220 22468 article Path: news.gmane.org!not-for-mail From: leni536@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: std::vector (and other containers) to have a perfect forwarding constructor Date: Wed, 11 Nov 2015 09:09:44 -0800 (PST) Lines: 146 Approved: news@gmane.org Message-ID: References: <14fb89ea-443b-4696-93ec-29889f642bca@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2761_1580705303.1447261784243" X-Trace: ger.gmane.org 1447261788 3614 80.91.229.3 (11 Nov 2015 17:09:48 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 11 Nov 2015 17:09:48 +0000 (UTC) Cc: mwoehlke.floss@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDXJR44QUQHBBWPMRWZAKGQETZKOD4Y@isocpp.org Wed Nov 11 18:09:48 2015 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-io0-f197.google.com ([209.85.223.197]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1ZwYu3-0002jU-IV for gclcip-std-proposals@m.gmane.org; Wed, 11 Nov 2015 18:09:47 +0100 Original-Received: by iofh3 with SMTP id h3sf68261561iof.1 for ; Wed, 11 Nov 2015 09:09:46 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp_org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:content-type:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=tbbkvZF+dopy8CNXQtViqlL+s2BzpaKqbzX27jLd11Y=; b=gyOQ9NvJuknsqCECf3nc6Ucl0yoL2H/9lxDblgfoupKi1s5YrmPkKrb0cD0jUwCojg a9O0SZ+/vgHPglIHrffS60uMGXhdT7ltfVozXgR+vIukh5MoffuwdnI4m2lKxIkIW7Kl hDR5atGhPlpw37pdud2FTnXKlLwUwpLa/8bOuxKGSPqDwFbvYmMiEbOVvRQXM+oNFgun J5x5HlfVYdSDv+KYe2REbElimsYgs3ktROPic8FH2b1zc/d6yVpN+63S3lSdkTImJuS3 eOSVrE7920DiTPgGQs7im+YBwgYK5mYl1hXvPF2ZVthJh6UhOnTL0N1ugbDATerZbcSE h6vg== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:content-type:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=tbbkvZF+dopy8CNXQtViqlL+s2BzpaKqbzX27jLd11Y=; b=cMhb3shwwdP6Z8wUAp9+D5JIr5zWPXM5q4+mti3P6DtDoq4EKBiKta+Yirgv/sGxSS yqFIXcH2PMeUZ3LxsuD3mu+KubvtTTrrZwPAVgKd2tyWxl+sXaAZJnI6oKjFH0IFjObS dCTO0WFWk5h6+ekWmV6MU+KSGgFwKAw8IDfv+EpRIEikCCAQUiQB2Z7xHF1Nm+dR61QY FSt/70G5R9Ze8bNmRvzKA7+lEgFFYixQt/dXOR0q2zv1wOM1wA/5V29Pa2mWGXGpFiO7 kp14g+ZmDALzMV1cU0mzCWF2RYZD1nVB04UUUcIMlBqlWdeIYaDUBU2f4CLEeh5oUL5Q doXw== 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:cc:message-id:in-reply-to :references:subject:mime-version:content-type:x-original-sender :reply-to:precedence:mailing-list:list-id:x-spam-checked-in-group :list-post:list-help:list-archive:list-subscribe:list-unsubscribe; bh=tbbkvZF+dopy8CNXQtViqlL+s2BzpaKqbzX27jLd11Y=; b=azcDO1KwgV0P52x8uTLfpHq4wovD45+736BsLBeCBWGH6trEwx2ZJoGDdvlH6pl1/x 5rnbsB05tJmGIvgIKnDs6xeXgXu8+t0jGz2q39ZagmPgwMghehdR+imwxucfM0nFsUUd m5Ei0REI3xkEqQsoT+5BN937x++fVFSfU11YqYrtWh1jhC4m6qU6NOVcajBi7DneRDi8 FKkhe9BNt44kdeLpVtWu4xuHW82U9N6aBPhgMWLrNIvZrbwtJD62Ih1q8O0wmwA0N/TE cbYDcSBnZP8C3rrtQa2MwpG63YQdXETZ87rm7ENDMSDTuvFMUyPeLwUO8JprTYG09S4D 2W7w== X-Gm-Message-State: ALoCoQl0hd5zAdKXNrzwCm1CfZXMUwm3O3vh9eznd+PjA2RwU2Mw1YS0wWC3Jm5AutkS8Fsw5F5t X-Received: by 10.107.8.221 with SMTP id h90mr9663889ioi.29.1447261786534; Wed, 11 Nov 2015 09:09:46 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.2.22 with SMTP id 22ls195756ioc.58.gmail; Wed, 11 Nov 2015 09:09:45 -0800 (PST) X-Received: by 10.50.3.41 with SMTP id 9mr790284igz.7.1447261785150; Wed, 11 Nov 2015 09:09:45 -0800 (PST) In-Reply-To: X-Original-Sender: leni536@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:22468 Archived-At: ------=_Part_2761_1580705303.1447261784243 Content-Type: multipart/alternative; boundary="----=_Part_2762_1956493067.1447261784243" ------=_Part_2762_1956493067.1447261784243 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Wednesday, November 11, 2015 at 3:36:38 PM UTC+1, Matthew Woehlke wrote: > > On 2015-11-11 03:52, len...@gmail.com wrote:=20 > > I didn't find a topic like this, so here it is. Something like this=20 > would=20 > > be nice:=20 > >=20 > > template =20 > > vector(size_type n,Args&&... args);=20 > >=20 > > 1) It could be faster than vector(n,value_type(args)) if value_type's= =20 > copy=20 > > constructor is more expensive than its constructor with "args"=20 > parameters=20 > > (I don't know a good example though).=20 > > Interesting idea. And since vector(n, value_type(...)) invokes=20 > value_type's copy ctor, it is in fact a subset of the proposed syntax,=20 > i.e. that form could be deprecated / removed in favor of the proposed=20 > "emplace" version. I don't think it's possible because of the optional "const allocator_type&"= =20 argument. I don't know if it's explicitly forbidden or not, but my hunch is= =20 that optional and variadic parameters don't mix well.=20 I think I made mistake in the usage of my "forward_for_copy" type trait,=20 one can use the following function instead: template constexpr decltype(auto) forward_for_copy(std::remove_reference_t& x) noexcept { using ReturnType=3Dstd::conditional_t< std::is_rvalue_reference{}, const T&,=20 T &>;=20 return static_cast(x); } Use similarly to "std::forward". What it does: 1. Leaves lvalue references alone. 2. Static casts all rvalue references to const lvalue references. - L=C3=A9n=C3=A1rd --=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/. ------=_Part_2762_1956493067.1447261784243 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable

