220 17081 <11B5438424E807459A28BD01C92339BA759E907C@XMB122CNC.rim.net> article Path: news.gmane.org!not-for-mail From: Tony Van Eerd Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: RE: virtual constexpr fields Date: Wed, 18 Mar 2015 20:52:59 +0000 Lines: 321 Approved: news@gmane.org Message-ID: <11B5438424E807459A28BD01C92339BA759E907C@XMB122CNC.rim.net> References: <532355906.19321.1426710347371.JavaMail.yahoo@mail.yahoo.com> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="_000_11B5438424E807459A28BD01C92339BA759E907CXMB122CNCrimnet_" X-Trace: ger.gmane.org 1426712025 31519 80.91.229.3 (18 Mar 2015 20:53:45 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 18 Mar 2015 20:53:45 +0000 (UTC) To: "c++std-ext@accu.org" , "'std-proposals@isocpp.org'" , "ehsan@mozilla.com" Original-X-From: std-proposals+bncBCOJZ74BXIBRBMGLU6UAKGQEVKKCXOY@isocpp.org Wed Mar 18 21:53:39 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yh0-f70.google.com ([209.85.213.70]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1YYKxe-0007vj-JR for gclcip-std-proposals@m.gmane.org; Wed, 18 Mar 2015 21:53:07 +0100 Original-Received: by yhoa41 with SMTP id a41sf31263734yho.2 for ; Wed, 18 Mar 2015 13:53:05 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:from:to:subject:thread-topic:thread-index:date :message-id:references:in-reply-to:accept-language:content-language :content-type:mime-version:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=scoXnUwxI/FgM7mWZT5xvux0UiQVNPsQ70USevP9uaQ=; b=FW+7eS80klRJ9L2f2kUqXpTMw4bNahvMagsB8GYAozFRJw6Oe4XyA2HWgHsHQWz+RP F4UfNZtDFjZxwDCNYLfRzadMmAoueWUlcGNEgWLkbTiWqx4fDmbKjtxLfZ6LpYAayniY GsY3sF9Z/HlRFtEa3QEqoa5HYU/jnGx0eGnWZHOZ9dNwKGFEgxaJWNnG3HljhRZxliGy qtLyR4JzTo0366Oam/Gbx5Zqw9hrk23QlfzDMU2eQ+6OyNguR1Tz9Ro2K9pdVnWt5bmp URO+XKw/909v0TggTbpwGF/bBXm/RE8Ox4/hKbF1RgI5wIIR2 X-Gm-Message-State: ALoCoQmosxZ81utWBH2NJ2Qzry4rI5iemSqellwQ6stjN3EEdkO4LFT2pCb7EVukopVtF6BHwKCM X-Received: by 10.236.40.112 with SMTP id e76mr85812518yhb.5.1426711985241; Wed, 18 Mar 2015 13:53:05 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.10.220 with SMTP id 89ls631588iok.5.gmail; Wed, 18 Mar 2015 13:53:04 -0700 (PDT) X-Received: by 10.42.27.14 with SMTP id h14mr12426272icc.19.1426711984579; Wed, 18 Mar 2015 13:53:04 -0700 (PDT) Original-Received: from smtp-p01.blackberry.com (smtp-p01.blackberry.com. [208.65.78.88]) by mx.google.com with ESMTP id r2si3109092igh.60.2015.03.18.13.53.04 for ; Wed, 18 Mar 2015 13:53:04 -0700 (PDT) Received-SPF: none (google.com: tvaneerd@blackberry.com does not designate permitted sender hosts) client-ip=208.65.78.88; Original-Received: from xct108cnc.rim.net ([10.65.161.208]) by mhs212cnc.rim.net with ESMTP/TLS/AES128-SHA; 18 Mar 2015 16:53:01 -0400 Original-Received: from XMB122CNC.rim.net ([fe80::28c6:fa1c:91c6:2e23]) by XCT108CNC.rim.net ([fe80::8dc1:9551:6ed8:c618%17]) with mapi id 14.03.0210.002; Wed, 18 Mar 2015 16:53:00 -0400 Thread-Topic: [std-proposals] virtual constexpr fields Thread-Index: AQHQYbxwdGJu00ca1kWgvOjM6ZmGsJ0itz7A In-Reply-To: Accept-Language: en-CA, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.65.160.251] X-Original-Sender: tvaneerd@blackberry.com X-Original-Authentication-Results: mx.google.com; spf=none (google.com: tvaneerd@blackberry.com does not designate permitted sender hosts) smtp.mail=tvaneerd@blackberry.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:17081 Archived-At: --_000_11B5438424E807459A28BD01C92339BA759E907CXMB122CNCrimnet_ Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable I would say it needs to be constexpr or static =E2=80=93 at least for it to= be stored in the vtable. (ie assuming the vtable is shared by all instances of the class means that = the value is shared by all instances of the class) From: Gabriel Dos Reis [mailto:gdr@microsoft.com] Sent: Wednesday, March 18, 2015 4:42 PM To: 'std-proposals@isocpp.org'; C++std-ext; ehsan@mozilla.com Subject: [c++std-ext-16622] Re: [std-proposals] virtual constexpr fields Does the non-static data member need to be constexpr if it is virtual? Why= ? -- Gaby From: 'Botond Ballo' via ISO C++ Standard - Future Proposals [mailto:std-pr= oposals@isocpp.org] Sent: Wednesday, March 18, 2015 1:26 PM To: C++std-ext; ehsan@mozilla.com; std-proposals@= isocpp.org Subject: [std-proposals] virtual constexpr fields There are cases where you would like to associate constants with a given ty= pe. Here is how you can currently do it in C++: struct Merchandise { virtual int price() { return 0; } }; struct Shoe : Merchandise { virtual int price() { return 10; } If you could do it this way instead: struct Merchandise { virtual constexpr int price =3D 0; }; struct Shoe : Merchandise { virtual constexpr int price =3D 10; } then the compiler could store the constant value directly in the vtable. Th= is would eliminate the double indirection involved in dereferencing a funct= ion vtable entry, the cost of running that function at runtime, and the cod= e size of the function in the compiled binary. Virtual fields would be limited to being constexpr, so that the value to be= placed in the vtable, like other values in the vtable, can be determined a= t compile time. Has this, or something similar, ever been proposed? Is there any interest i= n it? -- Botond and Ehsan -- --- 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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --=20 ---=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. Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposa= ls/. --_000_11B5438424E807459A28BD01C92339BA759E907CXMB122CNCrimnet_ Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

