220 30469 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: enum_cast proposal Date: Tue, 10 Jan 2017 11:09:42 -0800 (PST) Lines: 139 Approved: news@gmane.org Message-ID: References: <57258435-5d82-48e7-8ed7-3c38682418fb@isocpp.org> <58750288.5080004@gmail.com> <587512E1.1050708@gmail.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_1667_1998639733.1484075382524" X-Trace: blaine.gmane.org 1484075391 20143 195.159.176.226 (10 Jan 2017 19:09:51 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 10 Jan 2017 19:09:51 +0000 (UTC) Cc: mwoehlke.floss@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBB57C2TBQKGQEFS3JC7Y@isocpp.org Tue Jan 10 20:09:45 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f197.google.com ([209.85.192.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cR1nf-00040K-RQ for gclcip-std-proposals@m.gmane.org; Tue, 10 Jan 2017 20:09:40 +0100 Original-Received: by mail-pf0-f197.google.com with SMTP id z128sf300288254pfb.4 for ; Tue, 10 Jan 2017 11:09:44 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version: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=Lg7ZbAaT/+fBxV/owIG3WErgNriIbmfx3COHdYMQ0yc=; b=UpUjrOss6nrTJJWMC0lIqg3doJ03hzHEAHcGmNwp6dRB+S0kXsHiGJiZPgAyAhwwU4 cVQBNB/Mv6KbhEBmYgoBB5k78NfVVupJdBUt4tPXm95XdGxjcepOkGslzilqvOZtGyFb wYI1cD/KCqS5CtPcuZeRHL6LTWe7MlNCQJFncga3+MOLlidQ+jcKjHRF14OmYUvPrT6w 9gez8kTkgZH59K/Fc61yDQrV0Uuh0Vio7t+sm/56aNxUOD2AGdUznJszPhFimkgSsxd8 sjvvhTxD+NG0ypXY6DKPxGOsPNDOtHxSDk4/OWb+ps7/j3orNLpFjcdInWLwzcKbDQt/ cmzQ== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version: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=Lg7ZbAaT/+fBxV/owIG3WErgNriIbmfx3COHdYMQ0yc=; b=vAbI8zHnwg17qu90SE/q6qDtk9rtCqtqmj+EJd+h68S88JmG8JnU1O8HYHxgLsoJo/ f/0E3bdc8KgFiWBGkaEQf2Km9DR7Ch1Nxb1nwLbCGTcw5r2DsxLB1lmcMiCuE9uKjzz9 lEjlnwapRuf/JPVeBVFQ9uUultWwPqHHF2hOL1kmBtIcDp7/GifAP7DALhmMI3es9Dvu efLtErt5JpFk+Pn/0Q+aHq7JnVhN+oBdXJ+qTyicmwIXERUs3XEhVoeqCzVrUtW09726 EYIaazsSg2R/v5bTUSfQA8AtrCcfwYxd0+OHPnwe7Xz8ayLLZePCuLU1wjFs2GkBwcCL 9Djg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:message-id:in-reply-to :references:subject:mime-version: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=Lg7ZbAaT/+fBxV/owIG3WErgNriIbmfx3COHdYMQ0yc=; b=n/3iSx5mEq8zSriD5mP2NtPeaIfXHAgQkZ6pIWvZXBNcuDX2/rF3IFibI5fTUbyJGZ ujnvDsNwE//fSWtHz+HI3eKl1rYT9pGOp4tD+omtnpSrdzyRj17NFXl69nhamqdBDlch +x9wSjODuR804gFImuTcQs8bJGqJxph/NiMy436xSNC2feq1TVJRxua4BuicBmS/gTAR 2MbJmn0wkN2TOupNtDDkOfQbkqXHjDeKRO5DblTXF7nDYtza/ORSrXXvQVn0+e01zTJD MS6TKcq/QaGrf6s+P5i9L60h0ZEM1zHAwoW9zdQRSHkKV2KIFGr9X8AKX45mesbznfkW oUpw== X-Gm-Message-State: AIkVDXKPpNl4XYD2uPSwNBJCLLOv1R6qO7Rx37MWSVdGJyBMXGxkYy9wprB8mZSc/rbSZw== X-Received: by 10.99.102.68 with SMTP id a65mr1641829pgc.122.1484075383764; Tue, 10 Jan 2017 11:09:43 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.17.154 with SMTP id v26ls2311047otf.0.gmail; Tue, 10 Jan 2017 11:09:43 -0800 (PST) X-Received: by 10.157.18.211 with SMTP id g77mr321738otg.14.1484075383037; Tue, 10 Jan 2017 11:09:43 -0800 (PST) In-Reply-To: <587512E1.1050708@gmail.com> X-Original-Sender: jmckesson@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:30469 Archived-At: ------=_Part_1667_1998639733.1484075382524 Content-Type: multipart/alternative; boundary="----=_Part_1668_1585808523.1484075382525" ------=_Part_1668_1585808523.1484075382525 Content-Type: text/plain; charset=UTF-8 On Tuesday, January 10, 2017 at 11:59:16 AM UTC-5, Matthew Woehlke wrote: > > On 2017-01-10 11:20, Nicol Bolas wrote: > > On Tuesday, January 10, 2017 at 10:49:34 AM UTC-5, Matthew Woehlke > wrote: > >> On 2017-01-10 10:31, m.ce...@gmail.com wrote: > >>> For sure baseline API must be exception-free. > >>> Do we really want to additionally support API that throws on error? > >> > >> I *really* hope we can use something like std::expected; this is a > >> textbook use case for that, and makes it trivial to have a > >> throw-on-error API (just grab the value from the expected without > >> checking it first; this will turn around and throw if the value is not > >> there). > > > > But the fact that it's throwing the wrong exception makes it useless > from > > an exception handling perspective. > > Huh? I'm talking about std::expected, not std::optional. If you didn't > get a value, you get whatever error/exception (preferably an exception > in this case, since it doesn't need more storage than the enum and thus > doesn't cost extra) the API stuffed in instead, which would presumably > be a std::range_error or std::bad_enum_cast or whatever. IOW, the *SAME* > exception that an API that throws right away would throw. > Actually, you can't do that. Well, not in P0323 `expected`. It has a specific requirement that `E` is no-throw move constructible (in order to ensure never-empty without bloat). But `std::range_error` has no such guarantee on its move constructor. Also, it throws `bad_expected_access`, not `E` itself. Even ignoring that, you want the API to return `std::expected`. Well, `std::range_error` derives from `std::runtime_error`. And that internally stores a string. Not necessarily `std::string`, but it stores a sequence of characters it dynamically allocates, along with the means to delete it. And whatever that storage may be, it's almost certainly bigger than `sizeof(Enum)`. Which now means that your return type is needlessly bloated. If you had returned `std::optional`, there would be no bloat. Generally speaking, you should never use the *actual exception type* that you would have otherwise thrown as the non-expected value in an `expected`. `expected` errors are intended to be *lightweight*: error codes and the like. Not things that own memory and so forth. By contrast, exceptions can be quite heavy. -- 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/d974c090-2898-4397-8d42-bbc1682158c7%40isocpp.org. ------=_Part_1668_1585808523.1484075382525 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Tuesday, January 10, 2017 at 11:59:16 AM UTC-5, Matthew= Woehlke wrote:
On 2017-01-10 1= 1:20, Nicol Bolas wrote:
> On Tuesday, January 10, 2017 at 10:49:34 AM UTC-5, Matthew Woehlke= wrote:
>> On 2017-01-10 10:31, m.ce...@gmail.com <javascript:&= gt; wrote:=20
>>> For sure baseline API must be exception-free.=20
>>> Do we really want to additionally support API that throws = on error?=20
>>
>> I *really* hope we can use something like std::expected; this = is a=20
>> textbook use case for that, and makes it trivial to have a=20
>> throw-on-error API (just grab the value from the expected with= out=20
>> checking it first; this will turn around and throw if the valu= e is not=20
>> there).
>=20
> But the fact that it's throwing the wrong exception makes it u= seless from=20
> an exception handling perspective.