On Wednesday, November 11, 2015 at 3:36:38 PM UTC+1, Matthew Woehlk= e wrote:
On 2015-11-11 03:52, <= a href=3D"javascript:" target=3D"_blank" gdf-obfuscated-mailto=3D"nEAqKGcxF= wAJ" rel=3D"nofollow" onmousedown=3D"this.href=3D'javascript:';retu= rn true;" onclick=3D"this.href=3D'javascript:';return true;">len...= @gmail.com wrote:
> I didn't find a topic like this, so here it is. Something like= this would=20
> be nice:
>=20
> template <class... Args>
> vector(size_type n,Args&&... args);
>=20
> 1) It could be faster than vector(n,value_type(args)) if value_typ= e's copy=20
> constructor is more expensive than its constructor with "args= " parameters=20
> (I don't know a good example though).

Interesting idea. And since vector(n, value_type(...)) invokes
value_type's copy ctor, it is in fact a subset of the proposed synt= ax,
i.e. that form could be deprecated / removed in favor of the proposed
"emplace" version.

I don'= t think it's possible because of the optional "const allocator_typ= e&" argument. I don't know if it's explicitly forbidden or= not, but my hunch is that optional and variadic parameters don't mix w= ell.=C2=A0

I think I made mistake in the usage of = my "forward_for_copy" type trait, one can use the following funct= ion instead:
template<typename T>
constexp= r
decltype(auto)
forward_for_copy(std::remove_reference_t<T>& x) noexc= ept
{
=C2=A0 =C2=A0 =C2=A0 =C2=A0 using ReturnType=3Dstd::conditional_t<
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 = =C2=A0 =C2=A0 std::is_rvalue_reference<T&&>{},
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2= =A0 const T&,=C2=A0
=C2=A0 =C2=A0 = =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 T &>;=C2=A0
=C2=A0 =C2=A0 =C2=A0 =C2=A0 return static_cast<Retu= rnType>(x);
}

Use similarly to "std::forward"= .. What it does:
  1. Leav= es lvalue references alone.
  2. Static casts all rvalue references to const lvalue references.<= /li>
- L=C3=A9n=C3=A1rd

--

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