220 29482 article Path: news.gmane.org!.POSTED!not-for-mail From: Tom Honermann Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allowing string literals to match `char...` packs in templates Date: Fri, 18 Nov 2016 16:42:12 -0500 Lines: 137 Approved: news@gmane.org Message-ID: References: <9de07b5f-f870-4a02-882f-54f9fdd12ffd@isocpp.org> <35fe3945-71cf-4958-8f92-59df0ef98fae@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="------------8E17D3790B21A96AB15C4D24" X-Trace: blaine.gmane.org 1479505338 12946 195.159.176.226 (18 Nov 2016 21:42:18 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 18 Nov 2016 21:42:18 +0000 (UTC) User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDBNXSHG6UDBBOHLXXAQKGQECQ327TQ@isocpp.org Fri Nov 18 22:42:14 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-oi0-f71.google.com ([209.85.218.71]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1c7qvF-0002xO-LS for gclcip-std-proposals@m.gmane.org; Fri, 18 Nov 2016 22:42:13 +0100 Original-Received: by mail-oi0-f71.google.com with SMTP id j198sf254331284oih.5 for ; Fri, 18 Nov 2016 13:42:17 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=subject:to:references:from:message-id:date:user-agent:mime-version :in-reply-to:x-original-sender:x-original-authentication-results :reply-to:precedence:mailing-list:list-id:x-spam-checked-in-group :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=5trYI+OcEvVaZSqg3sWd91vwPbKhVQTcbtU1zXDHmZo=; b=gdY1Jl/pF6MOIjcdpSDSFe3PrGDQ6e7Vy2ybhJko/fHr01KS6R96zTX2LZzJ3JpJpZ lWX2Xx3S0q3NwE6zEdU5K+56FZLsC/5D49OP01r7mTVOrhyCQri807SDy0y41GQ26Jnm tPL+QFyBuDEgTdovoWvT2ek/Cf7gAjkh6bT1v7m/Nx0yzgOANUr+/W/gH+StMJOxde8t kbjOH1NZObDI73/B2Bjmc0YJszr+wMZgqKT8GLF6jE6EC05WsGVbDiU2GXsPkZCFC1DB U4EVfW4z9/pfamuSp83ba2SZ73QF9utMWcLbBMXjagUlzsES5DiHXBj/MVyD4U7rfSac fhqA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:subject:to:references:from:message-id:date :user-agent:mime-version:in-reply-to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=5trYI+OcEvVaZSqg3sWd91vwPbKhVQTcbtU1zXDHmZo=; b=TiB4ix190Bx30hNigTelhMAmraUZF+We6ZT1xrfO+CycAnJ+v5n4KHrfnVBeeylb7i Xe9QXAecBLw8DBQ6eYYhDayOgKf8brJWvlzIWek/HlwQePdNWb0hwR+ueOabU8Sax0VJ BRegxPN/9X5NYn89ZbHqTVA8W3NWSbUAnIO06xkREWw9xJbK8rDaXoyUZ0mnHcSl7hSc w2uR3w1vKmON728I6vMxWSIvSzhmznyAqqP+5ue/81RoIpnH3Gt1vaJs8/5yWgrMQ5Si ysYsu54w0aey5I+IX5hLd3panCk/17RLy8CrNHf0GXUuWdJ0O1HWZLSb7dC6LD7ec4Sk PHmw== X-Gm-Message-State: AKaTC00aMbTwwJ9ZSrUaVdwIztKtMWLScqV1QK/hUw5Xm/70yifmxav44wjLZZKD4WeQLw== X-Received: by 10.157.11.193 with SMTP id 59mr625791oth.118.1479505336950; Fri, 18 Nov 2016 13:42:16 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.18.246 with SMTP id g109ls8672894otg.7.gmail; Fri, 18 Nov 2016 13:42:16 -0800 (PST) X-Received: by 10.233.237.147 with SMTP id c141mr2234055qkg.175.1479505336037; Fri, 18 Nov 2016 13:42:16 -0800 (PST) Original-Received: from smtp83.iad3a.emailsrvr.com (smtp83.iad3a.emailsrvr.com. [173.203.187.83]) by mx.google.com with ESMTPS id h24si2930382qtf.266.2016.11.18.13.42.15 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 18 Nov 2016 13:42:15 -0800 (PST) Received-SPF: neutral (google.com: 173.203.187.83 is neither permitted nor denied by best guess record for domain of tom@honermann.net) client-ip=173.203.187.83; Original-Received: from smtp27.relay.iad3a.emailsrvr.com (localhost [127.0.0.1]) by smtp27.relay.iad3a.emailsrvr.com (SMTP Server) with ESMTP id BE0F124CE5 for ; Fri, 18 Nov 2016 16:42:15 -0500 (EST) X-Auth-ID: tom@honermann.net Original-Received: by smtp27.relay.iad3a.emailsrvr.com (Authenticated sender: tom-AT-honermann.net) with ESMTPSA id A6CE624B7F for ; Fri, 18 Nov 2016 