I would say it needs to b= e constexpr or static =E2=80=93 at least for it to be stored in the vtable.=

(ie assuming the vtable i= s shared by all instances of the class means that the value is shared by al= l instances of the class)

 <= /p>

 <= /p>

From: Gabriel = Dos Reis [mailto:gdr@microsoft.com]
Sent: Wednesday, March 18, 2015 4:42 PM
To: 'std-proposals@isocpp.org'; C++std-ext; ehsan@mozilla.co= m
Subject: [c++std-ext-16622] Re: [std-proposals] virtual cons= texpr fields

 

Does the non-static data = member need to be constexpr if it is virtual?  Why?<= /p>

 <= /p>

-- Gaby=

 <= /p>

From: 'Boton= d Ballo' via ISO C++ Standard - Future Proposals [mailto:std-proposals@isocpp.org]
Sent: Wednesday, March 18, 2015 1:26 PM
To: C++std-ext; ehsan@m= ozilla.com; std-proposals@isocpp.org
Subject: [std-proposals] virtual constexpr fields<= /p>

 

There are cases where you would like to associate constants with a given t= ype. Here is how you can currently do it in C++:

struct Merchandise {
    virtual int price() { return 0; }
};

struct Shoe : Merchandise {
    virtual int price() { return 10;
}

If you could do it this way instead:

struct Merchandise {
    virtual constexpr int price =3D 0;
};

struct Shoe : Merchandise {
    virtual constexpr int price =3D 10;
}

then the compiler could store the constant value directly in the vtable. Th= is would eliminate the double indirection involved in dereferencing a funct= ion vtable entry, the cost of running that function at runtime, and the cod= e size of the function in the compiled binary.

Virtual fields would be limited to being constexpr, so that the value to be= placed in the vtable, like other values in the vtable, can be determined a= t compile time.

Has this, or something similar, ever been proposed? Is there any interest i= n it?

-- Botond and Ehsan

 

--

---
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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals&#= 43;unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
Visit this group at http://groups.google.com/a/isocpp.org/group/std-proposals/.<= /p>

--

---
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
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