220 29632 article Path: news.gmane.org!.POSTED!not-for-mail From: David Krauss Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Use stateless lambdas without capturing Date: Sat, 3 Dec 2016 19:47:07 +0800 Lines: 89 Approved: news@gmane.org Message-ID: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 (Mac OS X Mail 9.1 \(3096.5\)) Content-Type: multipart/alternative; boundary="Apple-Mail=_0E270FA7-5D96-41CA-BA51-E99B482376E1" X-Trace: blaine.gmane.org 1480765633 24707 195.159.176.226 (3 Dec 2016 11:47:13 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sat, 3 Dec 2016 11:47:13 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCW25A7E3QCRBPXBRLBAKGQE6A3QPWA@isocpp.org Sat Dec 03 12:47:09 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 1cD8ma-0005Py-Jv for gclcip-std-proposals@m.gmane.org; Sat, 03 Dec 2016 12:47:08 +0100 Original-Received: by mail-pf0-f198.google.com with SMTP id 144sf433786799pfv.5 for ; Sat, 03 Dec 2016 03:47:12 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=from:subject:message-id:date:to:mime-version: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=gXP98QtbXdb8Zv5MFe+J3M0Mbi5CJEw/7pUZLIieVwk=; b=acCPy8v6UZx1YyewCTwNnBnZhOKfe8Uqw9xYhzKebqY8DnCLzHrVWzcnbH9fX7ET1n rvvyWn86ydkX3IEwFBj4vcXNWombInfPmaDcGs+MyHI9lklhKR5pdmpOPcb1lwnRJXjY UbsfAo3w1BZABsa704tL3K17gwJfqYVPFQRuCAPJIW71wTmvJt4L7w+UCZqWAMzoeRwW OSjShmTchsYG115of1sQwO9FQJdv2xXW1VTygjRo4yjCK1+jDngt0mGNAutrucnvTiWD g9l+XsmJfADK9mFXz98lEWj6XmlWLvQ7z9BSNezOQDzZF5VrH6pg+2BZbNERfuzLHQLA jwiw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:subject:message-id:date:to:mime-version :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=gXP98QtbXdb8Zv5MFe+J3M0Mbi5CJEw/7pUZLIieVwk=; b=SfiKXa2e1BXbZ4H/0in3Q/XDXaIWl28Asmpic7rnmx9HIew2ySsqhkrX+Fj11SQ6BF PTA4o9KDZcb6/OWKnwX6zI5MBKf/eNT0gvz/LeuOzAcPYIs+xyzz02WxjJC3S2kz9HQY mBJ4G2p6byA+JFoIe87CZ1ayIKo9UbzoguPAsp96UkgHjIndGJ56aYL8337xacm7zgwF jUi5lMbkjONbP3ICQ68pTYIlOuwMtCA0DuV3FXg1oAdCMn1VA4PatWbBJi8QhKGDPn/a NW9BnEW+EEI5i8JVg0fbM6zsuIHWak2fhiunjJH4Ey27GjFNvyY6/S3axXxyRuK7umKh KL7g== X-Gm-Message-State: AKaTC00hvJ/0vzgfNV6xZpzG+DHEhhOnTDyb0afr/dUcmhgLkB1rBa33KSCgvIlgr3E8nQ== X-Received: by 10.99.53.5 with SMTP id c5mr22227040pga.127.1480765632306; Sat, 03 Dec 2016 03:47:12 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.169.209 with SMTP id f78ls6734857ioj.29.gmail; Sat, 03 Dec 2016 03:47:10 -0800 (PST) X-Received: by 10.99.113.68 with SMTP id b4mr88076569pgn.33.1480765630646; Sat, 03 Dec 2016 03:47:10 -0800 (PST) Original-Received: from mail-pg0-x230.google.com (mail-pg0-x230.google.com. [2607:f8b0:400e:c05::230]) by mx.google.com with ESMTPS id q17si8514858pgh.96.2016.12.03.03.47.10 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 03 Dec 2016 03:47:10 -0800 (PST) Received-SPF: pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c05::230 as permitted sender) client-ip=2607:f8b0:400e:c05::230; Original-Received: by mail-pg0-x230.google.com with SMTP id p66so117673297pga.2 for ; Sat, 03 Dec 2016 03:47:10 -0800 (PST) X-Received: by 10.99.228.5 with SMTP id a5mr88085264pgi.1.1480765630315; Sat, 03 Dec 2016 03:47:10 -0800 (PST) Original-Received: from [172.20.10.2] ([175.158.200.160]) by smtp.gmail.com with ESMTPSA id o29sm14515070pgn.28.2016.12.03.03.47.08 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Sat, 03 Dec 2016 03:47:09 -0800 (PST) X-Mailer: Apple Mail (2.3096.5) X-Original-Sender: potswa@gmail.com X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of potswa@gmail.com designates 2607:f8b0:400e:c05::230 as permitted sender) smtp.mailfrom=potswa@gmail.com; dmarc=pass (p=NONE dis=NONE) header.from=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:29632 Archived-At: --Apple-Mail=_0E270FA7-5D96-41CA-BA51-E99B482376E1 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 int main() { auto a =3D []{ return 5; }; auto b =3D []{ +a; a(); }; // Error, =E2=80=9Ca=E2=80=9D not captured. } A stateless lambda object is empty, so it=E2=80=99s unfortunate that they m= ust be captured for use by another lambda. In this case, b cannot be statel= ess. C++17 constexpr lambdas offer a partial workaround: int main() { constexpr auto a =3D +[]{ return 5; }; auto b =3D []{ +a; a(); }; // OK, =E2=80=9Ca=E2=80=9D is a constant id-= expression used with L-to-R conversion. } This is a bit esoteric, and it can possibly cost some performance. Perhaps it should be permissible to call, convert, and even copy a stateles= s lambda from an enclosing scope. (Binding to a reference or taking an addr= ess are genuinely problematic, though. Pass-by-reference would be broken, b= ut fortunately and such pass by value.) --=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/DA325640-1898-4835-844F-290FC7A482BA%40gmail.com= .. --Apple-Mail=_0E270FA7-5D96-41CA-BA51-E99B482376E1 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=UTF-8
int main() {
    auto a =3D []{ return 5; };
    = auto b =3D []{ +a; a(); }; // Error, =E2=80=9Ca=E2=80=9D not captured.
}

A stateless lambda object is empty, so it= =E2=80=99s unfortunate that they must be captured for use by another lambda= .. In this case, b cannot be statel= ess.

C++17 constexpr la= mbdas offer a partial workaround:

int main= () {
  &= nbsp; constexpr auto a =3D +[]{ return 5; };
    auto b =3D []{ +a; a(); }; //= OK, =E2=80=9Ca=E2=80=9D is a constant id-expression used with L-to-R conve= rsion.
}

This is = a bit esoteric, and it can possibly cost some performance.

Perhaps it should be permissible= to call, convert, and even copy a stateless lambda from an enclosing scope= .. (Binding to a reference or taking an address are genuinely problematic, t= hough. Pass-by-reference would be broken, but fortunately <algorithm> and such pass by valu= e.)

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