220 41351 article Path: news.gmane.org!.POSTED!not-for-mail From: Andrew Tomazos Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: `bit_cast` and when UB happens. 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[209.85.220.41]) by mx.google.com with SMTPS id y86-v6sor50169531lje.18.2019.01.13.18.18.16 for (Google Transport Security); Sun, 13 Jan 2019 18:18:16 -0800 (PST) Received-SPF: pass (google.com: domain of andrewtomazos@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a2e:12d0:: with SMTP id 77-v6mr12683190ljs.132.1547432295351; Sun, 13 Jan 2019 18:18:15 -0800 (PST) In-Reply-To: X-Original-Sender: andrewtomazos@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=FUUOxCTv; spf=pass (google.com: domain of andrewtomazos@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=andrewtomazos@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:41351 Archived-At: --000000000000474ad5057f61a8b8 Content-Type: text/plain; charset="UTF-8" https://stackoverflow.com/q/54175056/1131467 On Sun, Jan 13, 2019 at 7:53 PM T. C. wrote: > > > On Sunday, January 13, 2019 at 7:27:23 PM UTC-5, Nicol Bolas wrote: >> >> >> >> On Sunday, January 13, 2019 at 6:31:49 PM UTC-5, T. C. wrote: >>> >>> >>> >>> On Sunday, January 13, 2019 at 3:12:16 PM UTC-5, Nicol Bolas wrote: >>>> >>>> P0476R2 specifies when a `bit_cast` yields UB >>>> : >>>> >>>> If there is no value of type `To` corresponding to the value >>>>> representation produced, the behavior is undefined. >>>>> >>>> >>>> OK, sure. But what does this really mean? Consider the following type: >>>> >>>> class specific_bits >>>> { >>>> public: >>>> specific_bits() = default; >>>> explicit specific_bits(int j) : j_(j) {} >>>> >>>> int get_i() const {return i_;} >>>> >>>> private: >>>> int j_; >>>> int i_ = 5; >>>> }; >>>> >>>> Now, consider the following operation: >>>> >>>> auto sb = bit_cast(pair{10, 20}); >>>> >>>> Let's assume that the sizes of these types are the same. `pair` is >>>> TriviallyCopyable, as is `specific_bits`. So, is this well-defined behavior? >>>> >>>> Well, `specific_bits` is technically just a pair of integers, so it >>>> seems like it should be well-defined. However, it is my understanding that >>>> it is essentially impossible to change the `i_` member of any >>>> `specific_bits` object to any other value. At least, not from outside of >>>> the class; anything which would do so would run into UB at some point. >>>> >>> >>> Access control is not watertight enough for that. There are ways you can >>> get your hands on a pointer to member to specific_bits::i_, at which point >>> you can modify the field ( >>> http://coliru.stacked-crooked.com/a/62dd7c4bfeb1eb96). >>> >> >> Why isn't `template struct accessor;` >> a compile error? Doesn't it mention a name which is not accessible from >> that location? >> > > Access checking is not applied to explicit instantiations (with exceptions > not relevant here). See [temp.spec]/6 in the current WP. > > -- > 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 > email to std-proposals+unsubscribe@isocpp.org. > To post to this group, send email to std-proposals@isocpp.org. > To view this discussion on the web visit > https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/e9d6281d-0256-437a-832f-62d2b885c552%40isocpp.org > > . > -- 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 email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CAB%2B4KHL-Kgk3rM2pujOs4yhrRw0EU8_7Hx%3DkiMqF-oRthj14MA%40mail.gmail.com. --000000000000474ad5057f61a8b8 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
<= /div>
On Sun, Jan 13, 2019 a= t 7:53 PM T. C. <rs2740@gmail.com> wrote:


On Sunday, January 13, 2019 at 7:27:23 PM UTC-5, Nico= l Bolas wrote:


On Sunday, January 13, 2019 at 6:31:49 PM UTC-5, T. C. wrote:=


= On Sunday, January 13, 2019 at 3:12:16 PM UTC-5, Nicol Bolas wrote:
P0476R2 specifies = when a `bit_cast` yields UB:

If there is no value of type `To` corresponding to the value= representation produced, the behavior is undefined.

OK= , sure. But what does this really mean? Consider the following type:
class specific_bits
= {
public:=C2=A0 =C2=A0 specific_bits()<= /span> =3D default;
=C2=A0 =C2=A0 explicit sp= ecific_bits(int j) : j_(j)<= /span> {}
=C2=A0 =C2=A0
= =C2=A0 =C2=A0 int get_i(= ) const {return= i_;}

private:
=C2=A0 =C2=A0 int = j_;
=C2=A0 =C2=A0 int= i_ =3D 5;
};

Now, consider the following opera= tion:

auto sb =3D bit_cast<specific_bi= ts>(pair<= int, int>{= 10, 20});=

Let's assume that the sizes of these types are t= he same. `pair` is TriviallyCopyable, as is `specific_bits`. So, is this we= ll-defined behavior?

Well, `specific_bits` is technically just a pai= r of integers, so it seems like it should be well-defined. However, it is m= y understanding that it is essentially impossible to change the `i_` member= of any `specific_bits` object to any other value. At least, not from outsi= de of the class; anything which would do so would run into UB at some point= ..

Access control is not watertigh= t enough for that. There are ways you can get your hands on a pointer to me= mber to specific_bits::i_, at which point you can modify the field (http://coliru.stacked-crooked.com/a/62dd7c4bfeb1eb96)= ..

Why isn't `template struc= t accessor<specific_bits, &specific_bits::i_>;` a compile error? = Doesn't it mention a name which is not accessible from that location?

Access checking is not appli= ed to explicit instantiations (with exceptions not relevant here). See [tem= p.spec]/6 in the current WP.=C2=A0

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