220 29857 <902b08de-322b-4a97-8ba2-dfd8d3eb88e6@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: szollosi.lorand@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Default constructor for lambdas Date: Fri, 16 Dec 2016 15:21:12 -0800 (PST) Lines: 130 Approved: news@gmane.org Message-ID: <902b08de-322b-4a97-8ba2-dfd8d3eb88e6@isocpp.org> References: <13497310-30de-4542-9ba4-fb1fbd730cb6@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_712_106992050.1481930472538" X-Trace: blaine.gmane.org 1481930476 7217 195.159.176.226 (16 Dec 2016 23:21:16 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 16 Dec 2016 23:21:16 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDR7PROGSALRB2PN2HBAKGQE3EBP6NA@isocpp.org Sat Dec 17 00:21:10 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f199.google.com ([209.85.161.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cI1oL-0000e6-G3 for gclcip-std-proposals@m.gmane.org; Sat, 17 Dec 2016 00:21:10 +0100 Original-Received: by mail-yw0-f199.google.com with SMTP id b66sf73568070ywh.2 for ; Fri, 16 Dec 2016 15:21:14 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=auVXPDKyixz/y1ecYatVDx5IapwRhXBkfPh18O/VLKY=; b=Ok/g+7eThu2kTTqhQn/xmIaLnRhr4vNjjCFgzmlwpE8wBE9lHQr6EuytnjbJ8EiLa5 yqdKbJlKTtdak9n/tr5r8RjyqV32NOfiZaPUYxMmHR7IRJVajzedvBVHjWhdGrV6oDeT 8BonbtEzaQc1mu66RIBXzn6BEiyfDJbwKv+6urzrYDaJN5XGNb9Aq33chVFBgw55WVcx fsH3AG3u8+261F5hIdZ+KZoKoEtbViAE3nHnaYDVl3qciy4cTpmwaJw78EQFggilRuAj VOwtDSJrT8RxxGS8s1HDgFMfHTEvUW3K2yhwar770e8TxHPt1Q2PGcu30vpJh2BNbVDy g6VA== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=auVXPDKyixz/y1ecYatVDx5IapwRhXBkfPh18O/VLKY=; b=A5tneNZ2NgUsgn2JtF4PoNJSGaqm0EfKcuFS8PDXRqdRgk9fA8V8XUWRzq+GRA0GhR PONsvH8mbIagfe6ZkrGUJfNaJC+pkCEavDbjAYnlLbtdDlDGWtmZhsTKgtSjhdS5yJiW BTn0lV6U4f8Qlosm+DVHWki3QwfiYwwBbLVyPdxcgBl6CTGCNlCsUT3Hty1T3SV1RFSR s9ftGwkNUiUEODzfC7OTIGRupEY6oI+LwZ0MPiL9sKwd5k059rfKWxsg/G7qUDCugsK1 Zh1xEgXTVLWHPv0n/r9w1ZpAXs2Bo56/eHbicnGnnkCayrv+dGhT13mSSIxXwc9Cj5kq sCWw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=auVXPDKyixz/y1ecYatVDx5IapwRhXBkfPh18O/VLKY=; b=oKjh8xH9rr4dgUIYFCZPDuglY+SKB1nMVAPJYtTHcefpUsIBe/k3EgbFPJP8xMZP7+ uYnf6+LunAKKcl9EYxMqNs7rkChl72kp+LkyenSQ990wpEDx6K+NyZDPbo7gV/aFQh6f Er6QKlVJLrC89RFXNyQxqICh4O/Tl/9fX3MhbDjX8fVydwH8WgBBmPfM155y3pIwniOj Qs9LzJywnpW3h9XF3mN1ZdEm8oLTOPvydXQcSZG7V0Vxv/FcC5igiyGoWnUdtfxyu6Mn qOGYq1Vu/opgyjP2Kk9uuF/JRNyvV7TmlLGeTLjUs2704uxiSm1iR1ELPWm1JN4avS2f DYcQ== X-Gm-Message-State: AIkVDXKpFFMf6ZKQ04UoCsgFMA9qB9q1nHNZG85ztqFMWLRV7MBy7yGthVsg77dbi/uiXw== X-Received: by 10.129.198.2 with SMTP id l2mr1475284ywi.75.1481930473720; Fri, 16 Dec 2016 15:21:13 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.39.136 with SMTP id c8ls9314408otb.9.gmail; Fri, 16 Dec 2016 15:21:12 -0800 (PST) X-Received: by 10.157.37.247 with SMTP id q110mr355670ota.5.1481930472953; Fri, 16 Dec 2016 15:21:12 -0800 (PST) In-Reply-To: <13497310-30de-4542-9ba4-fb1fbd730cb6@isocpp.org> X-Original-Sender: szollosi.lorand@gmail.com 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:29857 Archived-At: ------=_Part_712_106992050.1481930472538 Content-Type: multipart/alternative; boundary="----=_Part_713_1238446629.1481930472538" ------=_Part_713_1238446629.1481930472538 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Hi, Minor thing - it will only work if all the captures of the lambda are=20 copyable. Even then it's non-trivial sometimes to choose schematics. Thanks, -lorro 2016. december 16., p=C3=A9ntek 23:47:45 UTC+1 id=C5=91pontban Vincent Reve= rdy a=20 k=C3=B6vetkez=C5=91t =C3=ADrta: > > Hello. > > What is the reason why lambdas have no default constructor? Is there any= =20 > technical reason behind that, or is it a pure design decision? > > I have a use case where not having a default constructor for lambda is=20 > annoying. > > Given C++17 class template deduction, I would like to be able to do: > > template > class object > { > constexpr object(F) {} // Constructor > constexpr void execute() {F()();} // Default-construct the lambda,=20 > and run it > }; > > object myobject([](){std::cout<<"Hello world!"< myobject.execute(); > > What do you think? Remarks? Comments? > > Thank you very much. > Best, > Vincent Reverdy > --=20 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 e= mail 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/902b08de-322b-4a97-8ba2-dfd8d3eb88e6%40isocpp.or= g. ------=_Part_713_1238446629.1481930472538 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
Hi,

Minor thing - it will only work if all the capt= ures of the lambda are copyable. Even then it's non-trivial sometimes t= o choose schematics.

Thanks,
-lorro

2016. december 16., p= =C3=A9ntek 23:47:45 UTC+1 id=C5=91pontban Vincent Reverdy a k=C3=B6vetkez= =C5=91t =C3=ADrta:
Hello.

What is the reason why lambdas have no default construct= or? Is there any technical reason behind that, or is it a pure design decision?

I ha= ve a use case where not having a default constructor for lambda is annoying= ..

Given C++17 class template deduction, I would like to be able to d= o:

template <class F>
class object
{
=C2=A0 =C2=A0 conste= xpr obj= ect(F) = {} =C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0// Constru= ctor
=C2=A0 =C2=A0 constexpr void execute() {F()();} // Default-construct the lambda, and run it
};

object myobject([](){std::cout<<= ;"Hello world!"<<std= ::endl<= span style=3D"color:#660">;});
myobjec= t.execu= te();

What do you think? Remarks? Comments?
Thank you very much.
Best,
Vincent Reverdy

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