220 27071 article Path: news.gmane.org!not-for-mail From: Richard Smith Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Proposal - non allocating std::function Date: Fri, 15 Jul 2016 11:27:53 -0700 Lines: 92 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: multipart/alternative; boundary=001a11499a74eb59f50537b0c6c2 X-Trace: ger.gmane.org 1468607282 22283 80.91.229.3 (15 Jul 2016 18:28:02 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Fri, 15 Jul 2016 18:28:02 +0000 (UTC) To: "std-proposals@isocpp.org" Original-X-From: std-proposals+bncBDVNBJG4YAIBBK6WUS6AKGQEO5YDZOI@isocpp.org Fri Jul 15 20:28:02 2016 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pa0-f69.google.com ([209.85.220.69]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1bO7q9-0000g4-M9 for gclcip-std-proposals@m.gmane.org; Fri, 15 Jul 2016 20:27:57 +0200 Original-Received: by mail-pa0-f69.google.com with SMTP id q2sf198597351pap.1 for ; Fri, 15 Jul 2016 11:27:56 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:mime-version:sender:in-reply-to:references:from :date:message-id:subject:to:x-original-sender :x-original-authentication-results:reply-to:precedence:mailing-list :list-id:x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=VEjmyMO5LMIyiS57xHtDjQPjv4x56aQZ28ZNkcMMLKE=; b=e7zJqBUOgDjEVsFKM8BeAOKRnA6W93/KbDUhSfvrWsX0cPEzllrENDbvCr59y/Mpjf cAJGGn9/K/N9Z1E2Pgm4bU7eB6Btwruek62yPfiXy52mni58FWfYoy4Wi/ajqezb9TzK qVZmB3cZ7GmxdufBArUYZ559KTDruEh/geupC3Ut7AiOUsWMHeBqrWjNzr3iOqPSg3+6 1gK4nauRsfd/RTTSwhP8bKaNahR4vOQz+R2+N7KtyopTz0T56VW0opFtv0gHWfkYbNO0 pIfQSer0UbpnG2V8J4EVoD6obmmY+TonUN0ZQaKm/MSZzfLWXct/0s31MMtzh8FRwX8z 5LJA== X-Gm-Message-State: ALyK8tLV3/Cn8NESFvAtJDMYEltyLSDqbsEWfX2wPvQrsSMMDQtfJQMT8GEbJn3FW2ruzQ== X-Received: by 10.98.100.141 with SMTP id y135mr11677259pfb.6.1468607276296; Fri, 15 Jul 2016 11:27:56 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.54.140 with SMTP id h12ls4148999otc.40.gmail; Fri, 15 Jul 2016 11:27:55 -0700 (PDT) X-Received: by 10.55.109.198 with SMTP id i189mr27382830qkc.140.1468607275251; Fri, 15 Jul 2016 11:27:55 -0700 (PDT) Original-Received: from mail-qk0-x22b.google.com (mail-qk0-x22b.google.com. [2607:f8b0:400d:c09::22b]) by mx.google.com with ESMTPS id s64si6126681qka.43.2016.07.15.11.27.55 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 15 Jul 2016 11:27:55 -0700 (PDT) Received-SPF: pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400d:c09::22b as permitted sender) client-ip=2607:f8b0:400d:c09::22b; Original-Received: by mail-qk0-x22b.google.com with SMTP id s63so109052815qkb.2 for ; Fri, 15 Jul 2016 11:27:55 -0700 (PDT) X-Received: by 10.55.204.3 with SMTP id r3mr27567226qki.91.1468607274634; Fri, 15 Jul 2016 11:27:54 -0700 (PDT) Original-Sender: metafoo@gmail.com Original-Received: by 10.200.56.152 with HTTP; Fri, 15 Jul 2016 11:27:53 -0700 (PDT) In-Reply-To: X-Original-Sender: richard@metafoo.co.uk X-Original-Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com; spf=pass (google.com: domain of metafoo@gmail.com designates 2607:f8b0:400d:c09::22b as permitted sender) smtp.mailfrom=metafoo@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org 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:27071 Archived-At: --001a11499a74eb59f50537b0c6c2 Content-Type: text/plain; charset=UTF-8 On Fri, Jul 15, 2016 at 2:12 AM, Carl Cook wrote: > On Friday, 15 July 2016 02:35:42 UTC+2, Richard Smith wrote: >> >> llvm::function_ref doesn't capture anything, and is not supposed to -- >> it's a *non-owning* handle to a callable that outlives the handle. >> (llvm::function_ref is to std::function as std::string_view is to >> std::string.) This is a feature and a design goal of llvm::function_ref, >> but I think it means it's addressing a fundamentally different problem than >> the one you're tackling here (which seems to be essentially, "let me >> control the size and alignment in std::function's small function >> optimization" >> > > Understood. What caught me out is that compilers can aggressively limit > the scope of lambdas. Hence I believe the following is not legal usage: > > int non_const_var = 0xf; > function_ref f_ref = [&]() { return non_const_var;}; > return f_ref(); > Right, it's exactly as broken as this: string g() { string_view s_v = string("blah blah"); return s_v; } In both cases, you've bound an object with reference semantics to a (non-lifetime-extended) temporary object. -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. 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On F= ri, Jul 15, 2016 at 2:12 AM, Carl Cook <carl.cook@gmail.com> wrote:
On Friday, 15 July 2016 02:35:42 UTC+2, Richard Smith wrote:
llvm::function_ref doesn't capture anything, and is not supposed to = -- it's a *non-owning* handle to a callable that outlives the handle. (= llvm::function_ref is to std::function as std::string_view is to std::strin= g.) This is a feature and a design goal of llvm::function_ref, but I think = it means it's addressing a fundamentally different problem than the one= you're tackling here (which seems to be essentially, "let me cont= rol the size and alignment in std::function's small function optimizati= on"

Understoo= d. What caught me out is that compilers can aggressively limit the scope of= lambdas. Hence I believe the following is not legal usage:

<= /div>
int non_const_var = =3D 0xf;
function_ref<int()> f_ref =3D [&]() { return n= on_const_var;};
return f_ref();

Right, it's exactly as broken as th= is:

string g() {
=C2=A0 string_view s_v = =3D string("blah blah");
=C2=A0 return s_v;
}=

In both cases, you've bound an object with re= ference semantics to a (non-lifetime-extended) temporary object.

--
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