220 29444 article Path: news.gmane.org!.POSTED!not-for-mail From: "'Johannes Schaub' via ISO C++ Standard - Future Proposals" Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Allowing string literals to match `char...` packs in templates Date: Thu, 17 Nov 2016 16:50:23 +0100 Lines: 125 Approved: news@gmane.org Message-ID: References: <9de07b5f-f870-4a02-882f-54f9fdd12ffd@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 X-Trace: blaine.gmane.org 1479397832 21499 195.159.176.226 (17 Nov 2016 15:50:32 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 17 Nov 2016 15:50:32 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCQPNCEZSQNBBQ5DW7AQKGQEB5GHZVY@isocpp.org Thu Nov 17 16:50:27 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f198.google.com ([209.85.217.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1c7OxF-0004F2-3w for gclcip-std-proposals@m.gmane.org; Thu, 17 Nov 2016 16:50:25 +0100 Original-Received: by mail-ua0-f198.google.com with SMTP id d33sf144213866uad.2 for ; Thu, 17 Nov 2016 07:50:28 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=mime-version:in-reply-to:references:from:date:message-id:subject: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=63sA99KxXedbHueUffpZ9c/mjrr/OBlv8RiPyGG4Ru8=; b=BS72HEk4u5T+yzz+cMhhDGAolHysx2bxDLd3QjIYDtytnwkT2V93mLww05Q9eo57tI Pnz/5eq/cvQs4i0OgCFI1ZI0HYPzFuhzQ3P96qaVQSy5d3w8szdu+fi0EQaNMAxWRE1b oXeOIoMa24P9SL3BDK232ATB/6OJUEjCTQBmV2kMxLarNq4PApAUxOure5nJ2cogZJuN sQW5zOGGIGLlHrpCppm9C0bdusZCjXt8NYpTxosCSLx22fPhH5GpaKScJKg1Y9W/otxW I+kQoFyVJUFDPQyA5iQKPtM9YAG7F0QTc1lsOlhQOTR/pVle1ZzVjiEsLgWxZXzIKZZR Z42w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject: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=63sA99KxXedbHueUffpZ9c/mjrr/OBlv8RiPyGG4Ru8=; b=QhGgwK+KZ/KJPpKHOT6DCki3Th1xFIQNxC2N0QrLb5FPeE7eO+Sj+HusidcSbvzO7G 32soH+opQ0rwTbco2v9xaJXKQCEiRs4N0+VtPQifslM23p6jCQG19wclsmvF7haERmfS VGyZ38eiAH8Sqt36szmQqSaM+6XQuzi6xjYVmDPo+riow9PR06vINjxblaZcy/lP2NLk Dqmg93jtUiGaGKQdwEI36Jq1sZcwYJZdQj+mmPAQ5S3LzW7WdsEPFF8dUlbUTd0anFE4 Igx4q+VC7rlXkvYpnrNKAeKCyCJDN9i0wxQe2g7pwTk81DTZgVdycPO/HbwZl+uI/Yma CL1w== X-Gm-Message-State: AKaTC02Gni1cPwJ3b4DDgZhJLuw9BhPw4hHZvlXRQ0F4CxwUkPhAbBGvyRsiPHIQA+unaA== X-Received: by 10.157.34.199 with SMTP id y65mr1284124ota.136.1479397828202; Thu, 17 Nov 2016 07:50:28 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.130.85 with SMTP id e82ls1224855iod.40.gmail; Thu, 17 Nov 2016 07:50:27 -0800 (PST) X-Received: by 10.36.81.198 with SMTP id s189mr4975490ita.117.1479397827043; Thu, 17 Nov 2016 07:50:27 -0800 (PST) Original-Received: from mail-it0-x244.google.com (mail-it0-x244.google.com. [2607:f8b0:4001:c0b::244]) by mx.google.com with ESMTPS id h129si2730921itb.78.2016.11.17.07.50.26 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 17 Nov 2016 07:50:26 -0800 (PST) Received-SPF: pass (google.com: domain of schaub.johannes@googlemail.com designates 2607:f8b0:4001:c0b::244 as permitted sender) client-ip=2607:f8b0:4001:c0b::244; Original-Received: by mail-it0-x244.google.com with SMTP id c20so16001074itb.0 for ; Thu, 17 Nov 2016 07:50:26 -0800 (PST) X-Received: by 10.107.128.194 with SMTP id k63mr3541223ioi.223.1479397823838; Thu, 17 Nov 2016 07:50:23 -0800 (PST) Original-Received: by 10.107.175.15 with HTTP; Thu, 17 Nov 2016 07:50:23 -0800 (PST) In-Reply-To: X-Original-Sender: schaub.johannes@googlemail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@googlemail.com; spf=pass (google.com: domain of schaub.johannes@googlemail.com designates 2607:f8b0:4001:c0b::244 as permitted sender) smtp.mailfrom=schaub.johannes@googlemail.com; dmarc=pass (p=QUARANTINE dis=NONE) header.from=googlemail.com X-Original-From: Johannes Schaub 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:29444 Archived-At: And you need something to guarantee that decltype<0> is not the same type as decltype<1> in order for the templates that receive the types to be different for different constants ^_^ Perhaps you can make it a hidden compiler type and the user could simply write the above concepts-thingy as template<> struct Variable> { }; To say "some string", you could catch the type type and require that it's for a string-literal template requires StringLiteralConstant struct Variable { }; 2016-11-17 16:41 GMT+01:00 Johannes Schaub : > Perhaps pass it not as a value, but as a (class-) type with a new kind > of type specifier. Then you can use a type template parameter, and > need to invent only syntax to denote these types. The type would have > a "value" static constexpr member that denotes the value. For string > literals, that would be a char array. Notice that this approach will > be compatible with passing std::integral_constant as a type, just that > the actual type will be some "magic" type. Using the "decltype" > keyword here, I'm making an example > > template > struct Variable { > void print() { std::cout << Name::value; } > }; > > Variable> v; > v.print(); // prints "Hello folks" (without quotes) > > Variable > v42; > v42.print(); // prints 42 > > If we expose the type (bikeshed warning) as "std::constant", we could > also specialize on it > > template<> > struct Variable> { }; > > I can imagine that with concepts, one could also specialize on the > contained value, although I'm not familiar with the exact syntax and > whether it actually could work > > template // C will be char[N] > requires constexprCompareFunction(C::value, "Hello folks") > struct Variable> { }; > > All kind of literals are allowed. Perhaps one can extend it to all > kinds of constant expressions even. > > 2016-11-17 6:28 GMT+01:00 Louis Dionne : >> 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. >> >> So on Saturday, we tried looking into passing arrays as template arguments, >> but >> that becomes messy because arrays are already supported, but they decay to >> pointers so we'd need to support a new syntax. We then tried passing >> std::array, >> but doing this cleanly would require passing arbitrary literal types as >> template >> arguments, which is also non-trivial of a change. >> >> I have not started writing a paper yet, because I don't have any solution in >> mind >> at this time to be honest. However, this is one of the most valuable >> problems to >> be solved for metaprogramming, so I'll take anything that works. >> >> Louis >> >> >> On Wednesday, 16 November 2016 15:18:23 UTC+1, Matt Calabrese wrote: >>> >>> On Wed, Nov 16, 2016 at 9:04 AM, Vittorio Romeo >>> wrote: >>> > Currently string literals are not allowed as template arguments because >>> > the >>> > standard explicitly prevents that (see 14.3.2 Template non-type >>> > arguments). >>> >>> At the end of the Issaquah C++ standards meeting last week some of us >>> worked toward another approach to arrays as value-kind template >>> parameters (Louis Dione, Jeff Snyder, James Touton, myself, and >>> others), specifically because of wanting to easily pass compile-time >>> strings. It's a harder problem than it seems and implementors voiced >>> concern regarding representation as a pack of char even though the >>> desired semantics are similar. I'm not sure, but Louis Dionne might >>> already be starting on a new paper in this area. There are a few of us >>> who really want something like this. >> >> -- >> 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/d9433fac-152f-4515-b518-985a78768c03%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/CANu6V4W8v_rJNw0-mNbFbqE5OvqvxD8T3yWOjzQdKQbR_qzCog%40mail.gmail.com. .