220 39296 article Path: news.gmane.org!.POSTED!not-for-mail From: Richard Hodges Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Standardising a type-erased iterator Date: Mon, 23 Jul 2018 10:14:44 +0100 Lines: 158 Approved: news@gmane.org Message-ID: References: <09db196a-e818-4d7a-9af1-48f07b2f4c3d@isocpp.org> <24d8f121-aba9-4ead-b107-bc4b72014870@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="00000000000032dd380571a714ce" X-Trace: blaine.gmane.org 1532337171 26170 195.159.176.226 (23 Jul 2018 09:12:51 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Mon, 23 Jul 2018 09:12:51 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBD4PBM7UWAHRBEFZ23NAKGQEF2QACAA@isocpp.org Mon Jul 23 11:12:47 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-it0-f71.google.com ([209.85.214.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fhWta-0006gY-Oh for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id e23-v6sor3429411jaj.121.2018.07.23.02.14.56 for (Google Transport Security); Mon, 23 Jul 2018 02:14:56 -0700 (PDT) Received-SPF: pass (google.com: domain of hodges.r@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a02:579a:: with SMTP id b26-v6mr10682292jad.107.1532337295906; Mon, 23 Jul 2018 02:14:55 -0700 (PDT) In-Reply-To: X-Original-Sender: hodges.r@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=NFenCtZs; spf=pass (google.com: domain of hodges.r@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=hodges.r@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:39296 Archived-At: --00000000000032dd380571a714ce Content-Type: text/plain; charset="UTF-8" On Mon, 23 Jul 2018 at 09:01, Thomas Russell wrote: > The motivation is similar to having any standard type-erased facility; it > gives a concrete interface that accepts an arbitrary set of classes that > conform to the required concepts. > > The closest existing parallel in the standard library is `std::function`, > modelling the Callable concept. > > Having such a facility as a vocabulary type provides various benefits over > function templates accepting Callables: > - Reduced generated code-size (this can even go some way towards reducing > the performance penalty). > - Works with the OOP paradigm (virtual functions cannot be function > templates) > - Reduced compile-time > - Can work across dynamically-linked library boundaries, providing the > correctly corresponding build parameters are met. > > I think these are good motivations, but am interested to hear opposing > opinions. > I think it would be fair to say that they are theoretical or hypothesised motivations, not actual concrete use cases where there is a demonstrable benefit in providing a standardised type-erased iterator suite. I'm thinking in terms of "here's some current software that sucks because we don't have type-erased iterators, and presented here is a better version using my implementation of such iterators". I say this not to be argumentative. I say it because like you and many others, I have played with similar ideas because they were fun or interesting ideas. Including for example a type-erased iterator for iterating across a tuple (operator* returns a boost::variant of references). However, in 30 years on and off writing commercial software, I have never encountered an actual use case while using c++. This is probably because I reach for c++ when seeking concrete guarantees on type safety, and python, perl or javascript when I'm seeking type flexibility and expressiveness. But it could be that I am thinking myopically. I have experimented in the past with providing more "script-like" library objects to the odd project. Again, out of curiosity more than need. You see what I'm saying? Is this in reality a fun personal project, or is there a tangible benefit to me if I find this in the std toolbox? > > -- > 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/c9664ef6-f769-4942-8e44-56761d091efc%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/CALvx3haDm01D7jCvJJA3JJycraXVZ_WHz9A4md5h1WXhCR29cg%40mail.gmail.com. --00000000000032dd380571a714ce Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable


On Mon= , 23 Jul 2018 at 09:01, Thomas Russell <thomas.russell97@gmail.com> wrote:
The motivation is similar to having any standard type-= erased facility; it gives a concrete interface that accepts an arbitrary se= t of classes that conform to the required concepts.

The closest existing parallel in the standard library is `std::function`, m= odelling the Callable concept.

Having such a facility as a vocabulary type provides various benefits over = function templates accepting Callables:
- Reduced generated code-size (this can even go some way towards reducing t= he performance penalty).
- Works with the OOP paradigm (virtual functions cannot be function templat= es)
- Reduced compile-time
- Can work across dynamically-linked library boundaries, providing the corr= ectly corresponding build parameters are met.

I think these are good motivations, but am interested to hear opposing opin= ions.

I think it would be fair to say = that they are theoretical or hypothesised motivations, not actual concrete = use cases where there is a demonstrable benefit in providing a standardised= type-erased iterator suite.

I'm thinking in t= erms of "here's some current software that sucks because we don= 9;t have type-erased iterators, and presented here is a better version usin= g my implementation of such iterators".

I say= this not to be argumentative.

I say it because li= ke you and many others, I have played with similar ideas because they were = fun or interesting ideas. Including for example a type-erased iterator for = iterating across a tuple (operator* returns a boost::variant of references)= ..

However, in 30 years on and off writing commerci= al software, I have never encountered an actual use case while using c++.

This is probably because I reach for c++ when seeki= ng concrete guarantees on type safety, and python, perl or javascript when = I'm seeking type flexibility and expressiveness.

But it could be that I am thinking myopically. I have experimented in th= e past with providing more "script-like" library objects to the o= dd project. Again, out of curiosity more than need.=C2=A0

You see what I'm saying? Is this in reality a fun personal proj= ect, or is there a tangible benefit to me if I find this in the std toolbox= ?
=C2=A0

--
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org.
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