From -8367296928398200760 X-Google-Thread: 7894ca11fe,f5fa8c2235f87223 X-Google-Attributes: gid7894ca11fe,public,usenet X-Google-NewGroupId: yes X-Google-Language: ENGLISH,ASCII-7-bit Path: g2news2.google.com!news1.google.com!border1.nntp.dca.giganews.com!nntp.giganews.com!news.alt.net!frodo.cs.rpi.edu!not-for-mail From: =3D?ISO-8859-1?Q?Daniel_Kr=3DFCgler?=3D Newsgroups: comp.std.c++ Subject: Re: Trivial copy/move operations Date: Thu, 8 Apr 2010 00:23:32 CST Organization: http://groups.google.com Lines: 69 Sender: cppmods@cs.rpi.edu Approved: austern@google.com Message-ID: <40e66b13-1602-4ca0-a28a-a76602d3649c@30g2000yqi.googlegroups.com> References: NNTP-Posting-Host: netlab.cs.rpi.edu Content-Type: text/plain; charset=ISO-8859-1 To: (Usenet) Return-Path: X-Original-Date: Wed, 7 Apr 2010 14:29:30 -0700 (PDT) X-Submission-Address: std-c++@netlab.cs.rpi.edu Xref: g2news2.google.com comp.std.c++:2394 On 1 Apr., 02:00, Scott Meyers wrote: > N3053 (about implicitly-generated move functions) says "Unions have implicit > deleted moves if a members has a non-trivial moves. This rule is the sam= e > as for copy, and means that a user-defined union copy will not shadow a > move." > > Can somebody please summarize for me what it means for a move or copy > operation to be trivial? I'm not sure that I understand this question properly. Are you asking for the conditions/criteria that must be satisfied for a move or copy function to be considered as trivial or are you asking for a summary of all special rules that require such a trivial operation? Without any guidance I reply to the first question: 1) For a trivial copy/move constructor ([class.copy]/13) or assignment operator (/27) the requirements are that the c'tor is not user- provided nor deleted, that no virtual functions nor virtual base classes exists, and that all non-static data member and base class operation that will be used for this copy/move are also trivial. It is note-worthy to mention, what a user-provided special member function is (I assume that the term "deleted function" is sufficiently clear): It means that the function is user-declared and not explicitly defaulted on its first declaration, e.g. struct S { S(const S&) = default; // not user-provided S(S&) = delete; // explicitly deleted, therefore user-provided }; It is also note-worthy to mention that a special member function may be deleted without any explicit user-declaration. For example the core language requires that the copy/move assignment operator is deleted, if the class type has a non-static data member of reference type. 2) Finally the standard distinguishes a "trivially copyable" type. This is more than a type with trivial copy and trivial move operations (and a trivial destructor as well), specifically there must be *no* non-trivial copy and move operations, e.g. the type struct Funny { Funny(const Funny&) = default; Funny(Funny&) = delete; }; is *not* trivially copyable, because the second copy constructor is not trivial. HTH & Greetings from Bremen, Daniel Kr=FCgler -- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@netlab.cs.rpi.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ]