220 29443 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 16:41:32 +0100 Lines: 109 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 1479397296 25792 195.159.176.226 (17 Nov 2016 15:41:36 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Thu, 17 Nov 2016 15:41:36 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCQPNCEZSQNBBLM7W7AQKGQE7VBXWEQ@isocpp.org Thu Nov 17 16:41:32 2016 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 1c7Ood-0005fY-84 for gclcip-std-proposals@m.gmane.org; Thu, 17 Nov 2016 16:41:31 +0100 Original-Received: by mail-it0-f71.google.com with SMTP id l8sf129102783iti.6 for ; Thu, 17 Nov 2016 07:41:35 -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=zqPySpfjrtrVzJtjQ4vhFgMQSHYTxp2Lj4cPad6KKjs=; b=LDy7HvoFBnJf0ShQegCIRBDmeGDZYEAdwhEtkwHB4RbsFL37Y1GoNqgpc9C+OcLlQg xXk+tRoqWTxxAhdk5WI/MJCjv5je0QEZwuYvLYIKkIsj2NLsBWORo1j84Ow4SwxbaN6X w8MiN9QvyXPjktquXKwm5r5zRkJvwtKBJEWwKSTfc/u+A9Mi2lkBx+9I3uKv5GcPHe3Z skXwg1FkbJrwtVGBlkpgac2A5QbAXlGs6c0j6+yur+xOPPah9H/kP7ThSmloHv4rdU71 jvC3zoNVohXZ8Eorj7m8Y+0jai+5t00kV/5M36ECOtrkSsi3DEjDs4tXx1WPraTzwK1T QbfA== 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=zqPySpfjrtrVzJtjQ4vhFgMQSHYTxp2Lj4cPad6KKjs=; b=PZVusR3/58G03ZRB8QKv5df9bluo+6bsuVODmKCbr/mGC2b88xgo9v9EhyFnAImJrc kNWCrxMhZd7BldZvjCUiCZuCvJaji+9UU73DPq0N/KiVFX6q/NomEYzA4uiJkAqSGRZk If33slDX5ors4cxXHq6dZVSe1oQHVVurolfz3MDWdk5S0YV/ZldganoYoWUQowvGRmHb 3331k/XYJMbvLjnE3jf5AM+J82YFb4HdtMQ3iIJATOQg+owNjvMaoKpp0T3jsKIBJH9v th9V7yxDNwxUvzLIlRZPJgu2TFqtySS1T2Tbgagq+gDg82/8RL1mccOc+5bOE2SSbUsS AQbw== X-Gm-Message-State: AKaTC01uJ9zqHbYsDl55U2YNSGROBew2EsY23LVdGg0B1PfUQIspbzAfYguqwfK+c8a5FA== X-Received: by 10.107.20.4 with SMTP id 4mr749861iou.22.1479397294530; Thu, 17 Nov 2016 07:41:34 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.136.230 with SMTP id s99ls1271414ioi.49.gmail; Thu, 17 Nov 2016 07:41:33 -0800 (PST) X-Received: by 10.107.1.138 with SMTP id 132mr3381268iob.72.1479397293525; Thu, 17 Nov 2016 07:41:33 -0800 (PST) Original-Received: from mail-it0-x243.google.com (mail-it0-x243.google.com. [2607:f8b0:4001:c0b::243]) by mx.google.com with ESMTPS id w93si2701918ioi.177.2016.11.17.07.41.33 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 17 Nov 2016 07:41:33 -0800 (PST) Received-SPF: pass (google.com: domain of schaub.johannes@googlemail.com designates 2607:f8b0:4001:c0b::243 as permitted sender) client-ip=2607:f8b0:4001:c0b::243; Original-Received: by mail-it0-x243.google.com with SMTP id c20so15947814itb.0 for ; Thu, 17 Nov 2016 07:41:33 -0800 (PST) X-Received: by 10.36.245.9 with SMTP id k9mr13467836ith.65.1479397293035; Thu, 17 Nov 2016 07:41:33 -0800 (PST) Original-Received: by 10.107.175.15 with HTTP; Thu, 17 Nov 2016 07:41:32 -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::243 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:29443 Archived-At: 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/CANu6V4WLfjDJF%2BNjRQcOE0vO2Fd%3DFnEtOgTQg9N8%2BmcyZ%2BvX7A%40mail.gmail.com. .