220 33798 article Path: news.gmane.org!.POSTED!not-for-mail From: Ville Voutilainen Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: properties? 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[2607:f8b0:400c:c05::242]) by mx.google.com with ESMTPS id e30si3715328uag.42.2017.08.10.09.35.57 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 10 Aug 2017 09:35:57 -0700 (PDT) Received-SPF: pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400c:c05::242 as permitted sender) client-ip=2607:f8b0:400c:c05::242; Original-Received: by mail-vk0-x242.google.com with SMTP id z187so547838vkd.3 for ; Thu, 10 Aug 2017 09:35:57 -0700 (PDT) X-Received: by 10.31.206.133 with SMTP id e127mr7831807vkg.145.1502382957501; Thu, 10 Aug 2017 09:35:57 -0700 (PDT) Original-Received: by 10.103.103.65 with HTTP; Thu, 10 Aug 2017 09:35:57 -0700 (PDT) In-Reply-To: <5d2c2db8-9774-4099-82d6-c97b909363ad@isocpp.org> X-Original-Sender: ville.voutilainen@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=rBGeu30l; spf=pass (google.com: domain of ville.voutilainen@gmail.com designates 2607:f8b0:400c:c05::242 as permitted sender) smtp.mailfrom=ville.voutilainen@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:33798 Archived-At: On 10 August 2017 at 19:21, Nicol Bolas wrote: > On Thursday, August 10, 2017 at 10:40:58 AM UTC-4, Ville Voutilainen wrote: >> >> On 10 August 2017 at 17:08, Matthew Woehlke wrote: >> > Ah. Of course, mixins are sufficiently wide-spread that figuring those >> > out is almost certainly worthwhile in its own right. (I've always >> > thought so, anyway.) >> > >> > Maybe someone would be interested in taking up the torch on those? >> >> >> I daresay we should see how much we can get via reflection, and then >> figure out whether we want >> to add specific facilities. > > > This is not a problem reflection can solve. Or at least, not the problem I > was encountering with CRTP-based mixins. That problem being the fact that > the derived class (the class being mixed into) is incomplete at the point > where the mixin is being instantiated. > > That's the catch-22: you need a way to augment a type with new stuff, but > you also need a way to access the stuff that was previously added to the > type. And as I understand, you cannot invoke reflection on an incomplete > type. You can run metaprograms on a type that is so-called almost-complete, and the metaprogram completes it. See http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0707r1.pdf. > And even if you could reflect over a partially complete type, you wouldn't > be able to access members that haven't been declared yet. And since Except that you can, if you postpone instantiation of such code to a time when the members are available. You generate a template, and then instantiate the template with the member. The member does need to be a parameter, but that doesn't change the mixed-in code. > A proper mixin facility needs the following three properties: > > 1: The ability for a type to add the mixin to itself, such that the mixin's > declarations are able to have access to any previously-declared members in > the type. Right, none of the metaprogramming proposals I've seen thus far support exactly this. The mixin must accept the type it's going to be mixed in as a parameter of some kind. We don't have 'inline templates' where we can just explode code into a context, and have the exploded code expect certain declarations to be valid. > 2: The ability for a type that gets mixed-into to access the mixin's > declarations as if they were its own. This is another aspect which is not that direct in the proposals I've seen. > 3: The ability to convert a pointer/reference to a type into a > pointer/reference to the mixin it uses, providing access only to the mixin's > interface. This is easy to do, just generate a conversion function or an operator. With the proposed metaprogramming facilities, we can fairly easily express "make type X behave like type Y, without requiring an inheritance relationship". We can generate delegation code. What we don't have is "make the code in type Y behave differently depending on its surrounding context, without having any readiness for that in type Y". -- 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/CAFk2RUakgF_ZAt81uhcwn4KzLcbjC04LpqyREHWoKNFe6njDfQ%40mail.gmail.com. .