220 18237 <62FE5462-BE3F-4C9E-B03D-322CB0CFF753@gmail.com> article Path: news.gmane.org!not-for-mail From: Miro Knejp Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: Core Language feature: Multiple assignments from multiple return values via tuple Date: Wed, 27 May 2015 10:51:50 +0200 Lines: 218 Approved: news@gmane.org Message-ID: <62FE5462-BE3F-4C9E-B03D-322CB0CFF753@gmail.com> References: <69a59613-6504-466a-9878-786653d6087f@isocpp.org> <4498da2a-af3a-457d-9bf7-e8bb3721d6b9@isocpp.org> <6ea4a1dc-bfb4-4219-93ea-606619a77c53@isocpp.org> <71a799fc-1897-4c43-adc6-5d26267688bb@isocpp.org> <500d24fe-73ba-439a-ac53-aeaa7919111e@isocpp.org> <32801228-1faf-4b73-8da1-9c8ce5bcb9ae@isocpp.org> <0b91248a-1994-4127-932e-9e1d97edadb2@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2098\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_3DF0C2DE-72E6-476C-99F6-D723A7EC6453" X-Trace: ger.gmane.org 1432716731 6512 80.91.229.3 (27 May 2015 08:52:11 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 27 May 2015 08:52:11 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC6ONSXJ54LBBLMLS2VQKGQEBJSSQOY@isocpp.org Wed May 27 10:52:04 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-la0-f71.google.com ([209.85.215.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YxX4C-00006Y-6D for gclcip-std-proposals@m.gmane.org; Wed, 27 May 2015 10:52:00 +0200 Original-Received: by lagv1 with SMTP id v1sf1013789lag.1 for ; Wed, 27 May 2015 01:51:58 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:content-type:message-id:mime-version :subject:date:references:to:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=UQUmUPmJof0GDvMuwZ2Ed0zrK77ZqL2On57RU9LRC+g=; b=MwQmEoVPRzolJFuk4OpVYn0cImbsnjfrOa/HChgAB+huZ7GTfyF98QjSKCJAKpug07 507Kq8oCEYUlmG8bomexPgP8+cFMxGiJ/KHaDYVGyG9xAvI2FZIP1Mh5t2fXuOjLQQ9H LCSQjl5rXMtPWR/X23q0oziXabFn/SbI9ZoCNeP75mRKG7yTxpOGVqGONlZYs+TUuxYR DpPj7hWsaZecHKx/vsVpbYDddaZGyLRY/VeOJKYj44E1cmKluEz6BAi+0fpWjeqtL5mP tolo1FcXVseaLzVjMpmY+0mnYsM3+GW4b7pl5+u22DUgTTZ+xjnThvrq+N4f+fk5WhcN c86w== X-Gm-Message-State: ALoCoQliEM+LD2FHrGbpWH8BktdKYOREmDVxot1gdo95XvPGiL3eRf3q1EOpgXF41C5aiXsD5Cpk X-Received: by 10.112.151.146 with SMTP id uq18mr31365587lbb.20.1432716718214; Wed, 27 May 2015 01:51:58 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.180.76.132 with SMTP id k4ls823396wiw.25.canary; Wed, 27 May 2015 01:51:56 -0700 (PDT) X-Received: by 10.181.6.37 with SMTP id cr5mr48515605wid.18.1432716716712; Wed, 27 May 2015 01:51:56 -0700 (PDT) Original-Received: from mail-wg0-x22e.google.com (mail-wg0-x22e.google.com. [2a00:1450:400c:c00::22e]) by mx.google.com with ESMTPS id s5si28403315wjz.147.2015.05.27.01.51.56 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 27 May 2015 01:51:56 -0700 (PDT) Received-SPF: pass (google.com: domain of miro.knejp@gmail.com designates 2a00:1450:400c:c00::22e as permitted sender) client-ip=2a00:1450:400c:c00::22e; Original-Received: by wgme6 with SMTP id e6so3229688wgm.2 for ; Wed, 27 May 2015 01:51:56 -0700 (PDT) X-Received: by 10.180.96.196 with SMTP id du4mr19940642wib.77.1432716712956; Wed, 27 May 2015 01:51:52 -0700 (PDT) Original-Received: from [172.16.1.100] (vpn.qmob.de. [80.147.156.178]) by mx.google.com with ESMTPSA id w11sm25205521wjr.48.2015.05.27.01.51.51 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Wed, 27 May 2015 01:51:52 -0700 (PDT) In-Reply-To: X-Mailer: Apple Mail (2.2098) X-Original-Sender: miro.knejp@gmail.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of miro.knejp@gmail.com designates 2a00:1450:400c:c00::22e as permitted sender) smtp.mail=miro.knejp@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-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:18237 Archived-At: --Apple-Mail=_3DF0C2DE-72E6-476C-99F6-D723A7EC6453 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 > On 27 May 2015, at 09:43 , Nicol Bolas wrote: >=20 >=20 > [int,long,double] foo(); > auto x =3D foo(); >=20 > Ok, if tuple is out then what is decltype(x) in the above example doing t= hings your way? >=20 > There are two defensible answers to this: >=20 > 1) A compilation error. You called a function that returns 3 things, but = you only captured one. That suggests you weren't paying attention. >=20 > 2) `int`. The other two types are dropped on the floor. >=20 > #2 is probably the way to go. Any sort of aggregate object type is not ev= en up for discussion. >=20 > Multiple return values means multiple return values, not "pack multiple v= alues into some kind of object and unpack them later". > =20 > Another kind of typelist which is standard layout compatible? Or are we n= o longer able to capture all of the return values into one object? >=20 > If you want to capture them all in an object, you just ask for it: >=20 > auto x =3D make_tuple(foo()); >=20 > That is neither hard nor particularly verbose. You can even stick them in= an array, if all of the returned values are of the same type: >=20 > auto x =3D make_array(foo()); >=20 > Or in an arbitrary data structure: >=20 > auto x =3D std::vector{foo()}; >=20 > But most people using multiple return values don't capture them in a big = object, so we don't make the syntax optimal for that case. >=20 I assume Matthew=E2=80=99s point comes from dealing with template code. template ??? contrived_example(F f) { return f(); } The author would probably expect that ??? can be plain =E2=80=9Cauto=E2=80= =9D, implying =E2=80=9Cin some places auto can expand to multiple types=E2= =80=9D. Then there=E2=80=99s the point of what generic code would require t= o do to consistently capture the return value(s) of a Callable without know= ing if there are one or multiple return values. template void contrived_example2(F f) { ??? x =3D f(); ... } Should ??? be something that can potentially capture a single or multiple v= alues depending on the initialization? If so what can be done with x down t= he pipeline? Or is a concept necessary to overload contrived_example2 for f= unctions with single or multiple return values? Is template code forced int= o packing every Callable=E2=80=99s invocation into make_tuple() just in cas= e it might return multiple values? --=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=_3DF0C2DE-72E6-476C-99F6-D723A7EC6453 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
On 27 May 2015, at 0= 9:43 , Nicol Bolas <jm= ckesson@gmail.com> wrote:


