220 29047 article Path: news.gmane.org!.POSTED!not-for-mail From: mihailnajdenov@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Re: [propagate_const] Implicit conversion to T Date: Sun, 23 Oct 2016 03:12:00 -0700 (PDT) Lines: 140 Approved: news@gmane.org Message-ID: References: <1f43ffe8-b267-4c8c-8834-f3e60035f18d@isocpp.org> <7f61be69-4f4c-40a5-9199-805edc90b428@isocpp.org> <13a71b20-8fd3-4a29-9279-716a2c9082a6@isocpp.org> <7abb6203-6ca7-4e04-ab6a-dc6a60e223ca@isocpp.org> <60ae1a96-7a31-491d-bcf8-c81590544006@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_900_55946508.1477217520702" X-Trace: blaine.gmane.org 1477217538 4407 195.159.176.226 (23 Oct 2016 10:12:18 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 23 Oct 2016 10:12:18 +0000 (UTC) Cc: jbcoe@me.com, joseph.thomson@gmail.com, mihailnajdenov@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCUJ3A7GRAPRB4UZWLAAKGQEHEAG2JQ@isocpp.org Sun Oct 23 12:12:14 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yw0-f199.google.com ([209.85.161.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1byFl3-0007Xw-Fk for gclcip-std-proposals@m.gmane.org; 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Sun, 23 Oct 2016 03:12:03 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.107.32.129 with SMTP id g123ls3552062iog.31.gmail; Sun, 23 Oct 2016 03:12:02 -0700 (PDT) X-Received: by 10.36.225.205 with SMTP id n196mr1719376ith.0.1477217522202; Sun, 23 Oct 2016 03:12:02 -0700 (PDT) In-Reply-To: X-Original-Sender: MihailNajdenov@gmail.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:29047 Archived-At: ------=_Part_900_55946508.1477217520702 Content-Type: multipart/alternative; boundary="----=_Part_901_1734623081.1477217520702" ------=_Part_901_1734623081.1477217520702 Content-Type: text/plain; charset=UTF-8 On Sunday, October 23, 2016 at 6:09:54 AM UTC+3, joseph....@gmail.com wrote: > > On Saturday, 22 October 2016 21:08:50 UTC+8, mihailn...@gmail.com wrote: >> >> ... >> >> Const-propagation only really has value in cases of composition, where >> you want to ensure that, when an object is only accessible via a reference >> to const, that the constness propagates not only to its data members, but >> also any objects those data members indirectly reference. If we restrict >> our use of propagate_const to the data members of a class, then we get >> natural copying syntax, and callers have the flexibility to do with their >> copies what they want. >> > Even if we restrict it to members only, the facts it forbids copy, kills "natural copying syntax" - one will have to write a copy ctor and assignment op, fiddling with get_underlying and what not. struct S { propagate_const> p; }; VS struct S { S() = default; S(S&&) = default; S& operator=(S&&) = default; S(const S& o) : _p(get_underlying(o._p)) {} S& operator=(const S& o) { if(&o != this) { _p = get_underlying(o._p); } return *this; }; propagate_const> _p; }; And this is just for a single propagator! I reckon this is hard to teach and reserved to non-junior programmers. And of course, assignment const on non-const is possible. Basically propagate_const will work correctly only if there is a deep copy or probably copy on write. -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/bebb500d-5b71-48e5-8878-0e5fae129c41%40isocpp.org. ------=_Part_901_1734623081.1477217520702 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable


On Sunday, October 23, 2016 at 6:09:54 AM UTC+3, j= oseph....@gmail.com wrote:
On Saturday, 22 October 2016 21:08:50 UTC+8, mihailn...@gma= il.com wrote:
= ....

Const-propagation only really has value in cases of composition,= where you want to ensure that, when an object is only accessible via a ref= erence to const, that the constness propagates not only to its data members= , but also any objects those data members indirectly reference. If we restr= ict our use of propagate_const to the data members of a class, then we get = natural copying syntax, and callers have the flexibility to do with their c= opies what they want.

Even if we restrict it to members only, the facts it forbids copy, = kills=C2=A0"natural copying syntax" - one will have to write a co= py ctor and assignment op, fiddling with get_underlying and what not.

struc= t S
{
=C2=A0propagate_co= nst<shared_ptr<int>> p<= span class=3D"styled-by-prettify" style=3D"color: #660;">;
};=

VS

struct S
{
=C2=A0S(= ) =3D default;
=C2=A0S(S&&) =3D default;
=C2=A0S& operato= r=3D(S&&) =3D default;
=C2=A0S(const S& o) : _p(get_underlyi= ng(o._p)) {}
=C2=A0S& operator=3D(const S& o) { if(&o !=3D t= his) { _p =3D get_underlying(o._p); } return *this; };=

=C2=A0propagate_const<shared_ptr<int>> _p;
};
<= /span>

And this is just for a single propagator! I reckon this is hard to te= ach and reserved to non-junior programmers.

And of= course, assignment const on non-const is possible.
Basically propagate_const will work correctly only if there is a= deep copy or probably copy on write.

--
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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/bebb500d-5b71-48e5-8878-0e5fae129c41= %40isocpp.org.
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