C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  returning object with private copy c-tor & private assignmen Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Dmitry Guest Posted: Thu Oct 28, 2004 1:43 pm    Post subject: returning object with private copy c-tor & private assignmen Hi I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Basicly, since the Return Ralue Optimization(RVO) is always enabled in modern compilers, there should be no need in copy c-tor when returning unnamed object but the compiler(VC 7.1) prints the error even if copy c-tor is not really used at all... Thanks, Dmitry [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ron Natalie Guest Posted: Fri Oct 29, 2004 2:11 am    Post subject: Re: returning object with private copy c-tor & private assig Dmitry wrote: Quote: Hi I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Basicly, since the Return Ralue Optimization(RVO) is always enabled in modern compilers, there should be no need in copy c-tor when returning unnamed object but the compiler(VC 7.1) prints the error even if copy c-tor is not really used at all... The compiler is obligated to check the access on the copy constructor even if no copy is made. Your object forbids copies, you can't return by value. You'll have to return a pointer or reference to the original object. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top jnorberg@gmail.com Guest Posted: Fri Oct 29, 2004 2:12 am    Post subject: Re: returning object with private copy c-tor & private assig Hi, I suggest that you return a pointer to the object (or rather a boost::shared_ptr) if possible. __ Jonas Norberg [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Barbati Guest Posted: Fri Oct 29, 2004 2:14 am    Post subject: Re: returning object with private copy c-tor & private assig Dmitry wrote: Quote: Hi I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? If you mean return *by-value* the answer is no. What you can do is to allocate the object on the heap and return an std::auto_ptr (or another smart pointer type of your choice) to it. Quote: Basicly, since the Return Ralue Optimization(RVO) is always enabled in modern compilers, there should be no need in copy c-tor when returning unnamed object but the compiler(VC 7.1) prints the error even if copy c-tor is not really used at all... RVO is, as the name says, an *optimization*. Therefore, the fact that it's used shall have no impact on the semantic of the program. As the copy happens "conceptually" the copy ctor must be accessible, whether the compiler decides to omit the call or not. In standardese (12.2/1): "[...] Even when the creation of the temporary object is avoided (12., all the semantic restrictions must be respected as if the temporary object was created. [Example: even if the copy constructor is not called, all the semantic restrictions, such as accessibility (clause 11), shall be satisfied. ]" BTW, who said that RVO is *always* enabled in modern compilers? How can you be so sure? ;-) Regards, Alberto [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top jakacki Guest Posted: Fri Oct 29, 2004 2:15 pm    Post subject: Re: returning object with private copy c-tor & private assig Quote: I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Create on heap and return by std::auto_ptr. If your function does not create the object, you can pass it in and return by reference (like in operator<<). Quote: Basicly, since the Return Ralue Optimization(RVO) is always enabled in modern compilers, there should be no need in copy c-tor when returning unnamed object but the compiler(VC 7.1) prints the error even if copy c-tor is not really used at all... Return Value Optimization is, well, optimization. Compiler can employ as it pleases. That would be disastrous if correctness of the code would rely on what a particular compiler pleases. BR Grzegorz -- Free C++ frontend library: http://opencxx.sourceforge.net China from the inside: http://www.staryhutong.com Myself: http://www.dziupla.net/gj/cv [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Fri Oct 29, 2004 2:22 pm    Post subject: Re: returning object with private copy c-tor & private assig Dmitry wrote: Quote: Hi I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Basicly, since the Return Ralue Optimization(RVO) is always enabled in modern compilers, there should be no need in copy c-tor when returning unnamed object but the compiler(VC 7.1) prints the error even if copy c-tor is not really used at all... private copy-ctor and op= are usually a clear indication that someone has gone to some effort to make sure that you cannot do that. Usually, it's because copying or assigning such object will