already talked to you
before regarding CV overloads but we were proposing the following syntax:

struct A

{

 Â Â Â  template <bool BC>

 Â Â Â      void foo() bool<BC>

 Â Â Â      {

 Â Â Â      }

};

And regarding the this generic overloading I still believe the following
syntax is cleaner:

struct A

{

 Â Â Â  template <typename T>

 Â Â Â      bool compare(T const &);

};

template <typename C, typename T>

 Â Â Â  void C::compare(C const &)

 Â Â Â  {

 Â Â Â      ...

 Â Â Â  }

Because with your syntax I could technically reassign "self":

struct A

{

 Â Â Â  template <typename C, typename T>

 Â Â Â      bool compare(this C & c, T const &);

};

template <typename C, typename T>

 Â Â Â  bool C::compare(this C & c, T const &) // redundant C here

 Â Â Â  {

 Â Â Â      static C sc = C();

 Â Â Â      c = sc; // in theory we could do this unless you patch the
compilers with a new error for that specific case

 Â Â Â  }

-- 
*Phil Bouchard*
Founder
C.: (819) 328-4743
Fornux Logo <http://www.fornux.com>
On 3/8/20 5:37 AM, Gašper Ažman wrote:
> I meant that you should consider the incremental value of your 
> proposal over p0847, which is on track for c++23 unless serious issues 
> crop up
>
> On Sat, Mar 7, 2020, 19:04 Phil Bouchard <phil_at_[hidden] 
> <mailto:phil_at_[hidden]>> wrote:
>
>     The syntaxes are way different:
>
>     http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p0847r4.html
>
>     In your case the compiler will have to change all of its rules for
>     the number of parameters "operator" overloads. For example:
>
>     struct A
>
>     {
>
>         int value;
>
>         ...
>
>         template <typename Self>
>
>             bool operator < (this Self && self, A const & a) // 2
>     parameters... ok
>
>             {
>
>                 return self.value < a.value;
>
>             }
>
>
>             bool operator < (A const & a) // 1 parameter... ok
>
>             {
>
>                 return value < a.value;
>
>             }
>
>     };
>
>
>     -- 
>
>     *Phil Bouchard*
>     Founder
>     C.: (819) 328-4743
>
>     Fornux Logo <http://www.fornux.com>
>
>
>     On 3/7/20 1:35 PM, Gašper Ažman wrote:
>>     P0847
>>
>>     On Sat, Mar 7, 2020, 18:33 Phil Bouchard via Std-Proposals
>>     <std-proposals_at_[hidden]
>>     <mailto:std-proposals_at_[hidden]>> wrote:
>>
>>         Alright, I'm pretty sure this is not implemented yet. Suppose
>>         you have:
>>
>>         struct A
>>
>>         {
>>
>>             int value;
>>
>>             ...
>>
>>             template <typename T>
>>
>>                 int compare(T const & t) const;
>>
>>         }
>>
>>
>>         struct B
>>
>>         {
>>
>>             int value;
>>
>>             ...
>>
>>             template <typename T>
>>
>>                 int compare(T const & t) const;
>>
>>         }
>>
>>
>>         Then a generic way to define the same functionality for all
>>         classes would be to have a "template 'this'":
>>
>>         template <typename C, typename T>
>>
>>                 inline int C::compare(T const & t) const
>>
>>                 {
>>
>>                     return value == t.value ? 0 : value < t.value ?
>>         -1 : 1;
>>
>>                 }
>>
>>
>>         (Please include my email address in your replies)
>>
>>
>>         -- 
>>
>>         *Phil Bouchard*
>>         Founder
>>         C.: (819) 328-4743
>>
>>         Fornux Logo <http://www.fornux.com>
>>         -- 
>>         Std-Proposals mailing list
>>         Std-Proposals_at_[hidden]
>>         <mailto:Std-Proposals_at_[hidden]>
>>         https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>>


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