220 4079 article Path: news.gmane.org!not-for-mail From: Christopher Jefferson Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Multiple return values Date: Mon, 29 Apr 2013 16:33:22 +0100 Lines: 174 Approved: news@gmane.org Message-ID: References: <269d4602-e158-4abc-a9a8-8c3cb453e013@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary=047d7b6d89a2deee8b04db819a6e X-Trace: ger.gmane.org 1367249604 30242 80.91.229.3 (29 Apr 2013 15:33:24 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Mon, 29 Apr 2013 15:33:24 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCF5NPH6QMFRBRNF7KFQKGQE3IAAEWY@isocpp.org Mon Apr 29 17:33:28 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-vc0-f198.google.com ([209.85.220.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1UWq51-0006fT-JG for gclcip-std-proposals@m.gmane.org; Mon, 29 Apr 2013 17:33:27 +0200 Original-Received: by mail-vc0-f198.google.com with SMTP id ha12sf6620426vcb.9 for ; Mon, 29 Apr 2013 08:33:26 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20120113; h=x-received:x-beenthere:x-received:received-spf:mime-version :x-received:in-reply-to:references:date:message-id:subject:from:to :x-gm-message-state:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-google-group-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=MwvZnVisfV3bxCjan3zbs4aUmIP/75iT1qWM5WimYeM=; b=hdvYeqPb5cNDpGNroJcDi1fMF5D8xxcmQSSjkS8MVGB6i2/3KqEgGRFp5/LHOgDlkq z1R9YqziuUjVal+i0tEGqqh2svfqiFw5zQdjYnJKRjn21fo7V0Iw11jE00ote7VOjfa/ omLDD2UzBzsgvzCKBPYKgy+64pQhIMIveWOpz7VLHQC/h9UScx+1ObLH/LE7lz7kNSYX MkFnj7xiTWariH1EYjYkH1L8sZv2GHdvTSKv4w9DZ4g8ooKeGm1tj4Ol17UndcpREd1q tdUNDxFW61FyAC9KAGxWkpIzaBJi/n3 X-Received: by 10.236.161.234 with SMTP id w70mr29482987yhk.22.1367249606644; Mon, 29 Apr 2013 08:33:26 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.87.103 with SMTP id w7ls2653996qez.83.gmail; Mon, 29 Apr 2013 08:33:22 -0700 (PDT) X-Received: by 10.224.105.146 with SMTP id t18mr52442513qao.65.1367249602783; Mon, 29 Apr 2013 08:33:22 -0700 (PDT) Original-Received: from mail-qe0-f41.google.com (mail-qe0-f41.google.com [209.85.128.41]) by mx.google.com with ESMTPS id ht6si5587181qab.2.2013.04.29.08.33.22 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 29 Apr 2013 08:33:22 -0700 (PDT) Received-SPF: neutral (google.com: 209.85.128.41 is neither permitted nor denied by best guess record for domain of chris@bubblescope.net) client-ip=209.85.128.41; Original-Received: by mail-qe0-f41.google.com with SMTP id b10so218583qen.28 for ; Mon, 29 Apr 2013 08:33:22 -0700 (PDT) X-Received: by 10.49.74.226 with SMTP id x2mr61559348qev.63.1367249602403; Mon, 29 Apr 2013 08:33:22 -0700 (PDT) Original-Received: by 10.49.120.194 with HTTP; Mon, 29 Apr 2013 08:33:22 -0700 (PDT) In-Reply-To: <269d4602-e158-4abc-a9a8-8c3cb453e013@isocpp.org> X-Gm-Message-State: ALoCoQnyfPjTAsGIyuaE+Ij2HTgXyGWYmMeuzLpuGBttPVcTdpBm/uVy9t3k9j+kQ5LJofn7gwcr X-Original-Sender: chris@bubblescope.net X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 209.85.128.41 is neither permitted nor denied by best guess record for domain of chris@bubblescope.net) smtp.mail=chris@bubblescope.net 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:4079 Archived-At: --047d7b6d89a2deee8b04db819a6e Content-Type: text/plain; charset=ISO-8859-1 On 29 April 2013 15:03, Hariharan Subramanian wrote: > Problem: > Having a single return value means we have to pass the other return values > as parameters as a reference or a pointer. This makes the programs > difficult to understand. Moreover we can solve most of the problems > mentioned in > http://www.openstd.org/JTC1/SC22/WG21/docs/papers/2013/n3538.html > . > I decided to have a go at seeing how much of this could be done with std::tuple. > > Solution: > (int; float) foo (int); > std::tuple foo(int); > Probably a contextual keyword returns can be added to specify that the > first pair of parentheses represents return values. So we will be passing > only in parameters and in out parameters to the function. All the out > parameters are only returned. > > With this we can write the code > > int a; > float b; > (a; b) = foo(a, b); > std::tie(a,b) = foo(a,b); > (int c; float d) = foo(a, b); > This one we have to write: int c; float d; std::tie(c, d) = foo(a,b); > > We can pass the return values to functions. We can write foo( foo(a, b) ); > Now you are automatically unpacking tuples. What happens if foo takes a tuple? This case doesn't work, but you have to be careful. > > We can compare the return values. The semantics can be defined as the > operation should be true for all the return values. > foo(a, b) == foo(c, d), foo(a, b) >= foo(c, d) etc., > Works with std::tuple already > > We cab do the following also > (c, d) == foo(a, b), (c, d) > foo(a, b) etc., > std::tie(c,d) == foo(a,b), std::tie(c,d) > foo(a,b); > > (a, b) = (b, a) swaps the values. It should generate the code equivalent to > This one, I wouldn't be confident doing with tie (but it might work). However, we have swap :) > There are some problem areas like type casting. Probably they can be > prohibited. Other issues may remain and need to be discussed. > The main problem (as I outline in my mail) is that many of these things can be done with std::tuple and std::tie. If this is just syntatic sugar over std::tuple and std::tie, then is it worth it, and if it is a seperate kind of tuple, we have to worry about the two interacting. -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/?hl=en. --047d7b6d89a2deee8b04db819a6e Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable



