220 11885 <4c42f7c9-0588-4ab9-9af5-b2e692513993@isocpp.org> article Path: news.gmane.org!not-for-mail From: tomaszkam@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: N4078: Rvalue reference overloads for value() method returns object by value Date: Sat, 12 Jul 2014 00:27:29 -0700 (PDT) Lines: 170 Approved: news@gmane.org Message-ID: <4c42f7c9-0588-4ab9-9af5-b2e692513993@isocpp.org> References: <4DA018C3-2327-4F55-B7F8-7988EC7B789A@gmail.com> <1a858a77-310b-4591-aaf5-960c40d9f5fc@isocpp.org> <66AF41C1-CB37-4311-98EA-D47DBC6068DC@gmail.com> <66c38b61-7a95-4ace-b34f-2346bb334fb9@isocpp.org> <6F4B3C72-9962-48DC-8527-0C89EA1B9710@gmail.com> <35873CE5-ED8C-4C03-A6A1-E4132DECD91F@gmail.com> <335948E6-8140-4673-B958-930356A52070@gmail.com> <76ACA175-5A44-4BE8-B49F-BAC6DA65E6B1@gmail.com> <4D3852AD-1AEB-44BA-941C-D645BAD19405@gmail.com> <6a4b6425-1167-40c1-acd8-3945c8cd4c74@isocpp.org> <13DD0D7A-A146-4667-9F30-6FB3739CDBE0@gmail.com> <6B925FF7-2940-44C7-BF1D-13209A703BA2@gmail.com> <944C9C01-865B-4215-8C91-95F01AEA256E@gmail.com> <8DC3E474-3957-40B2-B492-D4AF2FDDD451@gmail.com> <0ACAE525-4FB2-4932-BBD2-F0623C4EBBFB@gmail.com> <52187852-ff8e-4e59-a8cf-7a29fd0a405b@isocpp.org> <8209A2E9-D947-4C00-A1F3-38F5676A4E52@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; 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Sat, 12 Jul 2014 00:27:30 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.53.170 with SMTP id c10ls636229obp.49.gmail; Sat, 12 Jul 2014 00:27:29 -0700 (PDT) X-Received: by 10.182.108.165 with SMTP id hl5mr476obb.27.1405150049933; Sat, 12 Jul 2014 00:27:29 -0700 (PDT) In-Reply-To: <8209A2E9-D947-4C00-A1F3-38F5676A4E52@gmail.com> X-Original-Sender: tomaszkam@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:11885 Archived-At: ------=_Part_331_5533592.1405150049235 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable W dniu sobota, 12 lipca 2014 01:30:24 UTC+2 u=C5=BCytkownik David Krauss na= pisa=C5=82: > > > On 2014=E2=80=9307=E2=80=9311, at 6:57 PM, toma...@gmail.com wrote: > > Do we found any other argumentation for T member()&& overload except=20 > making auto&& s =3D f; work for some cases?=20 > > In the follwoing example: > template > delctype(auto) f(T&& t) { return std::addressof(*std::forward(t)); } > =20 > template > delctype(auto) g(T&& t) {=20 > auto&& ret =3D f(std::forward(t));=20 > //will create a dangling pointer, because the liftetime of result o= f=20 > f.value() is not longer same a liftme of the owner t > > > Once again you=E2=80=99ve omitted the parens after f in the comment. Plea= se=20 > proofread before posting. f is a naked function and f() is a naked pointe= r.=20 > Neither can have a member .value. You probably mean *f or f->value()=20 > under the assumption that f is type optional*. > > I agree with you, in the way the post was written I failed to present my=20 point. Sorry for that. =20 > A function that converts a possibly-owning smart pointer to merely an=20 > observing naked pointer is obviously not a candidate for move semantics, = so=20 > the example is not reasonable. > > Let me present my last point again, because I think it was never presented= =20 in the thread. I will begin with describing the issue: Having prvalue overloads (T=20 operator*()) for optional breaks the connection between expression of=20 type optional (opt_expr) and the lifetime of bounded result of=20 (opt_expr.value()) bound to the reference. This will only happen of opt_expr is rvalue, so the only situation when it= =20 may happen by accident is the generic context. Example: template void do_something(Nullable&& nullable) { pointer_type_t pointer =3D nullptr; if (nullable) { auto&& value =3D *std::forward(nullable); //Do something with value pointer =3D addressof(value); } function_taking_pointer(pointer); } 1.The above code works if the passed Nullable types does not define any=20 rvalue overloads for operator* - current status quo with library.=20 Because the lifetime of object bound by auto&& value =3D=20 *std::forward(nullable); is always bound to the lifetime of=20 nullable. 2. The above code work if the passed Nullable type define && overload of=20 operator* that returns by rvalue-reference. Same reasoning apply as above. 3. The above code does not work if if the passed Nullable type define &&=20 overload of operator* that returns by value. Lifetime of object bound by auto&& value =3D=20 *std::forward(nullable); is not the same as lifetime of nullable. --=20 ---=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. ------=_Part_331_5533592.1405150049235 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


