220 29483 <12f71e8a-b31f-4c68-be93-d4a513459daf@honermann.net> 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 16:48:42 -0500 Lines: 159 Approved: news@gmane.org Message-ID: <12f71e8a-b31f-4c68-be93-d4a513459daf@honermann.net> 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="------------997E246E3BA216A8AEFB6EE7" X-Trace: blaine.gmane.org 1479505726 23766 195.159.176.226 (18 Nov 2016 21:48:46 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 18 Nov 2016 21:48:46 +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+bncBDBNXSHG6UDBBO7OXXAQKGQEXJ5URWY@isocpp.org Fri Nov 18 22:48:42 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f199.google.com ([209.85.192.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1c7r1V-00056Y-HF for gclcip-std-proposals@m.gmane.org; Fri, 18 Nov 2016 22:48:41 +0100 Original-Received: by mail-pf0-f199.google.com with SMTP id 83sf154372061pfx.1 for ; Fri, 18 Nov 2016 13:48:45 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=subject:reply-to:references:to:from:message-id:date:user-agent :mime-version:in-reply-to:x-original-sender :x-original-authentication-results:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=3v4ZLHOQbwD05D4zNJHTd84lMAVqSgBzy54WoIP+QZY=; b=LMhVrZVmh0S3K2TCo5rMrr2QPRYRWKPYQhmpZehvyB4CoMgMsMe6cWr7yGJpUrz/or 110m6qUnbDAkKZ0+hOQISGwnD+nXY9uB1uEbqJvaQuj202Jy+pPM1HyIpjiIOvchBOGV Dh8HYw3ZRTADfaYffyaZCFeQoxBW7RlJxQKZUiVIUV1l7GKox8NbBxfzjJ1rEyulHwxz TWrYm6F54Z1Ejcya9dRI3QfHKp4xif8QvjG/r8yryNgsDFEmorRKl4BdKl7HM9/D7Jy0 KHFeEDj2vQLoL1QSV3NMXbOoHKZU8vgVvES6DQtWNntNKbvAZ44+yC2fFmgcK5Wh/bF6 6PWQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:subject:reply-to:references:to:from:message-id :date:user-agent:mime-version:in-reply-to:x-original-sender :x-original-authentication-results:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=3v4ZLHOQbwD05D4zNJHTd84lMAVqSgBzy54WoIP+QZY=; b=UvRugHTWjHyyWvzJkH5feGrIJTluE3WFTE4rkwYHRpwed4tma35VCihfA1vvltWSPi gtyr6CAiVQ9IU7s173MvtFGZxF15bELjJIfl/fPvMdE16xwj1vlrQi1V+ipjBITC1oDK y65SaKRh30yOLxwXs5VBZkrkuv78MTH+50/frjH2JtW1e34H8iDs+8QeC2XpERnp0o9f iI8vXvxGTTUaGq8G63NWTsFKV9EDub2bLbpYvpCl0Z43Dz7G5LL+d+3ACBQv+yMrJvmk c2DGUUydprd1Lwc9sHTCHATdkZ1N2UetHA/55UmECst0DQG8zGU2M2nRuMaDydrjctxq Sw3w== X-Gm-Message-State: AKaTC00PWZ6bMGNu/BK6VPXLkHRTSXuNbKyhE+SAofoK0ylpE12KACTWAkEANp2kYK4zUQ== X-Received: by 10.99.174.65 with SMTP id e1mr673623pgp.18.1479505724710; Fri, 18 Nov 2016 13:48:44 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.141.141 with SMTP id p135ls725263iod.36.gmail; Fri, 18 Nov 2016 13:48:43 -0800 (PST) X-Received: by 10.107.169.95 with SMTP id s92mr1706533ioe.65.1479505723617; Fri, 18 Nov 2016 13:48:43 -0800 (PST) Original-Received: from smtp67.ord1c.emailsrvr.com (smtp67.ord1c.emailsrvr.com. [108.166.43.67]) by mx.google.com with ESMTPS id e11si7078248ioi.152.2016.11.18.13.48.43 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 18 Nov 2016 13:48:43 -0800 (PST) Received-SPF: neutral (google.com: 108.166.43.67 is neither permitted nor denied by best guess record for domain of tom@honermann.net) client-ip=108.166.43.67; Original-Received: from smtp25.relay.ord1c.emailsrvr.com (localhost [127.0.0.1]) by smtp25.relay.ord1c.emailsrvr.com (SMTP Server) with ESMTP id 5905720353 for ; Fri, 18 Nov 2016 16:48:43 -0500 (EST) X-Auth-ID: tom@honermann.net Original-Received: by smtp25.relay.ord1c.emailsrvr.com (Authenticated sender: tom-AT-honermann.net) with ESMTPSA id 1B65F2034C for ; Fri, 18 Nov 2016 16:48:43 -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 16:48:43 -0500 In-Reply-To: X-Original-Sender: tom@honermann.net X-Original-Authentication-Results: mx.google.com; spf=neutral (google.com: 108.166.43.67 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:29483 Archived-At: This is a multi-part message in MIME format. --------------997E246E3BA216A8AEFB6EE7 Content-Type: text/plain; charset=UTF-8; format=flowed On 11/18/2016 4:42 PM, Tom Honermann wrote: > 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]) { ... } Of course, that doesn't provide the string-literal -> unique type mapping (based on string literal contents) that would be desired for at least some use cases. 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/12f71e8a-b31f-4c68-be93-d4a513459daf%40honermann.net. --------------997E246E3BA216A8AEFB6EE7 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On 11/18/2016 4:42 PM, Tom Honermann wrote:
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/V= jJzJi
https://godbolt.org/g/1= DVftM
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]) { ... }
Of course, that doesn't provide the string-literal -> unique type mapping (based on string literal contents) that would be desired for at least some use cases.

Tom.

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