220 5588 <03d81e38-a8ae-4c33-9bac-f5a4f9e66eac@isocpp.org> article Path: news.gmane.org!not-for-mail From: tomaszkam@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Grouping placeholders for bind Date: Sat, 27 Jul 2013 03:40:27 -0700 (PDT) Lines: 170 Approved: news@gmane.org Message-ID: <03d81e38-a8ae-4c33-9bac-f5a4f9e66eac@isocpp.org> References: <81385547-bb12-4ae6-9d12-82aaeb4b5ba0@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_37_12298667.1374921627853" X-Trace: ger.gmane.org 1374921628 5307 80.91.229.3 (27 Jul 2013 10:40:28 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 27 Jul 2013 10:40:28 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDNPVXXG6IGBBHGHZ2HQKGQELCH3E7Q@isocpp.org Sat Jul 27 12:40:31 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-qe0-f71.google.com ([209.85.128.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1V31vK-0007uN-Hu for gclcip-std-proposals@m.gmane.org; Sat, 27 Jul 2013 12:40:30 +0200 Original-Received: by mail-qe0-f71.google.com with SMTP id 1sf2830655qee.2 for ; Sat, 27 Jul 2013 03:40:29 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-beenthere:date:from:to:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:x-google-group-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=qf22kDN07IVwe3Fo+Lj08ZAHGEzr6HmsKkBDVD9uA2o=; b=pxhInYjdscongyeR0D9WBc5n0a50WmUMS3VuhkFEHLabpPf3ENpwhr7Em0vmvUG2f+ NEaTgosp4lM7fyfMd3S7lQXDKgM8zaqU4dmXSl8WDFTmRUT/61PbStm12+jw6lpI2YJm tlSJ7szHdGufxLHH71Xhk5OTYXI/g31ISDoVVDltSNt3Kj9l2AsLb8eQx4/w3CQiFFpE oozWT2oOdff8jZz/9i5cRD0Dnucn3sF38GuZmlnSypZJ+PPURLjuyhbEL4lJMlK5EE4Z mGbS7PXBngzYxi8FvwKTV9mCHlYwWcltv+5kEm8m36XZFhaA+vMsMjgD9sq9woX+1svY sWSQ== X-Received: by 10.224.163.14 with SMTP id y14mr54289874qax.3.1374921629236; Sat, 27 Jul 2013 03:40:29 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.129.233 with SMTP id nz9ls515471qeb.33.gmail; Sat, 27 Jul 2013 03:40:28 -0700 (PDT) X-Received: by 10.49.12.47 with SMTP id v15mr690430qeb.39.1374921628358; Sat, 27 Jul 2013 03:40:28 -0700 (PDT) In-Reply-To: X-Original-Sender: tomaszkam@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:5588 Archived-At: ------=_Part_37_12298667.1374921627853 Content-Type: text/plain; charset=ISO-8859-1 > The second argument N would mean actual number of argument provided to >> invocation of the bind (so we cane genereate index of all arguments) >> > Here is the problem. First, the number of (bound) arguments passed to > `std::bind` is irrelevant for the placeholders that are needed; these need, > if anything, to be determined from the passed callable object. Second, > notice that Jonathan specifically did *not* mention any indices. This > `std::placeholders::all` form requires a very different implementation that > simply forwards all arguments that are passed at the invocation of the > returned bind object. It cannot be done with the current interface and > numbered placeholders, simply because some functions don't even *have* an > upper bound on the number of arguments - variadic templates say hello. > > It can be done and to prove it I have prepared the almost standard conforming implementation of bind function in C++11 (tested on clang 3.2 and gcc 4.8.1) that supports customizable group placeholders. The thinks that are missing are: 1. bind - because its add nothing and implementation would be trivial 2. volatile, const volatile overloads of bind invoke operator - this is cause by lack of std::get overload for tuples which such qualification (const is supported). 3. _from is not variable template (not supported) The implementation can be found on: https://github.com/tomaszkam/proposals/tree/master/bind. Firstly the test cases can be found in test.cpp. That test includes varidatic template cases and also covers combination of group and non group placeholders. Firstly the new placeholders traits are placed in the file placeholder_traits.hpp. I decided to include new is_placeholder that forwards to std::is_placeholder traits to be sure that new group placeholders won't work with std::bind. You may also see default implementation of placeholder_positions that provides backward compatibility with single argument placeholders definied using only std::is_placeholder. To mark bind expression the std::is_bind_expression is used. The implementation of bind expression is placed in file bind.hpp. This implementation has no hardcoded placeholders types and depends only on placeholder traits. The definition of actual grouping placeholders can be found at placeholders.hpp. The _from is at this point alias template but should be changed to variable template in future. Are there any interest in defining group placeholders for the bind? If so I think the ability to define custom group placeholders by the user is must have, to avoid exponential grow of bind implementation. In the proposed approach implementation of _group<2, 3, 7> placeholder (which may be found pretty usefull) would look like: template struct group_placeholder {}; template struct is_placeholder> : true_type {}; template struct placeholder_positions, ArgCount> : integral_sequence { //static assert to avoid check if all Is are less ArgCount would be recomended but no actually needed. }; namespace placeholders { template group_placeholder _group; }; -- --- 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_37_12298667.1374921627853 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
The secon= d argument N would mean actual number of argument provided to invocation of= the bind (so we cane genereate index of all arguments)
Here is the problem. First, the number of (bound) arguments= passed to `std::bind` is irrelevant for the placeholders that are needed; = these need, if anything, to be determined from the passed callable object. = Second, notice that Jonathan specifically did not mention any indice= s. This `std::placeholders::all` form requires a very different implementat= ion that simply forwards all arguments that are passed at the invocation of= the returned bind object. It cannot be done with the current interface and= numbered placeholders, simply because some functions don't even *have* an = upper bound on the number of arguments - variadic templates say hello.
<= /div>
It can be done and to prove it I have prepared t= he almost standard conforming implementation of bind function in C++11 (tes= ted on clang 3.2 and gcc 4.8.1) that supports customizable group placeholde= rs. The thinks that are missing are:
  1. bind<R> - because i= ts add nothing and implementation would be trivial
  2. volatile, c= onst volatile overloads of bind invoke operator - this is cause by lack of = std::get overload for tuples which such qualification (const is supported).=
  3. _from<N> is not variable template (not supported)
Th= e implementation can be found on: https://github.com/tomaszkam/proposals/tree/mast= er/bind.

