220 28750 article Path: news.gmane.org!.POSTED!not-for-mail From: Larry Evans Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Ideas for extending std::tuple with field-name based access Date: Wed, 12 Oct 2016 14:21:24 -0500 Lines: 77 Approved: news@gmane.org Message-ID: References: <19048c95-770c-4413-b5ff-aa405c389621@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; format=flowed X-Trace: blaine.gmane.org 1476300111 3548 195.159.176.226 (12 Oct 2016 19:21:51 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 12 Oct 2016 19:21:51 +0000 (UTC) User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0 To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDAYTI72VMGRBNM27K7QKGQETAIWDHY@isocpp.org Wed Oct 12 21:21:47 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 1buP5g-0005uh-Et for gclcip-std-proposals@m.gmane.org; Wed, 12 Oct 2016 21:21:24 +0200 Original-Received: by mail-oi0-f71.google.com with SMTP id t73sf119452362oie.5 for ; Wed, 12 Oct 2016 12:21:26 -0700 (PDT) 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=FxNTh+sS1wcl4VvepNDTHmqvES5kRJI2lrW3PYhWYEI=; b=VLYHLypT4VjWrvamsnBfZXNFY0dnk7bftIE7aVAESsLxUNtjR0swkqvF8KUZcAQ2Gf KJ8QdG24SYTAduS4y0LfVeyxACvJzjEtW5YZ+LghByU6pOv8JxFHxE2u5KFhvqYlNO5G bOEF/YwXZ2BRzPnYEi4MbvXP91S0EzVy2FMYOZxhZ0FlVCpdUSWhdxw1WmU7egOH/Nz8 7pYuxZO0qPBwzoDC69ducspui7erpfl+9/AJu7o6J0WUTzEhSUc2q5I4/bwOyuCj7/LR y7DrOhzh0JVa5J+MoCatn/oKnEuH4K67cxRtNndwEPYIMYeS76yxnkCujl+OJ3f4rOQj Nbaw== 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=FxNTh+sS1wcl4VvepNDTHmqvES5kRJI2lrW3PYhWYEI=; b=OtNEx5GMycSULlcxLrEn9ubdPcCC6aWP66YD+Lhy2uAMJdEqguMEOVQ4RR0V+TOcph c49KUr9Y3E/KuYWoYMUSXM5RVs6NsPiyir0+yUUkL3W2YcNcCtbFBFM6I1pwWe3xiI0G 79m6vZP8BHhrQt6gttudhV8itCFQ82T4jwsC6VmToeARAaMpDqdAijdZCUVVRKrfLUiK S4goKZeUv4yrgxXWN6TfndfkA4GnQffltFmwTR4VJPHlP30aM3rwYej0KQyPMD83kuWO ostVkmp+g7xaT0O5mu+fk085E5mBZuIL5+z5JXxcwogH3w6M2eUVrk0iEKmh+cpX+Arc mSNQ== X-Gm-Message-State: AA6/9RkLAYde4PharbdR5wFpX1BohwdFXqb92+HDzcnJNQiDeeAGSWIOZpO212NUanSxmA== X-Received: by 10.157.58.35 with SMTP id j32mr690369otc.13.1476300086313; Wed, 12 Oct 2016 12:21:26 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.170.204 with SMTP id g73ls1615073ioj.2.gmail; Wed, 12 Oct 2016 12:21:25 -0700 (PDT) X-Received: by 10.98.57.84 with SMTP id g81mr4029109pfa.115.1476300085439; Wed, 12 Oct 2016 12:21:25 -0700 (PDT) Original-Received: from omta01.suddenlink.net (omta01.suddenlink.net. [208.180.40.71]) by mx.google.com with ESMTP id n5si7252079pgh.23.2016.10.12.12.21.25 for ; Wed, 12 Oct 2016 12:21:25 -0700 (PDT) Received-SPF: pass (google.com: domain of cppljevans@suddenlink.net designates 208.180.40.71 as permitted sender) client-ip=208.180.40.71; Original-Received: from [192.168.1.2] (really [50.24.16.38]) by dalofep01.suddenlink.net (InterMail vM.8.04.03.22 201-2389-100-167-20150619) with ESMTP id <20161012192124.ULNT15195.dalofep01.suddenlink.net@[192.168.1.2]> for ; Wed, 12 Oct 2016 14:21:24 -0500 In-Reply-To: <19048c95-770c-4413-b5ff-aa405c389621@isocpp.org> X-Cloudmark-Analysis: v=2.2 cv=GYRVpkfL c=1 sm=0 tr=0 a=IkcTkHD0fZMA:10 a=CH0kA5CcgfcA:10 a=yRZ3NqUWEgmhbTA5eQwA:9 a=QEXdDO2ut3YA:10 a=chvjmp5bT-K0Np4W8Gpx:22 X-Original-Sender: cppljevans@suddenlink.net X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of cppljevans@suddenlink.net designates 208.180.40.71 as permitted sender) smtp.mailfrom=cppljevans@suddenlink.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:28750 Archived-At: On 10/12/2016 08:09 AM, TONGARI J wrote: > On Wednesday, October 12, 2016 at 7:48:24 PM UTC+8, Johannes Schaub wrote: > > Hello all, > > Using type-tags to access tuple fields is a common technique to access > fields by "name" > > struct position_tag : type { }; > struct color_tag : type { }; > > my_tuple m; > > QColor c1 = std::get(m); > QColor c2 = std::get(m); > QColor c3 = std::get<1>(m); > > I was thinking about extending std::tuple with support for tags. This > can be done (i believe) most straight forward with string template > arguments, if we would have support for them in the future > > std::tuple, field> > getStop(); > > auto m = getStop(); > double pos = std::get<"position">(m); > > An alternative (but uglier) implementation would abuse unnamed struct > definitions in template-arguments with the "type" baseclass of above > to communicate the type > > std::tuple { type position; }, > struct : typed { type color; }> getStop(); > > Here, "type" is declared in base-class "typed" as a type-alias for its > template-parameter. When tuple sees a parameter has "typed" as a > base class, it inherits from it. Therefore you could write > > auto m = getStop(); > QColor c = m.color; > > std::get would cast the unnamed-struct to "struct_type::type", > requiring a standard-layout struct (can be tested-for with a > type-trait). So the unnamed-struct cannot "escape" to the users of the > tuple. > > What do you think about the two approaches? Are there existing papers > about perhaps other techniques? > > > I'm not aware of any existing proposal, but some experimental language > features I'm developing will allow you to do this: > > | > std::tuplegetStop(); > autom =getStop(); > doublepos =m.position; > QColorc =m.color; > | > > Let me know if you need more detail. What about something similar for variant? std::variant var_pos_color=std::tuple(1.2); then: assert(var_pos_color.position == 1.2); assert(var_pos_color.color == undefined); assert(var_pos_color.which == position); -- 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/bc065abb-acc5-ce9a-2b8a-2c114c2542a2%40suddenlink.net. .