220 7866 article Path: news.gmane.org!not-for-mail From: dacamara.cameron@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Proposal to add std::mem_to_fun Date: Tue, 19 Nov 2013 20:41:15 -0800 (PST) Lines: 286 Approved: news@gmane.org Message-ID: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_Part_2561_13557628.1384922475031" X-Trace: ger.gmane.org 1384922474 20530 80.91.229.3 (20 Nov 2013 04:41:14 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 20 Nov 2013 04:41:14 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBC63VLONXYMRB272WCKAKGQEQIXXN4I@isocpp.org Wed Nov 20 05:41:21 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f71.google.com ([209.85.220.71]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1VizbK-0007iM-FE for gclcip-std-proposals@m.gmane.org; Wed, 20 Nov 2013 05:41:18 +0100 Original-Received: by mail-pa0-f71.google.com with SMTP id kp14sf6087842pab.2 for ; Tue, 19 Nov 2013 20:41:17 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:message-id:subject:mime-version:x-original-sender :reply-to:precedence:mailing-list:list-id:list-post:list-help :list-archive:list-subscribe:list-unsubscribe:content-type; bh=6GmxszPHYekldt4wAhEaM0dtElB9Y2psxDU4ziLYXlo=; b=trxC0GgDik70T48s2vYQyKSUp3SjfaISlmYJzwk9smMW9I+7mNB3UfF/fKQL4Rbveo SOBhDqptC2wDFhNXmmjJckFyFGme/087qnavG10DiPgm9d6z3NKg837hcKGIXbxHruTq pCvRf/s3GJdcE1QeZ+HSKKaH0isYXQgqF84mSkaqUjhDSWJxZU3kQLV7Sa3G/dITRPWi jOXaLdY3HzGHLPsrDaNCvGhRyu3EwumwZyPvmjyZKfKmc1SrJCxU1IM6x00KzfTzqM8c 2zUxecMzCMz4bI7QYeByYHfdgo/GtDGmN0pFMNK3cR4jR4wkKIMz7VyPBGDsSS5JWod/ 5fCA== 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:message-id:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :list-post:list-help:list-archive:list-subscribe:list-unsubscribe :content-type; bh=6GmxszPHYekldt4wAhEaM0dtElB9Y2psxDU4ziLYXlo=; b=BqsqaD5AmAVLkh3v8y+yksX0D1olg4K4lGS5hXMr0Jb/XahKVABcS+QA2UMW6dObr9 a+7+c+qAByt67hp7BTLUMFcL58HxzzFElZGwJSzxs+CJODyo9C/kOmx0AMhOEK6vUyMZ LmBtIBKlF8JVNGx5huyWmb78K26NXUEPIFntrOq/VTIkBePyXB2KevMxMd3qQtvEuLVZ V9dxSDKzWPq8GLc2gOrjzY/Q5lLxnwq6Hnb3Kx6p3LHSYCCFt17cKKFjRn8XJTW5wPKu ke6QfpcsAkjEDgw6p1wwI7t7TB4S08T5XWPXFm/27XptkN0HvgELvpDhGFVYXksOCP2y hRcw== X-Gm-Message-State: ALoCoQm8xZPos5B5aINroCdzK8uGoNcC798jmhvJo2QMBbNS6h8DVeJwDgN8azxPWgq24J0rsrp+ X-Received: by 10.66.65.109 with SMTP id w13mr11314619pas.21.1384922477159; Tue, 19 Nov 2013 20:41:17 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.182.241.202 with SMTP id wk10ls2012134obc.0.gmail; Tue, 19 Nov 2013 20:41:15 -0800 (PST) X-Received: by 10.182.231.227 with SMTP id tj3mr75obc.40.1384922475755; Tue, 19 Nov 2013 20:41:15 -0800 (PST) X-Original-Sender: dacamaracameron@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:7866 Archived-At: ------=_Part_2561_13557628.1384922475031 Content-Type: text/plain; charset=ISO-8859-1 So I noticed browsing that there is currently not a way to create a function object that can return just a member of an object. There is a structure that converts a member function into a function object callable via the class as an argument (std::mem_fun_t). My question is why can't this exist for accessing members of a POD struct or even a class? That could be useful for standard algorithms when you need access to a specific member but don't wanna write out the lambda for it, eg: struct Books { std::string author; std::string title; }; // all we want is to make a set of titles of books from our library vector std::set uniqTitles; std::transform(library.begin(), library.end(), std::inserter(uniqTitles,uniqTitles ..end()), [] (const Books& b) -> std::string { return b.title; } ); // why can't there be a standard way to just grab the member? so really the implementation / interface would look a lot like std::mem_fun: template struct mem_to_fun_t { M T::*_mem; explicit mem_to_fun_t(M T::*mem): _mem(mem) { } const M& operator()(const T& c) const{return c.*_mem;} M& operator()(T& c) {return c.*_mem;} }; template inline auto mem_to_fun(M T::*mem) -> mem_to_fun_t { return mem_to_fun_t{mem}; } it doesn't have to be exactly like this but perhaps a similar implementation. Here is the ideone link if you wanna see how it could be used: http://ideone.com/3uoDPV -- --- 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_2561_13557628.1384922475031 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable
So I noticed browsing <functional> that there is cur= rently not a way to create a function object that can return just a member = of an object. There is a structure that converts a member function into a f= unction object callable via the class as an argument (std::mem_fun_t). My q= uestion is why can't this exist for accessing members of a POD struct or ev= en a class? That could be useful for standard algorithms when you need acce= ss to a specific member but don't wanna write out the lambda for it, eg:
struct Books {
=   std= ::string author;
  std::string title;
};


// all we want is to make a set of titles of books from o= ur library vector
std::set<= /span><= std::string> uniqTitles;
std::transform(library.begin<= /span>(),<= span style=3D"color: #000;" class=3D"styled-by-prettify"> library.end(), std::inserter(uniqTitles, = uniqTitles= ..end()),
    &nb= sp;          [] (const Books= & b) -> std::string
               {
      &nb= sp;           return b.title;
&n= bsp;              }
); // why can't there be a standard way to just grab the member?

so really the implementation / = interface would look a lot like std::mem_fun:

template= <typename T, typename M>
struct= mem_to_fun_t {
&= nbsp; M T:= :*_mem;
  explicit mem_to_fun_t(M T::*mem): _mem(mem) { }
 
  const M& operator()(const T& c) const{return c.*_mem;}=
 = M& operator()(T& c) {ret= urn c.*_mem;}
};


template <typename T= , <= span style=3D"color: #008;" class=3D"styled-by-prettify">typename M> inline
auto mem_to_fun(M T::*mem<= /span>) -> mem_to_fun_t<T,M> = {
 = ; return mem_to_fun_= t<T,M>{mem};
}

it does= n't have to be exactly like this but perhaps a similar implementation.

Here is the ideone link if you wanna see how it could = be used: http://ideone.com/3uoDPV=


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