220 35172 <169ba3dc-ee6f-4b2e-8e7f-c3c57e4b5529@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: tower120@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: std::vector must have strong move guarantees. Date: Mon, 30 Oct 2017 18:56:51 -0700 (PDT) Lines: 1102 Approved: news@gmane.org Message-ID: <169ba3dc-ee6f-4b2e-8e7f-c3c57e4b5529@isocpp.org> References: <0f1d296f-1d9c-4a63-bde0-bb1085b56e6c@isocpp.org> Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_7173_639982981.1509415011213" X-Trace: blaine.gmane.org 1509415032 1028 195.159.176.226 (31 Oct 2017 01:57:12 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Tue, 31 Oct 2017 01:57:12 +0000 (UTC) Cc: tower120@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBD27FEGT7AORBY5Q37HQKGQEWIKK55Q@isocpp.org Tue Oct 31 02:57:08 2017 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-ua0-f197.google.com ([209.85.217.197]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1e9LnT-0006QZ-NQ for gclcip-std-proposals@m.gmane.org; Tue, 31 Oct 2017 02:56:56 +0100 Original-Received: by mail-ua0-f197.google.com with SMTP id b11sf10916543uae.0 for ; Mon, 30 Oct 2017 18:57:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=vi9yPy65nIetDXclp4zOQVtJjFuodeXw1ruGl7HwpW8=; b=aPsy7RRmDRm/UY8ainhmd0NaxP9ssZdmntll6MP70hfqE82LQSLh/aQb66GcNTnyt3 GAVb7nYG0ZVwv+HUYljjovAo0XT134LGwmBmaPbXTjgFY0wnT3RwQBUJAC9MuGhmBBta ivZsfT09p2bf2fA87KBmLNUuEvugFcBn6OTj5HxVIoy/99jzX6cj9y3k2VrTklbOjhnb H56ibGy7mh/TwThlafoweVfeGFm7/8+gLVOFAQ0rvWBsz5mnvShvEXYxA5Nekx/m+6X5 nCjV0VfJMHO6sPIfz685l8OpivTVk1lE8pOHkTw+MPMgDAz7LdtSKlGtfzYrdoVwLDw2 cXBQ== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=vi9yPy65nIetDXclp4zOQVtJjFuodeXw1ruGl7HwpW8=; b=OsiamnK2iP6rj4UyeN7cCyu29mKsI5vSEKHZeMJOupwLN8iQ6XfDIEOnn81sRvciaJ o1tYr2+S3tYZuCFtI7ih4+xMYvbg/0E0qGvnwGJPMFyfwwy9C+I6jqXSSNyFS9zF3we7 6ZPHafljgv9Ow6ADAV2v/7M5aliqG00PrJeGR8Mp1tfR7hcRIANbkeWpTntXnGKx8jvM 7QtXPF89olyH1DtECJR6ylNbqPpR0GzFS2QEBEf6KFVBZdQPqp7nwaF+eh5K81asqN14 09zdKd/tKesS4QjqGNm+ZJC2mrLhpAF3vvf0i7KbTeArGUfHOq19+qG42jSJFqlGT6Xp DFWg== 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:cc: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=vi9yPy65nIetDXclp4zOQVtJjFuodeXw1ruGl7HwpW8=; b=DJBxUW1tehmsYRSiQyT0R21ELUGje4Kl2pp0nyZ3pi4Ei+HeL9BV5mfAwJKO2M5ciS YsrLuJwTHEUiJpXAWV+xUezRy8hB1QieLWFVfKmhNF8c8NVAsVc3Py+9Ks5FW3B6Miz0 3h1UB9qbiCUSdsOEz0ToGm0YykolL/IdjZCkat6SRWPoR+EJ8Mr9/Gf61wmUIniOhtZ6 hEbZMX9akBpvAo6HpHVpUtzUQt+K0Gt2VoTQUlFbdUx2wSxg9EBoPUOiekV8CH9dkgQV 3/zNs3m4B3NY39+fn7Puws+DVR7uLaybuldFjN0dzCfOnwjZYxKlVQJtOj/6sD+nHSoK l1Pw== X-Gm-Message-State: AMCzsaWgc9Cxb/ngt0GLl2fHDppFWLvae5QXxMzpjhzP6kHMyOyAIUzI nr3Nud7hfU7Jtwna+USD3S3HWA== X-Google-Smtp-Source: ABhQp+StBoH7kut5A5FpWHQsom4ue7bjidBHhBhyIoQ0ynAli9NMglLGHYkpfpC5Q/4QVrWFb3sX5g== X-Received: by 10.176.10.146 with SMTP id d18mr149995uak.25.1509415013036; Mon, 30 Oct 2017 18:56:53 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 10.31.160.76 with SMTP id j73ls3817233vke.12.gmail; Mon, 30 Oct 2017 18:56:51 -0700 (PDT) X-Received: by 10.31.153.21 with SMTP id b21mr17598vke.13.1509415011715; Mon, 30 Oct 2017 18:56:51 -0700 (PDT) In-Reply-To: <0f1d296f-1d9c-4a63-bde0-bb1085b56e6c@isocpp.org> X-Original-Sender: tower120@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:35172 Archived-At: ------=_Part_7173_639982981.1509415011213 Content-Type: multipart/alternative; boundary="----=_Part_7174_11375013.1509415011214" ------=_Part_7174_11375013.1509415011214 Content-Type: text/plain; charset="UTF-8" Okay, I think I start to understand your motivation. You think, that having both constructors available to use during reallocation phase will theoretically allow compiler to choose faster path? Sounds very vague... What motivation, then, to write move constructor in class, if it is slower beforehand? Other examples with cases were both the copy constructor and move constructor are noexcept, are: std::vector> std::vector std::vector If you need examples, where compiler will not be able to determine which constructor is faster at compile time, consider this: struct Data{ Data(){} std::function copy_ctr_hook; std::function move_ctr_hook; Data(const Data& other) noexcept { other.copy_ctr_hook(); } Data(Data&& other) noexcept { other.move_ctr_hook(); } ~Data() noexcept {} } I want to see how compiler will decide which constructor is faster. Kinda hypothetical, but if your main argument, that "compiler knows better"... Now, you have to brawl with compiler/std library to get required behavior (aka force compiler to use your move constructor). Because compiler allowed NOT TO use it, you have no idea what exactly will happened if you change compiler/platform. -- 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/169ba3dc-ee6f-4b2e-8e7f-c3c57e4b5529%40isocpp.org. ------=_Part_7174_11375013.1509415011214 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Okay, I think I start to understand your motivation. You t= hink, that having both constructors available to use during reallocation ph= ase will theoretically allow compiler to choose faster path? Sounds very va= gue... What motivation, then,=C2=A0 to write move constructor in class, if = it is slower beforehand?

