220 19982 <502d5927-226c-45cd-a18b-c3bd92455759@isocpp.org> article Path: news.gmane.org!not-for-mail From: Arthur O'Dwyer Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Address of a reference: determining the value category of what it refers? Date: Mon, 24 Aug 2015 12:09:57 -0700 (PDT) Lines: 182 Approved: news@gmane.org Message-ID: <502d5927-226c-45cd-a18b-c3bd92455759@isocpp.org> References: <76fe342f-3342-4355-af20-1c41b21e54aa@isocpp.org> <3183929.eXnTES4BH3@tjmaciei-mobl4> <13312961.n5JFXrPRMR@tjmaciei-mobl4> <1c31275e-3bac-4eb2-99cf-256b14e6f650@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_3154_1864411866.1440443397674" X-Trace: ger.gmane.org 1440443417 26801 80.91.229.3 (24 Aug 2015 19:10:17 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 24 Aug 2015 19:10:17 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDLZJYWNDQIINWHNVYCRUBCYS7ZBC@isocpp.org Mon Aug 24 21:10:01 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qk0-f200.google.com ([209.85.220.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZTx84-0000fb-LK for gclcip-std-proposals@m.gmane.org; Mon, 24 Aug 2015 21:10:00 +0200 Original-Received: by qkcu126 with SMTP id u126sf205689909qkc.0 for ; Mon, 24 Aug 2015 12:09:59 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :content-type:x-original-sender:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=va9ftjSAVOOXmL+XtB2yUVJHNT6bNYV37jfIHX7AYBk=; b=zswOw6YS9sumqjeSfGqhJ9qgn67agA/AXpKoRwiq5smUYVw+NL2/cFWFiYMeeE7YSv Z9tUT9TDsH5phiZ7wYVP89enleAw7lgP1ICw2mshhemXH219I6vwManN5fbsTV3YHAM+ V9kn6/7Obox06TMSdspcM1f/I4+DWyrkFgFVF2QqenKEy+cJLR8JegwbEiDcS9uv4ULP W/TSTAjwArF3JlrngYm2PadfU2ncQ2UxulUrSRI6/RmPQ3CZ+Zh7kWGWevVEwkABvc6A gGznNbRcx8DTtaH7ZoNaFrT5/44isn+L+V6ciSi4NwZTvUkcczjWe61QzgnC22dVAKJ+ b/Ew== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:content-type:x-original-sender:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=va9ftjSAVOOXmL+XtB2yUVJHNT6bNYV37jfIHX7AYBk=; b=cbcyFOyhIf0N4lytRrJtxmwTNqbu1Z5KJPI/F0wiSpSbM6PrCBKSeADqjObh1o+SNI 7u7bbNkHZwBDO95hJpGIR2cJTQp4plW+FdIUB4fUUt91GsUv3lamOV/z8mTiORgsy+7p yUeT9TWyrjnMYvWqXOJ1wEPfd7F8e107mnAHNI0u4zPXJuE/CdcL77/Xn/2BWALzlBaf 2Rigc+Rsx7tM7I5Bqxjpiv5WqHrpdVnnbyQPx0VjsIK9ebfVD0xreic8eXLMtpUQgN4S n4VccS7tIKR46UHN033axc5QD2/DjTwBE31xjf/YlrNgdpkmGPt7pwtcEzRzLqG9qcKK ZKIA== X-Gm-Message-State: ALoCoQkqkfTyzl0Ykwhyq2aANROnL2YkpVlk+VONzhrvf3JEOoZ9gN/yFA1gs57UxUG/8h6DNir1 X-Received: by 10.129.83.136 with SMTP id h130mr23912888ywb.23.1440443399624; Mon, 24 Aug 2015 12:09:59 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.129.38 with SMTP id c38ls1446903iod.15.gmail; Mon, 24 Aug 2015 12:09:58 -0700 (PDT) X-Received: by 10.50.62.68 with SMTP id w4mr273901igr.12.1440443398696; Mon, 24 Aug 2015 12:09:58 -0700 (PDT) In-Reply-To: <1c31275e-3bac-4eb2-99cf-256b14e6f650@isocpp.org> X-Original-Sender: arthur.j.odwyer@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:19982 Archived-At: ------=_Part_3154_1864411866.1440443397674 Content-Type: multipart/alternative; boundary="----=_Part_3155_218057942.1440443397674" ------=_Part_3155_218057942.1440443397674 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Friday, August 21, 2015 at 10:15:58 PM UTC-7, NDos Dannyu 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=ED=9B=84 1=EC=8B=9C 6=EB=B6=84 29=EC=B4=88 UTC+9, Thiago Macieira =EB= =8B=98=EC=9D=98 =EB=A7=90: >> >> >> What's is P and what is squaring the variable here? How should I=20 >> interpret the=20 >> equal sign above?=20 >> > P is probability function. It calculates the probability that the=20 > expression inside is true. > For example: > *#include * > *discrete_random_variable=20 > X(std::uniform_int_distribution(1,6));* > This is a variable that represents a die. > P(X=3D1) =3D 1/6, P(X=3D2) =3D 1/6, ... , P(X=3D6) =3D 1/6. > But what would P(X=3DX) be? > Of course a die will always be same number with itself, so P(X=3DX) =3D 1= ..=20 > But... > *discrete_random_variable Y(X);* > This is a copy of X. Now I have two dice. > Even Y is a perfect copy of X, the dice are different. > P(X=3DY) =3D 1/6. > You started off by claiming that you were trying to implement a "value=20 type"; but what you have here is definitely NOT a value type, because you= =20 want Y to be equal to X (i.e. Y is a copy of X), yet Y behaves differently= =20 from X. That's the very definition of "not a value type." You're basically trying to have the lexical name of the C++ variable=20 *holding* the value *be part of* the value: the fact that the variable=20 holding your object happens to be named X is part of the semantics of the= =20 object. I strongly recommend that you don't abuse C++ in this way. You want=20 something like this: discrete_random_variable=20 X(std::uniform_int_distribution(1,6)); discrete_random_variable identicalWithX =3D X; discrete_random_variable Y =3D X.makeIndependentCopy(); assert(P(X =3D=3D X) =3D=3D 1.0); assert(P(X =3D=3D Y) =3D=3D 0.16666666); =20 > Of course, C++ uses =3D=3D as equality, and C++ can't compare three or mo= re=20 > elements at once. > So, the expression P(X|Z|) would be written in C++ as someth= ing like=20 > this: > *compare(And{{X, std::less, bind_variable(std::pow, X, 2)}, {**bind_v= ariable(std::pow,=20 > X, 2), std::equal, Y}, {Y, std::greater, bind_variable(std::abs, Z)}});* > What a mess. > I'd prefer to write the above as discrete_random_variable B =3D (X < X*X && X*X =3D=3D Y && Y > ab= s(Z)); double p =3D P(B); which is perfectly doable with operator overloading. This kind of question ("how do I write this specific kind of numeric code= =20 in C++?") would be more suited for StackOverflow or=20 CodeReview.stackexchange.com, as opposed to this forum, which is more for= =20 proposed new language features and suchlike. HTH, =E2=80=93Arthur --=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_3155_218057942.1440443397674 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Friday, August 21, 2015 at 10:15:58 PM UTC-7, NDos Dannyu 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=ED=9B=84 1=EC=8B=9C 6= =EB=B6=84 29=EC=B4=88 UTC+9, Thiago Macieira =EB=8B=98=EC=9D=98 =EB=A7=90:<= blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;padding-= left:1ex;border-left-color:rgb(204,204,204);border-left-width:1px;border-le= ft-style:solid">
What's is P and what is squaring the variable here?= How should I interpret the=20
equal sign above?
P is probability function. It calculates the probabil= ity that the expression inside=C2=A0is true.
For example:
=C2=A0=C2=A0=C2=A0 #include <random>
= =C2=A0=C2=A0=C2=A0 discrete_random_variable<int> X(std::unifo= rm_int_distribution<int>(1,6));
This is a var= iable that represents a die.
P(X=3D1)=C2=A0=3D 1/6, P(X=3D2) =3D = 1/6, ... , P(X=3D6) =3D 1/6.
But what would P(X=3DX) be?
Of course a die will always be same number with itself, so P(X=3DX) =3D 1= .. But...
=C2=A0=C2=A0=C2=A0 discrete_random_variable<i= nt> Y(X);
This is a copy of X. Now I have two dice.
Even Y is a perfect copy of X, the dice are different.
P= (X=3DY) =3D 1/6.