break something. -- A. Kanawati [email]NO.antounk.SPAM (AT) comcast (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dmitry Guest Posted: Fri Oct 29, 2004 2:43 pm    Post subject: Re: returning object with private copy c-tor & private assig I need to return by value for the object to be in one state only. And returning pointer/reference leads to other complex issues.. Actually, I've found the workaround: I'am declaring the public copy c-tor without defining it and since it is not really needed the code compiles fine. As for RVO - there is absolutely no reason, why the compiler decides not to use it - it is even enabled when optimization is "off" in VC 7.1 I hope new versions of gcc has the same thing. Also, I consider RVO and even NRVO to be not an optimization but as all new compilers required feature. ;-) P.S. Thanks for all replies! Dmitry Sychov [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Dave Harris Guest Posted: Sun Oct 31, 2004 11:22 am    Post subject: Re: returning object with private copy c-tor & private assig [email]nospam (AT) nospam (DOT) com[/email] (Dmitry) wrote (abridged): Quote: I need to return the object which implements a private copy constructior and private assignment operator. You could try using placement-new, but it may be tricky to get correct and safe. So this: void test() { Object obj = proc(); } Object proc() { return Object(); } would become something like: void test() { char buf[ sizeof( Object ) ]; Object &obj = proc( buf ); obj.~Object(); } Object &proc( void *p ) { return *new(p) Object(); } which is roughly what the compiler does when it wants to apply the Return Value Optimisation. The tricky bits are getting the alignment of buf correct, and ensuring that obj gets destroyed exactly when it needs to be, even in the face of exceptions etc. And I may have the syntax wrong; I don't often write stuff like that for real. However, it seems to me that you are trying to do something which the object was specifically intended to prevent. You ought to review the design. If your code has a legitimate need, then the object class ought either to make the copy constructor public, or else make your routine a friend or member. Or maybe research one of the "move" or "pilfer" idioms that get discussed in this forum from time to time. Quote: Basicly, since the Return Ralue Optimization(RVO) is always enabled in modern compilers, there should be no need in copy c-tor when returning unnamed object but the compiler(VC 7.1) prints the error even if copy c-tor is not really used at all... The compiler is correct. -- Dave Harris, Nottingham, UK [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top steven11 Guest Posted: Sun Oct 31, 2004 12:05 pm    Post subject: Re: returning object with private copy c-tor & private assig Am 29 Oct 2004 10:15:17 -0400 schrieb jakacki <jakacki_hates_spam (AT) acm (DOT) org>: Quote: I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Create on heap and return by std::auto_ptr. he isn't able to do that because the ctor is private...i suggest he isn't the author of the class and so he can't modify the access of the ctor to public -- mfg Steven [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top jakacki Guest Posted: Mon Nov 01, 2004 11:48 am    Post subject: Re: returning object with private copy c-tor & private assig Quote: I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Create on heap and return by std::auto_ptr. he isn't able to do that because the ctor is private... To do what exactly? Create on heap or return by std::auto_ptr ? I do not see anything that would prevent him from doing either. BR Grzegorz -- Free C++ frontend library: http://opencxx.sourceforge.net China from the inside: http://www.staryhutong.com Myself: http://www.dziupla.net/gj/cv [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top jakacki Guest Posted: Mon Nov 01, 2004 11:49 am    Post subject: Re: returning object with private copy c-tor & private assig Quote: I need to return by value for the object to be in one state only. How can object be in two states? Quote: And returning pointer/reference leads to other complex issues.. Actually, I've found the workaround: I'am declaring the public copy c-tor without defining it and since it is not really needed the code compiles fine. As for RVO - there is absolutely no reason, why the compiler decides not to use it - it is even enabled when optimization is "off" in VC 7.1 I hope new versions of gcc has the same thing. If you are writting code that is to be used by you only and with one compiler only, that's perfectly OK. If your code is to be maintained or extended by others and possibly compiled with other compilers, then