From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=uo7a=3T=gmail.com=jwakely.gcc@sourceware.org>
Received: from mail-ed1-x52b.google.com (mail-ed1-x52b.google.com [IPv6:2a00:1450:4864:20::52b])
	by sourceware.org (Postfix) with ESMTPS id AA1BC3858D28
	for <gcc-help@gcc.gnu.org>; Sat, 19 Nov 2022 10:12:22 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org AA1BC3858D28
Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com
Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com
Received: by mail-ed1-x52b.google.com with SMTP id v17so10254819edc.8
        for <gcc-help@gcc.gnu.org>; Sat, 19 Nov 2022 02:12:22 -0800 (PST)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=gmail.com; s=20210112;
        h=cc:to:subject:message-id:date:from:in-reply-to:references
         :mime-version:from:to:cc:subject:date:message-id:reply-to;
        bh=ajCKLs7N7NtDA3tAon8WxrzhI8XH4hIx0Ol4sp00zDA=;
        b=hGqAl/HAwpI2PPlOarZiqUifxPonENoG6SsmS0rVpGBRUSX3z8jz4/C6Y2XFMUQO4e
         wUIYOZs0zmmc7E81+hS1Rje/hzD4QsIS32SYsDdsJ/24JazFo2aeHfzqMgEW4GLLsB2q
         veILU9bw2yYEfmUO5H4Qo6sLnBor7T3rhvwFFAbb6vRyvFxXl/tHtuvWqGDltnOgUp96
         hjchPA5P1w0shq2Z6t6EFdqMWRo69mXn1PmNIhKCpgj4t0fBThxdi4oickdv4IL5RZzz
         g1t/Fqu2+Fpd7PtWnjebWoxRKF6f0+MUKslLRYNZCslHVfm499V0RRssCRTROsN3OVMC
         mfTg==
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
        d=1e100.net; s=20210112;
        h=cc:to:subject:message-id:date:from:in-reply-to:references
         :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id
         :reply-to;
        bh=ajCKLs7N7NtDA3tAon8WxrzhI8XH4hIx0Ol4sp00zDA=;
        b=3JUUlDHYtXCdbieV3YnC4hAEv/QcGIE82IrweJwOOzldHC7+Ew5mkDcCeya2ppDJeY
         n0L70O7lRqIVk4Tu9xK2HSeVD/Xvjis+rFwkA/kD4BcARvYhjqKzbiBabhnytGxrVTso
         N3AjzHjoIJe4KirwFjr8voyKgaYGce0N4WkGzYaTSxw+G5OgJH/1JKYgSsV8gyRONcvo
         rOiWL7ZET37Rp5ug9QYnFKbAtL6KfFlw06/Umyt6xc20qP+Sadi+BOF3embfRiX9+/b3
         skGikGYMttwCOq7zoe4ZVp0lwK2EgodecC1W9t6exla3t2SYvipEb9chsjXRJMyvDjw0
         G0pg==
X-Gm-Message-State: ANoB5pmyl1uGxeOOTDhUc7EiXqzyxx79tK3dow/U9OWXhePwicJ6A6AW
	qDyjLZGwMWtjN7zBY501PyTMIfDDaXGYBlpI4n4=
X-Google-Smtp-Source: AA0mqf63ST2472UXtskr4EOMD7w9EtoW1Z8KTkpa/xrwMFe1Wq1hWtHLk69QmRO6XchxWqElT5g49XrD8mrWxPKgmpQ=
X-Received: by 2002:aa7:dbc7:0:b0:45f:b80f:1fe8 with SMTP id
 v7-20020aa7dbc7000000b0045fb80f1fe8mr8872197edt.118.1668852741485; Sat, 19
 Nov 2022 02:12:21 -0800 (PST)
MIME-Version: 1.0
References: <CAJYFCiO-gkmFEw+pyPigb-NC6bYaqjLU2Eb9ZoQE2mOSNw9fNA@mail.gmail.com>
In-Reply-To: <CAJYFCiO-gkmFEw+pyPigb-NC6bYaqjLU2Eb9ZoQE2mOSNw9fNA@mail.gmail.com>
From: Jonathan Wakely <jwakely.gcc@gmail.com>
Date: Sat, 19 Nov 2022 10:12:08 +0000
Message-ID: <CAH6eHdQ+Xsxp=msP9HLr7t+BiN1jkvkNE5NThbaC_mTMWuL_nQ@mail.gmail.com>
Subject: Re: Guaranteed copy elision
To: Yubin Ruan <ablacktshirt@gmail.com>
Cc: gcc-help <gcc-help@gcc.gnu.org>
Content-Type: multipart/alternative; boundary="000000000000d6b8f005edd00e50"
X-Spam-Status: No, score=-0.7 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,HTML_MESSAGE,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org
List-Id: <gcc-help.gcc.gnu.org>

--000000000000d6b8f005edd00e50
Content-Type: text/plain; charset="UTF-8"

On Tue, 15 Nov 2022, 05:48 Yubin Ruan via Gcc-help, <gcc-help@gcc.gnu.org>
wrote:

> Hi,
>
> As mentioned in cppreference:
> https://en.cppreference.com/w/cpp/language/copy_elision
>
> it is guaranteed in c++17 that copy elision must be applied for some cases
> like
>
>     SomeBigObject SomeBigObject::Factory(...) {
>         SomeBigObject local;
>          ...
>          return local;
>     }
>

No, this is not guaranteed. As the cppreference page explains, it's only
guaranteed for copying temporaries. This is not a temporary.

Elision here is allowed, but not required. It's commonly implemented though.




> (examples taken from https://abseil.io/tips/11 )
>
> but not for cases like
>
>     SomeBigObject SomeBigObject::Factory(...) {
>         SomeBigObject local1;
>         SomeBigObject local2;
>          ...
>
>          if (cond_1) {
>              return local1;
>          } else {
>              return local2;
>          }
>     }
>
> For a c++ user, it is somewhat difficult to be 100% sure that copy elision
> / NVO is applied to the functions' return value above.
>
> To be sure that a object would not be copied, we usually write something
> like
>
>          SomeBigObject obj;
>          func(&obj);
>
> while in most of the cases a one-liner like
>
>          SomeBigObject obj = func();
>
> would suffice.
>
> Is there any language facility to help us guarantee that at compile time
> (such as some kind of static_assert() ) so that we can be confident writing
> those one-liner ?
>

Make it cheap to move, and then it will be moved instead of copied. The
move isn't always elided, but if it's cheap then the code will still be
efficient.


> I know that marking the copy constructor deleted would do the good, but
> copy is needed in some cases.
>
> Thanks
> Yubin
>

--000000000000d6b8f005edd00e50--