You started of= f by claiming that you were trying to implement a "value type"; b= ut what you have here is definitely NOT a value type, because you want Y to= be equal to X (i.e. Y is a copy of X), yet Y behaves differently from X. T= hat's the very definition of "not a value type."
You're basically trying to have the lexical name of the C+= + variable holding the value be part of the value: the fact t= hat the variable holding your object happens to be named X is part of the semantics of the object.
<= div>
I strongly recommend that you don't abuse C++ in thi= s way. You want something like this:

=C2=A0 =C2=A0= discrete_random_variable<int> X(std::uniform_int_distribution<int= >(1,6));
=C2=A0 =C2=A0 discrete_random_variable<in= t> identicalWithX =3D X;
=C2=A0 =C2=A0 discrete_random_variabl= e<int> Y =3D X.makeIndependentCopy();
=C2=A0 =C2=A0 as= sert(P(X =3D=3D X) =3D=3D 1.0);
=C2=A0 =C2=A0 as= sert(P(X =3D=3D Y) =3D=3D 0.16666666);
=C2=A0
Of course, C= ++ uses =3D=3D as equality, and C++ can't compare three or more element= s at once.
So, the expression P(X<X=C2=B2=3DY>|Z|) would be= written=C2=A0in C++=C2=A0as something=C2=A0like this:
=C2=A0=C2= =A0=C2=A0 compare(And{{X, std::less, bind_variable(std::pow, X, 2)}= , {bind_variable(std::pow, X, 2), std::equal, Y}, {Y, std:= :greater, bind_variable(std::abs, Z)}});
What a mess.

I'd prefer to write the above= as

=C2=A0 =C2=A0 discrete_random_variable<bool= > B =3D (X < X*X && X*X =3D=3D Y && Y > abs(Z));
=C2=A0 =C2=A0 double p =3D P(B);

which is= perfectly doable with operator overloading.

This = kind of question ("how do I write this specific kind of numeric code i= n C++?") would be more suited for StackOverflow or CodeReview.stackexc= hange.com, as opposed to this forum, which is more for proposed new languag= e features and suchlike.

HTH,
=E2=80=93A= rthur

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