220 34496 article Path: news.gmane.org!.POSTED!not-for-mail From: Faisal Vali Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Auto return type deduction surprises Date: Tue, 26 Sep 2017 15:24:29 -0500 Lines: 102 Approved: news@gmane.org Message-ID: References: <7bf0d2563477e89a8faa3ca2daa18f67@kdab.com> <25067783-13f0-4943-9736-f7326f1567d7@isocpp.org> <072abf8e-251c-4fe5-a24f-80c9319e3326@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" X-Trace: blaine.gmane.org 1506457502 13908 195.159.176.226 (26 Sep 2017 20:25:02 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 26 Sep 2017 20:25:02 +0000 (UTC) To: "" Original-X-From: std-proposals+bncBDDKRIXX5YARBHHPVLHAKGQERKQEP5Y@isocpp.org Tue Sep 26 22:24:57 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oi0-f70.google.com ([209.85.218.70]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1dwwPW-0003Ei-KU for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id x11sor4132446oix.305.2017.09.26.13.25.00 for (Google Transport Security); Tue, 26 Sep 2017 13:25:00 -0700 (PDT) Received-SPF: pass (google.com: domain of faisalv@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 10.202.239.138 with SMTP id n132mr11962112oih.301.1506457500001; Tue, 26 Sep 2017 13:25:00 -0700 (PDT) Original-Received: by 10.157.22.167 with HTTP; Tue, 26 Sep 2017 13:24:29 -0700 (PDT) In-Reply-To: <072abf8e-251c-4fe5-a24f-80c9319e3326@isocpp.org> X-Original-Sender: faisalv@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=XIcDielJ; spf=pass (google.com: domain of faisalv@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=faisalv@gmail.com; dmarc=pass (p=NONE sp=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:34496 Archived-At: On Tue, Sep 26, 2017 at 2:04 PM, T. C. wrote: > On Tuesday, September 26, 2017 at 11:21:35 AM UTC-4, Nicol Bolas wrote: >> >> On Tuesday, September 26, 2017 at 4:54:21 AM UTC-4, Richard Smith wrote: >>> >>> 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. >> >> >> Personally, I think it's a good move. Not for self-recursion reasons but >> for sanity and error checking reasons. >> >> With the ?: operator, the two expressions are right next to each other. >> This means that if you can see one expression, you can see the other. As >> such, you can easily see from that one line what the overall expression >> resolves to. It requires more mental thought, but all of the information is >> right there in the same statement. >> >> Return statements can be pages apart, depending on function length. As >> such, it's much more difficult to know what `std::common_type` will resolve >> to; figuring that out requires bouncing between all of the return statements >> in the function. So in order to know what the function's return type is, you >> have to read the entire thing. > > > Also, we won't be able to use common_type's semantics, which is order > dependent for >2 arguments. That would be insane for this case. > > We'd have to generalize ?:'s semantics directly for >2 operands if we want > to go down that path. > Years ago, when i was interested in this, I had come up with something similar - my unsophisticated implementation (if i remember correctly) was O(N^2) in the number of return statements (but i think i felt this could be optimized...). Implementation: https://github.com/faisalv/clang/commits/cxx1z-conditional-return-type-deduction Test examples: https://github.com/faisalv/clang/blob/cxx1z-conditional-return-type-deduction/test/SemaCXX/cxx1z-deduced-return-type.cpp I also had some thoughts on how to merge return statements, returning different lambda expressions/or closure objects... But I think I agree with Richard, unless some very strong forces raise some very loud resonant noises, this ship might have sailed for good... > -- > 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/072abf8e-251c-4fe5-a24f-80c9319e3326%40isocpp.org. -- 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/CABsSThqzT3jY3hqUATysuMqz_hY5kB4Jmg5Z2izUn1VWqVKzvQ%40mail.gmail.com. .