220 7869 article Path: news.gmane.org!not-for-mail From: Jeffrey Yasskin Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposal to add std::mem_to_fun Date: Wed, 20 Nov 2013 00:51:58 -0500 Lines: 71 Approved: news@gmane.org Message-ID: References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 X-Trace: ger.gmane.org 1384926735 27070 80.91.229.3 (20 Nov 2013 05:52:15 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Wed, 20 Nov 2013 05:52:15 +0000 (UTC) To: std-proposals@isocpp.org Original-X-From: std-proposals+bncBDDM34EO6QDRBEU4WGKAKGQEIUX3HXA@isocpp.org Wed Nov 20 06:52:22 2013 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ie0-f200.google.com ([209.85.223.200]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1Vj0i4-00046N-EQ for gclcip-std-proposals@m.gmane.org; Wed, 20 Nov 2013 06:52:20 +0100 Original-Received: by mail-ie0-f200.google.com with SMTP id at1sf4415843iec.11 for ; Tue, 19 Nov 2013 21:52:19 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20120113; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :x-original-sender:x-original-authentication-results:reply-to :precedence:mailing-list:list-id:list-post:list-help:list-archive :list-subscribe:list-unsubscribe:content-type; bh=lma9El9NrFvyGGTU0kueiSVQOHlV9qeoEMudAwUuzCQ=; b=XSHsG2MfB7/VyMJbUFMg9Ow+YaqzDJPj5fu3vlpGdoaJ4R/sMf4n9IWyq/KBKmUYxh c+vu3XwWGFEnhnyHJIVB3+8s3MLeoyru8fJkCApQkzrxxTv8SFeu3oPtK2tScfkGWwhf 6cc7LcHgD0NCaQSJzBs1GvqYNAUuMsJJydd2D3vvVaicgflxF2uVRyl2l7TE8CF/4HyY t9PrnIaCs9PB7nK8CUpRJZtgioKEIz6FIaT+GjvGRJfzoOo+nBMqRdxhxEgf48AuPSfi VUMMtuHph9wU7TVkarMnSAp/r0SyK8oSw15vgYd5kfsvEfxKrRf3xrfpFeTosGgeTftA yEag== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe:content-type; bh=lma9El9NrFvyGGTU0kueiSVQOHlV9qeoEMudAwUuzCQ=; b=LlHepgULfIlU9r1hCxmJUsJbjaYRcGLvL0PifhZf5eXP1Tx2WqiPrfyOp7XDSyQ/LV LR8f/xai8nwHCis3VpL0y5AZ35FEb1hwl2Q3a/DgehbxlMyBzeVjne1ou+WfTKgp/MgB /wH9ElQfiovKsupX5zNh59NHdghd+yt/yjSH2ab0OnqYZNCuLzytkZzclnjdQqimvlUq wUXAiEw+KkrUOIPlG2EMctLboCzEbT1GTPQOObiNPSl1nEnNzZGA6dEu/8VdFng+B0DY jFl7AJiaq0CdXIXyUCmRA1zfR+OhcxA06HhAryCZyMwEfl6RJSQP4XvwZ/tAduPFcNSm ehJg== X-Gm-Message-State: ALoCoQn9ZMIEw8HwJ+DmyB4kl+cnRSvfmOLZxKtV/7Mw4YJ1D610t/1Egt37JAn29JJQNkFbFsut X-Received: by 10.182.66.193 with SMTP id h1mr18424obt.47.1384926739525; Tue, 19 Nov 2013 21:52:19 -0800 (PST) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.49.36.161 with SMTP id r1ls117830qej.71.gmail; Tue, 19 Nov 2013 21:52:18 -0800 (PST) X-Received: by 10.224.54.5 with SMTP id o5mr39091149qag.9.1384926738659; Tue, 19 Nov 2013 21:52:18 -0800 (PST) Original-Received: from mail-qa0-x232.google.com (mail-qa0-x232.google.com [2607:f8b0:400d:c00::232]) by mx.google.com with ESMTPS id k1si4992515qao.74.2013.11.19.21.52.18 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 19 Nov 2013 21:52:18 -0800 (PST) Received-SPF: pass (google.com: domain of jyasskin@google.com designates 2607:f8b0:400d:c00::232 as permitted sender) client-ip=2607:f8b0:400d:c00::232; Original-Received: by mail-qa0-f50.google.com with SMTP id i13so1910562qae.9 for ; Tue, 19 Nov 2013 21:52:18 -0800 (PST) X-Received: by 10.224.29.67 with SMTP id p3mr49434630qac.30.1384926738414; Tue, 19 Nov 2013 21:52:18 -0800 (PST) Original-Received: by 10.229.177.131 with HTTP; Tue, 19 Nov 2013 21:51:58 -0800 (PST) In-Reply-To: X-Original-Sender: jyasskin@google.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of jyasskin@google.com designates 2607:f8b0:400d:c00::232 as permitted sender) smtp.mail=jyasskin@google.com; dkim=pass header.i=@google.com; dmarc=pass (p=REJECT dis=NONE) header.from=google.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:7869 Archived-At: I think std::mem_fn already supports this: http://en.cppreference.com/w/cpp/utility/functional/mem_fn On Tue, Nov 19, 2013 at 11:41 PM, wrote: > 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/. -- --- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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