what you are doing is very fragile. The first thing the maintainer will do once linker reports missing symbol will be to write the copy ctor body ("he is returning by value, so clearly he must have missed the copy ctor body..."). This, however, will break assumptions that you now make when writting the code. In a nutshell: such code non-portable and easy to break. BR Grzegorz -- Free C++ frontend library: http://opencxx.sourceforge.net China from the inside: http://www.staryhutong.com Myself: http://www.dziupla.net/gj/cv [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Mon Nov 01, 2004 11:51 am    Post subject: Re: returning object with private copy c-tor & private assig steven11 wrote: Quote: Am 29 Oct 2004 10:15:17 -0400 schrieb jakacki <jakacki_hates_spam (AT) acm (DOT) org>: I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Create on heap and return by std::auto_ptr. he isn't able to do that because the ctor is private...i suggest he isn't the author of the class and so he can't modify the access of the ctor to public Earlier in this thread, he solved the problem by declaring the copy ctor public, and leaving it unimplemented. This allowed the compilation to proceed. The compiled code elides the copy construction anyway because of RVO, so the there were no unresolved references to the unimplemented copy ctor. So, I guess the fellow owns the code, or can modify it. -- A. Kanawati [email]NO.antounk.SPAM (AT) comcast (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Michiel Salters Guest Posted: Mon Nov 01, 2004 10:19 pm    Post subject: Re: returning object with private copy c-tor & private assig steven11 <steven11 (AT) gmx (DOT) de> wrote Quote: Am 29 Oct 2004 10:15:17 -0400 schrieb jakacki <jakacki_hates_spam (AT) acm (DOT) org>: I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Create on heap and return by std::auto_ptr. he isn't able to do that because the ctor is private...i suggest he isn't the author of the class and so he can't modify the access of the ctor to public The copy ctor and the assignment operator are private, but there may be more - or there is a factory returning heap-allocated objects. Roughly the only pattern in which you can't create an object is the singleton pattern - but in that case, you don't have to return it. HTH, Michiel Salters [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top steven11 Guest Posted: Tue Nov 02, 2004 1:35 pm    Post subject: Re: returning object with private copy c-tor & private assig Am 1 Nov 2004 06:51:44 -0500 schrieb Antoun Kanawati <NO.antounk.SPAM (AT) comcast (DOT) net>: Quote: steven11 wrote: Am 29 Oct 2004 10:15:17 -0400 schrieb jakacki [email]jakacki_hates_spam (AT) acm (DOT) org[/email]>: I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Create on heap and return by std::auto_ptr. he isn't able to do that because the ctor is private...i suggest he isn't the author of the class and so he can't modify the access of the ctor to public Earlier in this thread, he solved the problem by declaring the copy ctor public, and leaving it unimplemented. This allowed the compilation to proceed. The compiled code elides the copy construction anyway because of RVO, so the there were no unresolved references to the unimplemented copy ctor. So, I guess the fellow owns the code, or can modify it. sorry, it was my mistake . i should have read the whole answers... -- mfg Steven [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top David Abrahams Guest Posted: Tue Nov 02, 2004 1:36 pm    Post subject: Re: returning object with private copy c-tor & private assig Antoun Kanawati <NO.antounk.SPAM (AT) comcast (DOT) net> writes: Quote: steven11 wrote: Am 29 Oct 2004 10:15:17 -0400 schrieb jakacki <jakacki_hates_spam (AT) acm (DOT) org>: I need to return the object which implements a private copy constructior and private assignment operator. Is is possible? Some tricks maybe? Create on heap and return by std::auto_ptr. he isn't able to do that because the ctor is private...i suggest he isn't the author of the class and so he can't modify the access of the ctor to public Earlier in this thread, he solved the problem by declaring the copy ctor public, and leaving it unimplemented. This allowed the compilation to proceed. The compiled code elides the copy construction anyway because of RVO, so the there were no unresolved references to the unimplemented copy ctor. Of course you realize that's totally non-portable? -- Dave Abrahams Boost Consulting http://www.boost-consulting.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ Language (Moderated) All times are GMT Goto page 1, 2  Next Page 1 of 2   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.