220 5537 <1ba31ea2-e3a1-4c3f-9e46-e6150c1377cc@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: Tue, 23 Jul 2013 22:47:30 -0700 (PDT) Lines: 129 Approved: news@gmane.org Message-ID: <1ba31ea2-e3a1-4c3f-9e46-e6150c1377cc@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_376_302164.1374644850981" X-Trace: ger.gmane.org 1374644853 779 80.91.229.3 (24 Jul 2013 05:47:33 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 24 Jul 2013 05:47:33 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDNPVXXG6IGBB46UXWHQKGQEJPEFZNY@isocpp.org Wed Jul 24 07:47:36 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yh0-f72.google.com ([209.85.213.72]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1V1rvB-0003eJ-V0 for gclcip-std-proposals@m.gmane.org; Wed, 24 Jul 2013 07:47:34 +0200 Original-Received: by mail-yh0-f72.google.com with SMTP id z20sf2982518yhz.11 for ; Tue, 23 Jul 2013 22:47:33 -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=jMlv2Bt9mbxFhA2aBUPEsHG5L8U+G7KjOC48JtHklXM=; b=hZFAlpFyotWQhYsPF8P/MtEt95UA88A4r/Sz2eRRHf9yqawCK3HKNubcIp5cTT+zZi 70fH0l7/BniaZTNc6HJxFcyoXXsQ6M9Xcobgw81cEfOxaoRyemzuPoYa+5WbxMsyROkn 8C+iXoDPFGnITEKZwy6P220c27LkhLUtHnR9fSVxUV4TU48FnMZBQECbRNs7Y3l8oN18 7bo0YBx4L5FuFbrzW3bkNLSbg6jzvXiShPFpGXjWo3l69GEm/aDfVuIr5AhIqQrsqAxI 5scbAg9/b+aRK0AFVNx8bf280RwfiVjkp5nN/xf7HVzxzD0w0af3582rtJfUvomrXoHa qX7w== X-Received: by 10.224.86.200 with SMTP id t8mr49163735qal.0.1374644852814; Tue, 23 Jul 2013 22:47:32 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.48.170 with SMTP id m10ls90656qen.41.gmail; Tue, 23 Jul 2013 22:47:31 -0700 (PDT) X-Received: by 10.49.99.65 with SMTP id eo1mr254956qeb.3.1374644851489; Tue, 23 Jul 2013 22:47:31 -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:5537 Archived-At: ------=_Part_376_302164.1374644850981 Content-Type: text/plain; charset=ISO-8859-2 Content-Transfer-Encoding: quoted-printable W dniu wtorek, 23 lipca 2013 23:22:39 UTC+2 u=BFytkownik Xeo napisa=B3: > > I have played around with implementing such an `easy_bind` (as I call it)= =20 > before, which can be found hereand is really easy (especially in C++14 when you can just use=20 > `decltype(auto)` for the return type). > > You will notice that I used `std::function` instead of a=20 > generic `class F` in the public interface - the reason is that not all=20 > function objects can have the number of their parameters determined. The= =20 > `operator()` may be either overloaded, templated, or both, rendering any= =20 > attempt to get a specific signature moot. > > Of course, the overhead (if actually there) of `std::function` is likely= =20 > unwanted in such a general purpose function. However, there are really on= ly=20 > two callable entities where the signature can be inspected:=20 > function-pointers and member-function-pointers. As such, with the interfa= ce=20 > of `std::bind` today, you can't really do better than that. > > The second argument N would mean actual number of argument provided to=20 >> invocation of the bind (so we cane genereate index of all arguments) >> > Here is the problem. First, the number of (bound) arguments passed to=20 > `std::bind` is irrelevant for the placeholders that are needed; these nee= d,=20 > if anything, to be determined from the passed callable object. Second,=20 > notice that Jonathan specifically did *not* mention any indices. This=20 > `std::placeholders::all` form requires a very different implementation th= at=20 > simply forwards all arguments that are passed at the invocation of the=20 > returned bind object. It cannot be done with the current interface and=20 > numbered placeholders, simply because some functions don't even *have* an= =20 > upper bound on the number of arguments - variadic templates say hello. > The sentence should look like (I ommited it part while writing): The second argument N would mean actual number of argument provided to=20 the operator() of functor created as a result of bind. The expansion of the group placeholder is only done with the context when= =20 bind result is invoked so the number of arguments that was passed to it=20 would be enough to generate indicies that will forward alll of the and=20 placeholders::all can be implemented. --=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_376_302164.1374644850981 Content-Type: text/html; charset=ISO-8859-2 Content-Transfer-Encoding: quoted-printable

W dniu wtorek, 23 lipca 2013 23:22:39 UTC+2 u=BFytkownik Xeo napisa= =B3:
I have played around with = implementing such an `easy_bind` (as I call it) before, which can be found = he= re and is really easy (especially in C++14 when you can just use `declt= ype(auto)` for the return type).

You will notice that I used `std::function<R(FArgs...)>` instead of a generic `class F` in the public interface - the reason is that not all=20 function objects can have the number of their parameters determined. The `operator()` may be either overloaded, templated, or both, rendering=20 any attempt to get a specific signature moot.

Of course, the=20 overhead (if actually there) of `std::function` is likely unwanted in=20 such a general purpose function. However, there are really only two callabl= e entities where the signature can be inspected:=20 function-pointers and member-function-pointers. As such, with the interface= of `std::bind` today, you can't really do better than that.

The second argument N would mean actual number of argument provid= ed to invocation of the bind (so we cane genereate index of all arguments)<= /span>
Here is the problem. First, the number o= f (bound) arguments passed to `std::bind` is irrelevant for the placeholder= s that are needed; these need, if anything, to be determined from the passe= d callable object. Second, notice that Jonathan specifically did not= mention any indices. This `std::placeholders::all` form requires a very di= fferent implementation that simply forwards all arguments that are passed a= t the invocation of the returned bind object. It cannot be done with the cu= rrent interface and numbered placeholders, simply because some functions do= n't even *have* an upper bound on the number of arguments - variadic templa= tes say hello.
The sentence should look like (I ommi= ted it part while writing):
  The second argument N would mean actu= al number of argument provided to the operator() of functor created as a re= sult of bind.
The expansion of the group placeholder is only done with t= he context when bind result is invoked so the number of arguments that was = passed to it would be enough to generate indicies that will forward alll of= the and placeholders::all can be implemented.

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