[int,long,doub= le] foo();
auto x =3D foo();

Ok, if= tuple is out then what is decltype(x) in the above example doing things yo= ur way?

There are tw= o defensible answers to this:

1) A compilation= error. You called a function that returns 3 things, but you only captured = one. That suggests you weren't paying attention.

2) `int`. The other two types are dropped on the floor.
#2 is probably the way to go. Any sort of aggregate object typ= e is not even up for discussion.

Multiple retu= rn values means multiple return values, not "pack multiple values into some kind = of object and unpack them later".
 
Another kind of t= ypelist which is standard layout compatible? Or are we no longer able to ca= pture all of the return values into one object?

If you want to capture them all in an object, y= ou just ask for it:

auto x <= span class=3D"styled-by-prettify" style=3D"color: rgb(102, 102, 0);">=3D make_tuple(foo());

That is neither hard nor particularly verbose= .. You can even stick them in an array, if all of the returned values are of= the same type:

auto&n= bsp;x =3D=  make_array(foo());

Or in an arbitrary data structure:
=
auto x =3D std::vector<Type>{foo()};

But most people using multiple return values= don't capture them in a big object, so we don't make the syntax optimal fo= r that case.


I assume Ma= tthew=E2=80=99s point comes from dealing with template code.

template<class F>
??? contrived_example(F f) {
    ret= urn f();
}

<= div class=3D"">The author would probably expect that ??? can be plain =E2= =80=9Cauto=E2=80=9D, implying =E2=80=9Cin some places auto can expand to mu= ltiple types=E2=80=9D. Then there=E2=80=99s the point of what generic code = would require to do to consistently capture the return value(s) of a Callab= le without knowing if there are one or multiple return values.

template<class F>
=
void contrived_example2(F f) {
  =   ??? x =3D f();
    ...
}

Should ?= ?? be something that can potentially capture a single or multiple values de= pending on the initialization? If so what can be done with x down the pipel= ine? Or is a concept necessary to overload contrived_example2 for functions= with single or multiple return values? Is template code forced into packin= g every Callable=E2=80=99s invocation into make_tuple() just in case it mig= ht return multiple values?

--

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