220 34491 article Path: news.gmane.org!.POSTED!not-for-mail From: Richard Smith Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Auto return type deduction surprises Date: Tue, 26 Sep 2017 01:53:57 -0700 Lines: 180 Approved: news@gmane.org Message-ID: References: <7bf0d2563477e89a8faa3ca2daa18f67@kdab.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="001a113de226017dca055a13d248" X-Trace: blaine.gmane.org 1506416061 27278 195.159.176.226 (26 Sep 2017 08:54:21 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 26 Sep 2017 08:54:21 +0000 (UTC) To: "std-proposals@isocpp.org" Original-X-From: std-proposals+bncBDVNBJG4YAIBBO5LVDHAKGQEXFEHKQQ@isocpp.org Tue Sep 26 10:54:16 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f200.google.com ([209.85.128.200]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dwld7-0006Ws-3l for gclcip-std-proposals@m.gmane.org; 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[81.27.85.19]) by mx.google.com with ESMTPS id p3si6394720wrd.547.2017.09.26.01.54.18 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 26 Sep 2017 01:54:18 -0700 (PDT) Received-SPF: neutral (google.com: 81.27.85.19 is neither permitted nor denied by best guess record for domain of richard@metafoo.co.uk) client-ip=81.27.85.19; Original-Received: from mail-oi0-f51.google.com ([209.85.218.51]:56135) by uk12.easy-internet.co.uk with esmtpsa (TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128) (Exim 4.89) (envelope-from ) id 1dwldB-0000jq-M9 for std-proposals@isocpp.org; Tue, 26 Sep 2017 09:54:17 +0100 Original-Received: by mail-oi0-f51.google.com with SMTP id x85so10881096oix.12 for ; Tue, 26 Sep 2017 01:54:18 -0700 (PDT) X-Received: by 10.202.252.215 with SMTP id a206mr12646549oii.85.1506416057743; Tue, 26 Sep 2017 01:54:17 -0700 (PDT) Original-Received: by 10.74.91.133 with HTTP; Tue, 26 Sep 2017 01:53:57 -0700 (PDT) In-Reply-To: <7bf0d2563477e89a8faa3ca2daa18f67@kdab.com> X-Gmail-Original-Message-ID: X-AntiAbuse: This header was added to track abuse, please include it with any abuse report X-AntiAbuse: Primary Hostname - uk12.easy-internet.co.uk X-AntiAbuse: Original Domain - isocpp.org X-AntiAbuse: Originator/Caller UID/GID - [47 12] / [47 12] X-AntiAbuse: Sender Address Domain - metafoo.co.uk X-Get-Message-Sender-Via: uk12.easy-internet.co.uk: authenticated_id: metafooc/from_h X-Authenticated-Sender: uk12.easy-internet.co.uk: richard@metafoo.co.uk X-Source: X-Source-Args: X-Source-Dir: X-Original-Sender: richard@metafoo.co.uk X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@metafoo.co.uk header.s=default header.b=YPaUH7CV; spf=neutral (google.com: 81.27.85.19 is neither permitted nor denied by best guess record for domain of richard@metafoo.co.uk) smtp.mailfrom=richard@metafoo.co.uk 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:34491 Archived-At: --001a113de226017dca055a13d248 Content-Type: text/plain; charset="UTF-8" On 26 September 2017 at 00:53, Marc Mutz wrote: > Hi, > > Why do these semantically identical pieces of code have different syntax > rules? > > template > auto find(Map &map, const Lookup &key) > { > const auto it = map.find(key); > if (it == map.end()) > return std::nullopt; > else > return std::optional{*it}; // error: inconsistent deduction for > auto return type > } > > template > auto find(Map &map, const Lookup &key) > { > const auto it = map.find(key); > return it == map.end() ? std::nullopt : std::optional{*it}; // OK, > -> std::optional > } > > iow: Why do auto return types have to have identical types, and don't just > use the rules for the ternary operator? > Because some people thought it more important to allow self-recursive calls after the first return statement: auto f(int n) { if (n <= 1) return 1; return n * f(n - 1); // ok to call f here, return type already deduced } FWIW, I think we made the wrong tradeoff here, but this decision is unlikely to be reconsidered now. (We could use the ternary operator rules up until the first recursive use of the function within itself, but that risks major confusion and surprises when a recursive function is refactored and its return statements are reordered...) > And why, in the first case, can I not say > > auto find(Map &map, const Lookup &key) -> std::optional > { > > iow: why doesn't ctor template argument deduction work for trailing return > types? > An overabundance of conservatism. I expect this restriction will be relaxed as soon as someone argues for it in committee. Is/was this a conscious decision against, or would a proposal > adding/changing these be welcome? > > Thanks, > Marc > > -- > 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/is > ocpp.org/d/msgid/std-proposals/7bf0d2563477e89a8faa3ca2daa18f67%40kdab.com > . > -- 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/CAOfiQqnmBJ%2BH2a1R6pCeKkJqBYmOX3qK0ZFyNRMGngStSSChVA%40mail.gmail.com. --001a113de226017dca055a13d248 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On 2= 6 September 2017 at 00:53, Marc Mutz <marc.mutz@kdab.com> w= rote:
Hi,

Why do these semantically identical pieces of code have different syntax ru= les?

=C2=A0 template <typename Map, typename Lookup>
=C2=A0 auto find(Map &map, const Lookup &key)
=C2=A0 {
=C2=A0 =C2=A0 =C2=A0 const auto it =3D map.find(key);
=C2=A0 =C2=A0 =C2=A0 if (it =3D=3D map.end())
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return std::nullopt;
=C2=A0 =C2=A0 =C2=A0 else
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 return std::optional{*it}; // error: inc= onsistent deduction for auto return type
=C2=A0 }

=C2=A0 template <typename Map, typename Lookup>
=C2=A0 auto find(Map &map, const Lookup &key)
=C2=A0 {
=C2=A0 =C2=A0 =C2=A0 const auto it =3D map.find(key);
=C2=A0 =C2=A0 =C2=A0 return it =3D=3D map.end() ? std::nullopt : std::optio= nal{*it}; // OK, -> std::optional<Map::value_type>
=C2=A0 }

iow: Why do auto return types have to have identical types, and don't j= ust use the rules for the ternary operator?

=
Because some people thought it more important to allow self-recursive = calls after the first return statement:

auto f(int= n) {
=C2=A0 if (n <=3D 1) return 1;
=C2=A0 return n= * f(n - 1); // ok to call f here, return type already deduced
}<= /div>

FWIW, I think we made the wrong tradeoff here, but= this decision is unlikely to be reconsidered now.

(We could use the ternary operator rules up until the first recursive use = of the function within itself, but that risks major confusion and surprises= when a recursive function is refactored and its return statements are reor= dered...)
=C2=A0
And why, in the first case, can I not say

=C2=A0 =C2=A0auto find(Map &map, const Lookup &key) -> std::opti= onal
=C2=A0 =C2=A0{

iow: why doesn't ctor template argument deduction work for trailing ret= urn types?

An overabundance of conserva= tism. I expect this restriction will be relaxed as soon as someone argues f= or it in committee.

Is/was this a conscious decision against, or would a proposal adding/changi= ng these be welcome?

Thanks,
Marc

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