Other examples with cases were both = the copy constructor and move constructor are noexcept, are:
std::vector<= std::= vector<T>>
std::= vector<std::= string>
std::= vector<std::function>

If you need examples, wher= e compiler will not be able to determine which constructor is faster at com= pile time, consider this:

struct Data{
=C2=A0=C2=A0=C2=A0 Data(){= }

=C2=A0=C2=A0=C2=A0 std::function<void()> copy_ctr_hook;
= =C2=A0=C2=A0=C2=A0 std::function<void()> move_ctr_hook;

=C2=A0= =C2=A0=C2=A0 Data(const Data& other) noexcept {
=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0 other.copy_ctr_hook();
=C2=A0=C2=A0=C2=A0 }
= =C2=A0=C2=A0=C2=A0 Data(Data&& other) noexcept {
=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0 other.move_ctr_hook();
=C2=A0=C2=A0=C2=A0 }<= br>
=C2=A0=C2=A0=C2=A0 ~Data() noexcept {}
}

I want to see how= compiler will decide which constructor is faster. Kinda hypothetical, but = if your main argument, that "compiler knows better"...

Now= , you have to brawl with compiler/std library to get required behavior (aka= force compiler to use your move constructor). Because compiler allowed NOT= TO use it, you have no idea what exactly will happened if you change compi= ler/platform.

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To view this discussion on the web visit https://groups.google.= com/a/isocpp.org/d/msgid/std-proposals/169ba3dc-ee6f-4b2e-8e7f-c3c57e4b5529= %40isocpp.org.
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