220 38484 article Path: news.gmane.org!.POSTED!not-for-mail From: Gareth Lloyd Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Restricting the valid use of certain methods upon temporary objects Date: Tue, 5 Jun 2018 01:29:38 +0100 Lines: 153 Approved: news@gmane.org Message-ID: References: <81b39e17-fe78-41ab-b9c9-3b830e32dbf5@isocpp.org> <709f2ceb-5059-4217-8475-288a8f4ecd44@isocpp.org> <48ae754e-18c0-4350-922b-8b6faac0df16@isocpp.org> <73c26f59-2aa0-4d89-b783-ebf73282a0bf@isocpp.org> <1ff61a56-191f-49c2-849d-b3bb47b52930@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="000000000000489c8a056dda257a" X-Trace: blaine.gmane.org 1528158454 24688 195.159.176.226 (5 Jun 2018 00:27:34 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 5 Jun 2018 00:27:34 +0000 (UTC) Cc: florian.csdt@gmail.com To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBCTJTSELJQIPJMWX3ACRUBG7HCR2Y@isocpp.org Tue Jun 05 02:27:30 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-wr0-f197.google.com ([209.85.128.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fPzov-0006Km-OA for gclcip-std-proposals@m.gmane.org; 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[209.85.220.41]) by mx.google.com with SMTPS id f4-v6sor63084wmh.82.2018.06.04.17.29.39 for (Google Transport Security); Mon, 04 Jun 2018 17:29:39 -0700 (PDT) Received-SPF: pass (google.com: domain of gareth.andrew.lloyd@gmail.com designates 209.85.220.41 as permitted sender) client-ip=209.85.220.41; X-Received: by 2002:a1c:1588:: with SMTP id 130-v6mr4269332wmv.35.1528158579365; Mon, 04 Jun 2018 17:29:39 -0700 (PDT) Original-Sender: gareth.andrew.lloyd@gmail.com Original-Received: by 2002:adf:8e57:0:0:0:0:0 with HTTP; Mon, 4 Jun 2018 17:29:38 -0700 (PDT) In-Reply-To: <1ff61a56-191f-49c2-849d-b3bb47b52930@isocpp.org> X-Original-Sender: gareth@ignition-web.co.uk X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=TICKrWbS; spf=pass (google.com: domain of gareth.andrew.lloyd@gmail.com designates 209.85.220.41 as permitted sender) smtp.mailfrom=gareth.andrew.lloyd@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:38484 Archived-At: --000000000000489c8a056dda257a Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable So after a long walk today I finally have been able to point at what makes me uneasy about the lifetime extension approach. In short, if we depend upon more than one lifetimebound object which are temporaries, then as it currently stands they are unsequenced. Consider the following example: =E2=80=8B #include #include struct Foo { static int counter; std::string s; Foo(std::string const & _s) : s{_s + std::to_string(counter++)} { std::cout << "ctr " << s << '\n'; } ~Foo(){std::cout << "dtr " << s << '\n';} }; int Foo::counter =3D 0; auto f0(Foo const & foo1 /*lifetimebound*/, Foo const & foo2 /*lifetimeboun= d*/) { return [&](){std::cout << foo1.s << '-' << foo2.s << '\n';}; } int main() { using namespace std::string_literals; auto x =3D f0("a"s, "b"s); x(); } With or without lifetime extension, the temporaries that foo1 and foo2 are bound to have undefined sequence of construction. This is a problem for temporaries as it currently is, but if we enhance lifetime extension and encourage the use of temporaries then this will possibly be a common pitfall. --=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/CANgTfEgedZZj1%3D3ydxBcZf9AMNNZvXG4Ndk6zTFxBNUU5= X8e1w%40mail.gmail.com. --000000000000489c8a056dda257a Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
So after a long walk today I finally have been able to poi= nt at what makes me uneasy about the lifetime extension approach. In short,= if we depend upon more than one lifetimebound=20 object which are temporaries, then as it currently stands they are unsequen= ced. Consider the following example:
=E2=80=8B 
#include <string>
#include <iostream>

struct Foo
{
    static int counter;
   =20
    std::string s;
   =20
    Foo(std::string const & _s) : s{_s + std::to_string(counter++)}=20
    {=20
        std::cout << "ctr " &l=
t;< s << '\n';
    }
    ~Foo(){std::cout <<=
; "dtr "=
 << s << '\n';}
};

int Foo::counter =3D 0;

auto f0(Foo const & foo1 /*lifetime=
bound*/, Foo con=
st & foo2 /*lifetimebound*/)
{
    return [&](){std::cout << foo1.s << '-' << foo2.s << '\n';};
}

int main()
{
    using namespace std::string_literals;
    auto x =3D f0("a"s, "b"s);
    x();
}



With or without lifetime extension, the temporaries that fo=
o1 and foo2 are bound to have undefined sequence of construction. This is a=
 
problem for temporaries as it currently is, but if we enhance lifetime=
 extension and encourage the use of temporaries then this will 
possibly=
 be a common pitfall.

--
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