220 31729 <136fc743-bc63-48c8-b1d5-dc4084dd13cf@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: Nicol Bolas Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Complexities caused by using unique_ptr and move semantics Date: Fri, 24 Mar 2017 15:25:51 -0700 (PDT) Lines: 145 Approved: news@gmane.org Message-ID: <136fc743-bc63-48c8-b1d5-dc4084dd13cf@isocpp.org> References: <46a85060-631b-49e5-94f3-ab07429d8085@isocpp.org> <86aeb556-e0d7-4d68-b553-1b4163461473@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_2292_1276201536.1490394351178" X-Trace: blaine.gmane.org 1490394364 22908 195.159.176.226 (24 Mar 2017 22:26:04 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Fri, 24 Mar 2017 22:26:04 +0000 (UTC) To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBCEKFTV6ZUMBB35Z23DAKGQEX55AR5A@isocpp.org Fri Mar 24 23:25:59 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-pg0-f72.google.com ([74.125.83.72]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1crXeU-0004Im-SR for gclcip-std-proposals@m.gmane.org; Fri, 24 Mar 2017 23:25:47 +0100 Original-Received: by mail-pg0-f72.google.com with SMTP id 79sf4370750pgf.2 for ; Fri, 24 Mar 2017 15:25:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=LRXvFY0jOFnE3RopgapwHarT/JuX3i5sdPueaHXOzRg=; b=r1L9lOkp6mxC0CR1jO1GDbXukr4ljXLC0+MI+0Wdz43JT3BdeOIr277Gk5Uzsf6OtZ tl6XsZZv/U0sU2s0dpRlihYbRV7tZv3DtRl9mqgxS2wKxCKrNm7jdVKabfVv+s4Y3KIo aG1coAcDTGJNv/ZpVkyl0mwB8A5sJdyOdjYwIc41SGiGlFdtwJnkYXaj5mIyw8VrPTb3 gEODIs/AImYBVzvKRV9ypQakRA8HQvMVc5keFylUsqB1ToW/BL++d4tjJQeS+FcTpTc8 PkymBZM8U0/2HlYuJ5mFgRshKM1dmiYyw7UGBWMmbASJPr3goe3iMZivA9fm40eL8Nj3 uW+g== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:message-id:in-reply-to:references:subject:mime-version :x-original-sender:reply-to:precedence:mailing-list:list-id :x-spam-checked-in-group:list-post:list-help:list-archive :list-subscribe:list-unsubscribe; bh=LRXvFY0jOFnE3RopgapwHarT/JuX3i5sdPueaHXOzRg=; b=HOjkCgyHfAKLhjjbgmuYc2EbTXaOKHAVWD7wZTY87YHk4U4SNknnmuPUaVFCQtQl4W H/I7NaWGyumH6USxro8frReT1BxAudKbddmwhVrSlOv5sUW24KlJfUpCmLO98oQt8rNH vUi0Kd1PSJCIjPKCN4Ipasxy/gGoeGmRe6riAp5n+ZywxgTrcBpAqL4hN51wN70kZDyh AMH+35gImSFkJDsDYf1Vl5ZRV3HMmxtZytmI0qSC9gDLuqBj3U1WWJycRwdQRYEZymra WhfdvokQLn49C4gBRoB/VwDDeY4L0kXr7UdAU49RiF1/d3TrcDoDCK2VKcFKFc5FSovS 4pzg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:message-id:in-reply-to:references :subject:mime-version:x-original-sender:reply-to:precedence :mailing-list:list-id:x-spam-checked-in-group:list-post:list-help :list-archive:list-subscribe:list-unsubscribe; bh=LRXvFY0jOFnE3RopgapwHarT/JuX3i5sdPueaHXOzRg=; b=VjSOdayqNxJ709CphoJ5nlkWFLPQZ7/VwsbDgchnGaL4rlGAmogwRTWqT7rFyo243E 9amLifqXmfUbJ8KPnNTLn79XcML9oFQvRnEBp5o4I9OunxRKqoF6r7JPwYrViYeE6j9N 2RB3RCbnYnxn6/JBs4HDSzFOeKzSYc94hRWe2g9qQUpv+17oDwgYxZNpeRdFiTBpV461 JyGMkdy+UIjpJHfJMhVp4hzl99tb5vU0jgs0of7+l5GswEEC53gZEweQfjhbncLJeaqf 2S3wSjBrzjDHJOG5JnBAaKjSLhZSW3cw3131X3N4T7A5MEiJ4MmkZFQ9HktLhimPLDAm AGlg== X-Gm-Message-State: AFeK/H3FplVcEmKAXPybcQGpLnuto/H0exdWWU+q+/QbMnzotgckIUnZfqnkKUOJrS97aQ== X-Received: by 10.99.103.71 with SMTP id b68mr3483847pgc.28.1490394352614; Fri, 24 Mar 2017 15:25:52 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.157.43.209 with SMTP id u75ls3332743ota.13.gmail; Fri, 24 Mar 2017 15:25:51 -0700 (PDT) X-Received: by 10.157.13.75 with SMTP id 69mr1048556oti.3.1490394351688; Fri, 24 Mar 2017 15:25:51 -0700 (PDT) In-Reply-To: <86aeb556-e0d7-4d68-b553-1b4163461473@isocpp.org> X-Original-Sender: jmckesson@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:31729 Archived-At: ------=_Part_2292_1276201536.1490394351178 Content-Type: multipart/alternative; boundary="----=_Part_2293_1425852513.1490394351178" ------=_Part_2293_1425852513.1490394351178 Content-Type: text/plain; charset=UTF-8 On Friday, March 24, 2017 at 5:57:11 PM UTC-4, Sean Middleditch wrote: > > On Thursday, March 23, 2017 at 9:38:19 AM UTC-7, Matthew Fioravante wrote: >> >> 1. Its too easy to accidentally use a moved from object by mistake >> > > This is a problem with C++'s move semantics (e.g., non-destructive move > and unspecific states). These problems have been discussed plenty of times > before, particularly in destructive-move threads. > > One solution in the works may be contract programming. Some of the > proposals allow a function to set a post-condition state on an object which > can be verified by a compiler or static analysis in a pre-condition. e.g., > unique_ptr might have something like: > > [[post_condition(src): empty] > unique_ptr(unique_ptr&& src); > > [[pre_condition(this): !empty] > unique_ptr::operator->(); > > Such a setup can further hypothetically address other issues like > accidental dereferencing of default-constructed unique_ptrs. As well as a > wide host of other issues. > > >> 2. unique_ptr and T* being different types can cause pessimizations >> because of language rules. >> > > I personally most see this as a problem caused by a lack of generators, or > more specifically at the lack of range concepts (which of course are in the > works). > > You usually don't want to require specifically a vector nor specifically a > pointer of any kind. You want to return an object that can be enumerated to > produce references to T objects. The vector itself or unique_ptr vs raw > pointers is an implementation detail that callers should not have to care > about. > > With range concepts, this all still has to be a template of course, but > you don't have to over-worry about the specifics while using the interface. > > With generators, this can be abstracted without templates (though perhaps > at a runtime cost). > > Basically, you want to write something like: > > iterable foo::get_things() { return make_iterable(_things, [](auto& > p){ return *p; }); } > > That's a really bad tradeoff: type-erasing performance plus the cost of filtering, all just to gain some increased genericity. But Boost.Range already has something like this: `any_range `, coupled with transforming the input range by a function .. -- 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. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/136fc743-bc63-48c8-b1d5-dc4084dd13cf%40isocpp.org. ------=_Part_2293_1425852513.1490394351178 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Friday, March 24, 2017 at 5:57:11 PM UTC-4, Sean Middle= ditch wrote:
O= n Thursday, March 23, 2017 at 9:38:19 AM UTC-7, Matthew Fioravante wrote:
1. Its too easy to a= ccidentally use a moved from object by mistake
<= br>
This is a problem with C++'s move semantics (e.g., non-de= structive move and unspecific states). These problems have been discussed p= lenty of times before, particularly in destructive-move threads.
=
One solution in the works may be contract programming. Some = of the proposals allow a function to set a post-condition state on an objec= t which can be verified by a compiler or static analysis in a pre-condition= .. e.g., unique_ptr might have something like:

=C2= =A0 [[post_condition(src): empty]
=C2=A0 unique_ptr(unique_ptr&am= p;& src);

=C2=A0 [[pre_condition(this): !empty= ]
=C2=A0 unique_ptr::operator->();

Su= ch a setup can further hypothetically address other issues like accidental = dereferencing of default-constructed unique_ptrs. As well as a wide host of= other issues.
=C2=A0
2. unique_ptr<T> and T* being different types= can cause pessimizations because of language rules.

I personally most see this as a problem caused by a l= ack of generators, or more specifically at the lack of range concepts (whic= h of course are in the works).

You usually don'= ;t want to require specifically a vector nor specifically a pointer of any = kind. You want to return an object that can be enumerated to produce refere= nces to T objects. The vector itself or unique_ptr vs raw pointers is an im= plementation detail that callers should not have to care about.
<= br>
With range concepts, this all still has to be a template of c= ourse, but you don't have to over-worry about the specifics while using= the interface.

With generators, this can be abstr= acted without templates (though perhaps at a runtime cost).

<= /div>
Basically, you want to write something like:

=
=C2=A0 iterable<T&> foo::get_things() { return make_iterable= (_things, [](auto& p){ return *p; }); }


That's a really bad tradeoff: type-eras= ing performance plus the cost of filtering, all just to gain some increased= genericity.

But Boost.Range already has something like this: `any_range`, coupled with transforming the input range by a function.=

--
You received this message because you are subscribed to the Google Groups &= quot;ISO C++ Standard - Future Proposals" group.
To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposa= ls+unsubscribe@isocpp.org.
To post to this group, send email to std-proposals@isocpp.org.
To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/136fc743-bc63-48c8-b1d5-dc4084dd13cf= %40isocpp.org.
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