On 29 April 2013 15:03, Hariharan Subramanian <= ;tohari@gmail.com= > wrote:
Problem:
Having a single return value me= ans we have to pass the other return values as parameters as a reference or= a pointer. This makes the programs difficult to understand. Moreover we ca= n solve most of the problems mentioned in=A0http://ww= w.openstd.org/JTC1/SC22/WG21/docs/papers/2013/n3538.html.=A0

I decided to have a go at seeing how= much of this could be done with std::tuple.=A0

Solution:
(int; float) foo (int);
std::tuple<int, float> foo(int);=A0
Probably a contextual keyword returns can be added to specify that the= first pair of parentheses represents return values. So we will be passing = only in parameters and in out parameters to the function. All the out param= eters are only returned.

With this we can write the code

int a;
float b;
(a; b) =3D foo(a, b);
std::tie(a,b) =3D foo(a,b);=A0
(int c; float d) =3D foo(a, b);
This one = we have to write:
int c; float d;
std::tie(= c, d) =3D foo(a,b);=A0

We can pass the return values to functions. We can writ= e foo( foo(a, b) );
Now you are automatica= lly unpacking tuples. What happens if foo takes a tuple? This case doesn= 9;t work, but you have to be careful.=A0

We can compare the retur= n values. The semantics can be defined as the operation should be true for = all the return values.
foo(a, =A0b) =3D=3D foo(c, d), foo(a, b) >=3D foo(c, d) etc.,
Works with std::tuple already

We cab do the following also
(c, d= ) =3D=3D foo(a, b), (c, d) > foo(a, b) etc.,
std::tie(c,d) =3D=3D foo(a,b), std::tie(c,d) > foo(a,b);=A0

(a, b) =3D (b, a) swaps the values. It should generate = the code equivalent to
This one, I wouldn'= t be confident doing with tie (but it might work). However, we have swap :)= =A0

=A0
There = are some problem areas like type casting. Probably they can be prohibited. = Other issues may remain and need to be discussed.

The main problem (as I outline= in my mail) is that many of these things can be done with std::tuple and s= td::tie. If this is just syntatic sugar over std::tuple and std::tie, then = is it worth it, and if it is a seperate kind of tuple, we have to worry abo= ut the two interacting.

--
 
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