W dniu sobota, 12 lipca 2014 01:30:24 UTC+2 u=C5= =BCytkownik David Krauss napisa=C5=82:

On 2014=E2=80=930= 7=E2=80=9311, at 6:57 PM, toma...@gmai= l.com wrote:

Do we = found any other argumentation for T member()&& overload except maki= ng auto&& s =3D f; work for some cases?

In the follwoing ex= ample:
  template<typename T>
  delctype(auto) f(T&am= p;& t) { return std::addressof(*std::forward<T>(t)); }
&n= bsp; 
  template<typename T>
  delctype(auto) g= (T&& t) {
      auto&& ret =3D= f(std::forward<T>(t));
      //will cre= ate a dangling pointer, because the liftetime of result of f.value() is not= longer same a liftme of the owner t

<= div>Once again you=E2=80=99ve omitted the parens after f in the comment. Pl= ease proofread before posting. f is a naked function and f() is a naked poi= nter. Neither can have a member .value. You p= robably mean *f or f-&= gt;value() under the assumption that f is type optional*.


I= agree with you, in the way the post was written I failed to present my poi= nt. Sorry for that.
 
A function t= hat converts a possibly-owning smart pointer to merely an observing naked p= ointer is obviously not a candidate for move semantics, so the example is n= ot reasonable.


Let me present my= last point again, because I think it was never presented in the thread.
I will begin with describing the issue: Having prvalue overloads (T op= erator*()) for optional<T> breaks the connection between expression o= f type optional (opt_expr) and the lifetime of bounded result of (opt_expr.= value()) bound to the reference.
This will only happen of opt_expr is rv= alue, so the only situation when it may happen by accident is the generic c= ontext.

Example:
template<typename Nullable>
void do_som= ething(Nullable&& nullable)
{
  pointer_type_t<Nullab= le> pointer =3D nullptr;
  if (nullable)
  {
 &n= bsp;   auto&& value =3D *std::forward<Nullable>(nul= lable);
     //Do something with value
 &nbs= p;   pointer =3D addressof(value);
  }
  function= _taking_pointer(pointer);
}
1.The above code works if the passed Null= able types does not define any rvalue overloads for operator* - current sta= tus quo with library.
   Because the lifetime of object bound= by auto&& value =3D *std::forward<Nullable>(nullable); is al= ways bound to the lifetime of nullable.
2. The above code work if the pa= ssed Nullable type define && overload of operator* that returns by = rvalue-reference. Same reasoning apply as above.
3. The above code does = not work if if the passed Nullable type define && overload of opera= tor* that returns by value.
    Lifetime of object bound = by auto&& value =3D *std::forward<Nullable>(nullable); is not= the same as lifetime of nullable.

--

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