Huh? I'm talking about std::expected, not std::optional. If you did= n't
get a value, you get whatever error/exception (preferably an exception
in this case, since it doesn't need more storage than the enum and = thus
doesn't cost extra) the API stuffed in instead, which would presuma= bly
be a std::range_error or std::bad_enum_cast or whatever. IOW, the *SAME= *
exception that an API that throws right away would throw.

Actually, you can't do that. Well, not in P0323 `expected`.= It has a specific requirement that `E` is no-throw move constructible (in = order to ensure never-empty without bloat). But `std::range_error` has no s= uch guarantee on its move constructor. Also, it throws `bad_expected_access= <E>`, not `E` itself.

Even ignoring that, you want the API to = return `std::expected<Enum, std::range_error>`. Well, `std::range_err= or` derives from `std::runtime_error`. And that internally stores a string.= Not necessarily `std::string`, but it stores a sequence of characters it d= ynamically allocates, along with the means to delete it. And whatever that = storage may be, it's almost certainly bigger than `sizeof(Enum)`.
Which now means that your return type is needlessly bloated. If you had r= eturned `std::optional<Enum>`, there would be no bloat.

Genera= lly speaking, you should never use the actual exception type that yo= u would have otherwise thrown as the non-expected value in an `expected`. `= expected` errors are intended to be lightweight: error codes and the= like. Not things that own memory and so forth. By contrast, exceptions can= be quite heavy.

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/d974c090-2898-4397-8d42-bbc1682158c7= %40isocpp.org.
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