220 19957 <9CEC4FAB-0198-4CB9-929E-57FD48BA1D6B@gmail.com> article Path: news.gmane.org!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Address of a reference: determining the value category of what it refers? 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[2607:f8b0:400e:c03::231]) by mx.google.com with ESMTPS id p4si16374131pdo.42.2015.08.21.21.04.05 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 21 Aug 2015 21:04:05 -0700 (PDT) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::231 as permitted sender) client-ip=2607:f8b0:400e:c03::231; Original-Received: by pacgr6 with SMTP id gr6so6880664pac.0 for ; Fri, 21 Aug 2015 21:04:05 -0700 (PDT) X-Received: by 10.66.219.39 with SMTP id pl7mr23823049pac.148.1440216244906; Fri, 21 Aug 2015 21:04:04 -0700 (PDT) Original-Received: from [172.20.10.2] ([121.54.44.90]) by smtp.gmail.com with ESMTPSA id ob4sm9422593pbb.40.2015.08.21.21.04.01 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 21 Aug 2015 21:04:04 -0700 (PDT) In-Reply-To: X-Mailer: Apple Mail (2.2098) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c03::231 as permitted sender) smtp.mailfrom=potswa@gmail.com; dkim=pass header.i=@gmail.com; dmarc=pass (p=NONE 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:19957 Archived-At: --Apple-Mail=_62997BD6-83E0-4C29-B190-5FD5C466B56A Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 You=E2=80=99re basically reinventing expression templates. The solution to = your stated problem of differentiating lvalues from rvalues is to alternate= ly use member lvalue references (&) or rvalue references (&&), depending on= the operator overloads that build the expression. Another excerpt from my upcoming proposal: > Expression templates with rvalue semantics >=20 > Expression template libraries benefit from conserving memory and minimizi= ng intermediate temporaries. If temporaries could be retained as persistent= storage for results, expensive copy operations could be eliminated. >=20 > A library designed this way would also be able to recycle memory from nam= ed objects passed into an expression through std::move. >=20 It=E2=80=99s a hopeful direction, but the language isn=E2=80=99t there yet. But this optimization doesn=E2=80=99t seem to be what you=E2=80=99re after;= it looks like you want easy alias analysis. Alias analysis is never easy. = I think the best you can do is to compare object addresses. So, as for P(X<= X=C2=B2), there could be a rule simplifying P(X On 2015=E2=80=9308=E2=80=9322, at 10:42 AM, NDos Dannyu wrote: >=20 > 2015=EB=85=84 8=EC=9B=94 22=EC=9D=BC =ED=86=A0=EC=9A=94=EC=9D=BC =EC=98= =A4=EC=A0=84 11=EC=8B=9C 24=EB=B6=84 55=EC=B4=88 UTC+9, Thiago Macieira =EB= =8B=98=EC=9D=98 =EB=A7=90: >=20 > You're saying that the function doit above should return null if it was c= alled=20 > with a reference to a temporary. But if it does so, how will it print the= =20 > value that was referenced?=20 > Does printf need its address??=20 > Besides, we're missing an important aspect: why do you want this? What's = the=20 > motivation?=20 > I'm writing my story again: > I have a value class indicating a finite discrete random variable. > But I had a problem. For example, assume that I have a variable named X. > But how can P(X Things will get much more complicated if I have multiple variables, X, Y = and Z, > and I have to do like P(X|Z|) or something.=20 > So I decided to make a bind of variables. > It would be a tree structure, consisting of other binds or variables. > If it consists of references of variables, it will know what variables ar= e same(or dependant) and what variables are different(or indepentant). > But how can I deal with rvalues? For examples, 1 can be seen as a variabl= e which is always 1. > But it has no name. It is just 1. It is always indepentant from other var= iables. > So I made it to consist of const lvalue references, which can initialized= with lvalue or rvalue. > But how can I determine it refers to lvalue or rvalue?? I couldn't find a= ny solution from the standard library. --=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/. --Apple-Mail=_62997BD6-83E0-4C29-B190-5FD5C466B56A Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
You=E2=80=99re basically reinventing expression template= s. The solution to your stated problem of differentiating lvalues from rval= ues is to alternately use member lvalue references (&) or rvalue references (&&), depending= on the operator overloads that build the expression.

Another excerpt from my upcoming proposal:

Expression templates with rvalue semantics

Expression template libraries benefit from conserving memor= y and minimizing intermediate temporaries. If temporaries could be retained= as persistent storage for results, expensive copy operations could be elim= inated.

A library design= ed this way would also be able to recycle memory from named objects passed = into an expression through std::move.


It=E2= =80=99s a hopeful direction, but the language isn=E2=80=99t there yet.

But this optimization= doesn=E2=80=99t seem to be what you=E2=80=99re after; it looks like you wa= nt easy alias analysis. Alias analysis is never easy. I think the best you = can do is to compare object addresses. So, as for P(X<X=C2=B2), there co= uld be a rule simplifying P(X<Y*Z) to P(1 < abs(X)) when &X =3D=3D &Y && &Y =3D=3D = &Z. The optimizer should be able to evaluate such equality tests= at compile time when X, Y, and Z are a mix of temporaries and local object= s.


On 2015=E2=80=9308=E2=80=9322, at 10:42 AM, NDos Dannyu <ndospark320@naver.com> = wrote:

2015= =EB=85=84 8=EC=9B=94 22=EC=9D=BC =ED=86=A0=EC=9A=94=EC=9D=BC =EC=98=A4=EC= =A0=84 11=EC=8B=9C 24=EB=B6=84 55=EC=B4=88 UTC+9, Thiago Macieira =EB=8B=98= =EC=9D=98 =EB=A7=90:

You're sayi= ng that the function doit above should return null if it was called=20
with a reference to a temporary. But if it does so, how will= it print the=20
value that was referenced?
Does printf need its addre= ss?? 
Besides, we're missing an impo= rtant aspect: why do you want this? What's the=20
motivation?
I'm writing my story again:
I have a value class indicating a f= inite discrete random variable.
But I had a pro= blem. For example, assume that I have a variable named X.
But how can P(X<X=C2=B2) interpreted? X is still= X, even it is squared, isn't it?
Things will get much= more complicated if I have multiple variables, X, Y and Z,
and I have to do like P(X<X=C2=B2=3DY>|Z|) or something.&n= bsp;
So I decided to make a bind of variables.
It would be a tree structure, consisting of other&n= bsp;binds or variables.
If it consists&= nbsp;of references of variables, it will know what variables are same(or de= pendant) and what variables are different(or indepentant).
But how can I deal with rvalues? For examples, 1 can be= seen as a variable which is always 1.
But it has = ;no name. It is just 1. It is always indepentant from other varia= bles.
So I made it to consist of const lvalu= e references, which can initialized with lvalue or rvalue.
But how can I determine it refers to lvalue o= r rvalue?? I couldn't find any solution from the standard library.

--

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