220 14925 <356c4eaa-6d79-4938-869b-b3f8627cfa32@isocpp.org> article Path: news.gmane.org!not-for-mail From: sasho648 Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Earlier creation of function return-value. Date: Tue, 9 Dec 2014 14:45:23 -0800 (PST) Lines: 367 Approved: news@gmane.org Message-ID: <356c4eaa-6d79-4938-869b-b3f8627cfa32@isocpp.org> References: <3c0ee2e3-8706-44b0-a3d2-985e494dd8ac@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_5717_1363659495.1418165124119" X-Trace: ger.gmane.org 1418165132 22829 80.91.229.3 (9 Dec 2014 22:45:32 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Tue, 9 Dec 2014 22:45:32 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDBNZXHN2QPRBBHXTWSAKGQEROQZF4Q@isocpp.org Tue Dec 09 23:45:27 2014 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pd0-f198.google.com ([209.85.192.198]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1XyTX4-0005Yr-19 for gclcip-std-proposals@m.gmane.org; Tue, 09 Dec 2014 23:45:26 +0100 Original-Received: by mail-pd0-f198.google.com with SMTP id fp1sf8247338pdb.9 for ; Tue, 09 Dec 2014 14:45:25 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:content-type:x-original-sender:reply-to :precedence:mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=+xa8swto3OxBCBP/TUaVXtX9CI9lGFqBCRM6v35AYAQ=; b=TIo/mDo87F3iq7rcK2ZAPz3r3oKf9fF2GDHtRvZDZdhaQng8eeYhuX21R8CwuX/ndT oALzXwSc5diHGLoKwM9FEJR7MQx1cosVud56mTgMGJ7TiYiqGRJwrPf87fdn36wIW0HU VQl5d+9hDXG0rfDjgVoFKAqlgcHIMDWvl5TVctwjE2YsWp3f9FkjeKJjnoZDEp9nZ6XT swGKb4xmOYptOssr7Jh6l8OcZUUzUDuZ/O+c2/E9+n3KS0gXyU8V4KDTFQTYrF9KE2lX U6RapMapbPnl9D7WqH6Sn7wl12sOaDzOda/3RlVuH+g1FPP6NUlfSEXIrkDLHaIigd0m HCTg== X-Gm-Message-State: ALoCoQnqcp+dSA5bRgbqa3OuwnQtwnkGcZMT1ESUw8qrfmijCzy3CYjCZLHmM2Tbv2eC/ak3KXKu X-Received: by 10.66.248.229 with SMTP id yp5mr950553pac.12.1418165125291; Tue, 09 Dec 2014 14:45:25 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.140.105.202 with SMTP id c68ls359632qgf.62.gmail; Tue, 09 Dec 2014 14:45:24 -0800 (PST) X-Received: by 10.140.18.178 with SMTP id 47mr12503qgf.9.1418165124528; Tue, 09 Dec 2014 14:45:24 -0800 (PST) In-Reply-To: <3c0ee2e3-8706-44b0-a3d2-985e494dd8ac@isocpp.org> X-Original-Sender: sasho648@mail.bg 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:14925 Archived-At: ------=_Part_5717_1363659495.1418165124119 Content-Type: multipart/alternative; boundary="----=_Part_5718_1079651681.1418165124120" ------=_Part_5718_1079651681.1418165124120 Content-Type: text/plain; charset=UTF-8 What about - when we don't know exactly what local will be returned? In this case (at least my GCC compiler, even with '-02') haven't implemented any 'RVO'. By using a named variable which represents the return value, we can store one of those temporaries directly into it and in one of the cases no copying will be performed. Here is what I mean: struct S { S () { pSomeMemory = new char[250], cout << "S::S()" << endl; } S (const S &arg) : S() { memcpy(pSomeMemory, arg.pSomeMemory, sizeof( char) * 250), cout << "S::S(const S &arg)" << endl; } S (S &&arg) : S() { pSomeMemory = arg.pSomeMemory, arg.pSomeMemory = nullptr, cout << "S::S(S &&arg)" << endl; } ~S () { delete[] pSomeMemory; } char *pSomeMemory; }; S FunctionReturningS(bool b) { //the below 2 temporaries must be constructed S temp; S temp1; //store some data at 'temp' and 'temp1' temp.pSomeMemory[0] = '\0'; temp1.pSomeMemory[5] = '6'; return (b ? (S&&)temp : (S&&)temp1); // return either 'temp' or 'temp1' } Life example . Using my new syntax we can optimize 'FunctionReturningS' like this: S FunctionReturningS(bool b) return { //the below 1 temporarie + 'return' must be constructed S temp; //store some data at 'temp' and 'return' temp.pSomeMemory[0] = '\0'; return.pSomeMemory[5] = '6'; if(!b) return; //just exit the function (temp1 is replaced by 'return') return.~return(); //calling explicit destructor on 'return' new (&return) S((&&)temp); //placement new, re-constructs 'return' by 'temp' } As in my case if 'b' is 'false' no copy-construction will be done. -- --- 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 email 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-proposals/. ------=_Part_5718_1079651681.1418165124120 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
What about - when we don't know exactly what local will be= returned? In this case (at least my GCC compiler, even with '-02') haven't= implemented any 'RVO'. By using a named variable which represents the retu= rn value, we can store one of those temporaries directly into it and in one= of the cases no copying will be performed. Here is what I mean:

struct S
{
    S () = { pSomeMem= ory =3D new char[250], cout << "S::S(= )" = << endl; }

    S (const S &arg) : S() { memcpy(pSomeMemory, arg.pSomeMemory, sizeof(char) * <= /span>250<= span style=3D"color: #660;" class=3D"styled-by-prettify">), cout << "S::S(const S &arg)" << endl; }

    S (S &&arg= ) <= span style=3D"color: #660;" class=3D"styled-by-prettify">: S() { pSomeMemory =3D arg.pSom= eMemory, arg<= span style=3D"color: #660;" class=3D"styled-by-prettify">.pSomeMemory =3D nullptr, cout << "S::S(S &&arg)" << endl; = }

  &nbs= p; ~S () { delete[] pSomeMemory; }

   = char *pSomeMemory;
};


S FunctionReturningS(bool b)
{
  &nb= sp; //the = below 2 temporaries must be constructed

    S temp;

    S temp1<= span style=3D"color: #660;" class=3D"styled-by-prettify">;

    //store some da= ta at 'temp' and 'temp1'

    temp.pSomeMemory[0] =3D '\0';

    temp1.pSomeMemory[5] =3D = '6'= ;

    return<= span style=3D"color: #000;" class=3D"styled-by-prettify"> (b ? (S&&)temp :<= /span> (S&&)temp1); // return either 'temp' or 'temp1'
}

Life example.

<= div>Using my new syntax we can optimize 'Functi= onReturningS' like this:

S FunctionReturningS<= /span>(bool b) return
{
    //the below 1 temporarie + 'return' must be constructed=

  &n= bsp; S temp;

=     //store some data at 'temp' and 'return'

    temp.pSomeMemory[0] =3D = '\0';

    return= .pSomeMemory[5] =3D '= 6';=

  &n= bsp; if(!b) return; //just exit the function (temp1 is replaced by 'return')

    = return.~return(); //calling explicit destructor on 'return'<= /span>

&nb= sp;   new (&return) S((&&)temp); //placement new, re-constructs 'return' by 'temp'
}
=

As= in my case if 'b' is 'false' no copy-construction will be done.

--

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To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+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/<= /a>.
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