16:42:15 -0500 (EST) X-Sender-Id: tom@honermann.net Original-Received: from [10.12.196.91] (pool-71-176-249-9.rcmdva.fios.verizon.net [71.176.249.9]) (using TLSv1.2 with cipher DHE-RSA-AES128-SHA) by 0.0.0.0:587 (trex/5.7.12); Fri, 18 Nov 2016 16:42:15 -0500 In-Reply-To: <35fe3945-71cf-4958-8f92-59df0ef98fae@isocpp.org> X-Original-Sender: tom@honermann.net X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 173.203.187.83 is neither permitted nor denied by best guess record for domain of tom@honermann.net) smtp.mailfrom=tom@honermann.net 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:29482 Archived-At: This is a multi-part message in MIME format. --------------8E17D3790B21A96AB15C4D24 Content-Type: text/plain; charset=UTF-8; format=flowed On 11/18/2016 3:47 PM, inkwizytoryankes@gmail.com wrote: > > > On Thursday, November 17, 2016 at 6:28:26 AM UTC+1, Louis Dionne wrote: > > Let me just expand a bit on what Matt said. > > I think this is a brilliant idea, but I don't think it's going to > work, for the same reason > as my P0424R0 paper was not warmly received. The reason is that > implementers > don't want anything that encourages the use of large template > parameter lists, > because the representation of that inside the compiler is too > inefficient. They don't > want a single template parameter per character in the string, > because the structure > that represents a template parameter in the compiler is expensive. > Instead, they'd > rather have a single structure that represents all the characters. > I'm not sure why they > can't just represent a character pack in a more efficient manner, > but that's what I > was told when I presented in EWG. > > > Fact that storing string in template parameters list is ineffective > did not stop GCC and Clang from implementing this: > https://godbolt.org/g/VjJzJi > https://godbolt.org/g/1DVftM > Thanks to that we can test how far we need push compilers to break > when we use this UDL. I'd still prefer to be able to do something like this: template constexpr X operator""_udl(const char (&sl)[N]) { ... } Tom. -- 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/a0d1a03d-d330-eb1b-13b1-0d241107c8b4%40honermann.net. --------------8E17D3790B21A96AB15C4D24 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On 11/18/2016 3:47 PM, inkwizytoryankes@gmail.com wrote:


On Thursday, November 17, 2016 at 6:28:26 AM UTC+1, Louis Dionne wrote:
Let me just expand a bit on what Matt said.

I think this is a brilliant idea, but I don't think it's going to work, for the same reason
as my P0424R0 paper was not warmly received. The reason is that implementers
don't want anything that encourages the use of large template parameter lists,
because the representation of that inside the compiler is too inefficient. They don't
want a single template parameter per character in the string, because the structure
that represents a template parameter in the compiler is expensive. Instead, they'd
rather have a single structure that represents all the characters. I'm not sure why they
can't just represent a character pack in a more efficient manner, but that's what I
was told when I presented in EWG.


Fact that storing string in template parameters list is ineffective did not stop GCC and Clang from implementing this: https://godbolt.org/g/VjJzJi
https://godbolt.org/g/1DVftM
Thanks to that we can test how far we need push compilers to break when we use this UDL.

I'd still prefer to be able to do something like this:

template<std::size_t N>
constexpr X operator""_udl(const char (&sl)[N]) { ... }

Tom.

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