220 29862 article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Default constructor for lambdas Date: Fri, 16 Dec 2016 20:46:08 -0800 (PST) Lines: 161 Approved: news@gmane.org Message-ID: 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_763_984198195.1481949968238" X-Trace: blaine.gmane.org 1481949972 23738 195.159.176.226 (17 Dec 2016 04:46:12 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 17 Dec 2016 04:46:12 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBBEEG2PBAKGQEQ4ZP5FQ@isocpp.org Sat Dec 17 05:46:08 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pf0-f198.google.com ([209.85.192.198]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1cI6sn-0004r7-D2 for gclcip-std-proposals@m.gmane.org; Sat, 17 Dec 2016 05:46:05 +0100 Original-Received: by mail-pf0-f198.google.com with SMTP id j128sf151942271pfg.4 for ; Fri, 16 Dec 2016 20:46:10 -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=8fh81VltZx5byIxaIaFoH70H2L/5wysPokG9UxUJOII=; b=pSjbe4lJ8CZ671ADzJy8Ust5bvbKPT/8DayEQ8iuHmsPvaKnxLPnd7rqk2tdgp12g6 rD1/vXubnNGEtBP9qPt017h9N4CB/2lCLjykAETz/WaYZ/LlSzxoFbHv0Xz58csLGEb1 K+VyG4fWtgbcnFIBicF2pQBd4aUx24KOxnSze3gIPnKajeBQRiLpUPzeN+i4OoS+Dlfp lkVXoDlRjjldz8DndJ8KwlyHLvCL9H96IKL2PdoeHzTfkYc5UXB0pzF9fbYgruq93lpe IdVecYzt7bDCwv+CJdTv+/+1gBcTGtgE6kMAnEPUp210F9EAz/AqLF3v5SCw+w5ys9L5 8Erw== 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=8fh81VltZx5byIxaIaFoH70H2L/5wysPokG9UxUJOII=; b=d4zu8AFdC1XCVXrtmmz5QLrkyNBeCXWfMZArUbZBm0u5BfWwazRxh/x+jQkVU9hjru AAYDspjzV4Zssc1Hdz93PrNDSOmzagbThVvplpMBNC09W7PairkYEpYm1IO3RX74lDwV xGMUzUe67bAkKFDZ9as1RfVSPUMGz9s4m1YTx3/4j5v/GNdO5cDragttNFnVDswRoAoK /Loq8iRxl4Gm6EcfKvqJfto+Oet1yvynLqlG1qcECjq2KAlcszptyQ8jXPt10B25Ar5R htmI5/VGj4qmx/QY4jqwrLEcZ1YJ4dQhzWOzJjfL7ZtZMX8GDxthjdexyu+SF+CPJIOq d+qA== 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=8fh81VltZx5byIxaIaFoH70H2L/5wysPokG9UxUJOII=; b=XKnIH6ajjdPFO0+Mk4bjqYzREEuyfIceAXbtVfdRF97dD71G3S1LPR3ZEMcc950rQl 3Cj1lZ4gAs9urEn5xF7UgUmOw5rIXCCHKC4laSGCRwptYjI0ZvKoN46XgWRbVVdyTbob UU+I9zCs3hdvOyO2Jw0cvgAOp+ynyDjX1X3Z3XKOHM5pp1uLA58OkWNihQoSK30eDN4t 35oRoGLg5ntjdbTjaYOgL5DHrLw69Fd0OiGsC3+bXuoX+kqo+V4pwO0sxsY28o0+OHsp YFaFIySe6mxpRf1wWT8VRp3BlyFwhDbsS4IYLkD9d+d6gCxqmaPhHVvIjgKtegrB2dYQ JLHA== X-Gm-Message-State: AKaTC00wHuooGZNy1Jr5W0qFslSBP6S8gPxJvfMe7z4i5KKag/Kz0tXvLXy14kjHVL5VDA== X-Received: by 10.99.148.1 with SMTP id m1mr3221207pge.4.1481949969503; Fri, 16 Dec 2016 20:46:09 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.27.206 with SMTP id v14ls9686717otv.2.gmail; Fri, 16 Dec 2016 20:46:08 -0800 (PST) X-Received: by 10.157.8.134 with SMTP id 6mr369671otf.17.1481949968769; Fri, 16 Dec 2016 20:46:08 -0800 (PST) In-Reply-To: <13497310-30de-4542-9ba4-fb1fbd730cb6@isocpp.org> X-Original-Sender: jmckesson@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:29862 Archived-At: ------=_Part_763_984198195.1481949968238 Content-Type: multipart/alternative; boundary="----=_Part_764_95023109.1481949968238" ------=_Part_764_95023109.1481949968238 Content-Type: text/plain; charset=UTF-8 On Friday, December 16, 2016 at 5:47:45 PM UTC-5, Vincent Reverdy wrote: > > Hello. > > What is the reason why lambdas have no default constructor? Is there any > 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 > 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, > and run it > }; > > object myobject([](){std::cout<<"Hello world!"< myobject.execute(); > > What do you think? Remarks? Comments? > I think this would be much more reasonable as this: struct HelloWorld { void operator()() {std::cout<<"Hello world!"<On Friday, December 16, 2016 at 5:47:45 PM UTC-5, Vincent = Reverdy wrote:
Hello.

What is the reason why lambdas have no default constructor? = 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?

I think this would be much more reasonable as thi= s:

struct = HelloWorld= { void operator()() {= std::cout<<"Hello world!"= ;<<<= /span>std<= span style=3D"color: #660;" class=3D"styled-by-prettify">::endl;}};

object myobject(HelloWorld());

I generally consider= globally-scoped lambdas like these to be dubious constructs. There's n= othing to capture, so the only thing you're doing is making it sligh= tly more convenient to define a struct with an operator() overload.
=
That being said, I'm not against the feature, so long as it only= applies to lambdas that have no captures. There was a long thread on t= his forum about applying default constructors to capturing lambdas, and it = was generally agreed on that it broke the meaning of the lambda.
<= /div>

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