Firstly the test cases can be found in test.cpp. = That test includes varidatic template cases and also covers combination of = group and non group placeholders.

Firstly the new placeholders trait= s are placed in the file placeholder_traits.hpp. I deci= ded to include new is_placeholder  that forwards to std::is_placeholde= r traits to be sure that new group placeholders won't work with std::bind. = You may also see default implementation of placeholder_positions that provi= des backward compatibility with single argument placeholders definied using= only std::is_placeholder. To mark bind expression the std::is_bind_express= ion is used.

The implementation of bind expression is placed in file= bind.hpp. This implementation has no hardcoded placeholders types an= d depends only on placeholder traits.

The definition of actual group= ing placeholders can be found at placeholders.hpp. The _from&= lt;N> is at this point alias template but should be changed to variable = template in future.


Are there any interest in defining group pla= ceholders for the bind? If so I think the ability to define custom group pl= aceholders by the user is must have, to avoid exponential grow of bind impl= ementation. In the proposed approach implementation of _group<2, 3, 7>= ; placeholder (which may be found pretty usefull) would look like:

template<std::size_t.= .... Is>
struct group_placeholder {};

template<std::size_t..= ... Is>
struct is_placeholder<group_placeholder<Is...>> : = true_type {};

template<std::size_t.... Is, std::size_t ArgCount&g= t;
struct placeholder_positions<group_placeholder<Is...>, ArgCo= unt> : integral_sequence<std::size_t, Is...>
{
  = //static assert to avoid check if  all Is are less ArgCount would be = recomended but no actually needed.
};

namespace placeholders
{=
   template<std::size_t.... Is>
   group_p= laceholder<Is....> _group;
};

--
 
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