C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Simplifying cost-correctness          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message Sylvain Pion Guest Posted: Mon Apr 10, 2006 5:06 pm    Post subject: Simplifying cost-correctness When one writes const-correct classes, usually a lot of code has to be duplicated between the const and the non-const version of the member functions. I was wondering if there is some ongoing to work on attempting to enhance the language to simplify this task (e.g. some way of factorizing the code between the two versions in a compact manner) ? -- Sylvain --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Tomás Guest Posted: Mon Apr 10, 2006 6:06 pm    Post subject: Re: Simplifying cost-correctness Sylvain Pion posted: Quote: When one writes const-correct classes, usually a lot of code has to be duplicated between the const and the non-const version of the member functions. I was wondering if there is some ongoing to work on attempting to enhance the language to simplify this task (e.g. some way of factorizing the code between the two versions in a compact manner) ? Do any of you do something akin to the following: class Monkey { const Monkey& Climb() const { //Very complicated function containing //a thousand lines of code return *this; } Monkey& Climb() { const Monkey& untouchable = *this; return const_cast<Monkey&>( untouchable.Climb() ); } }; Seems like a very legitimate use of "const_cast" to me. -Tomás --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top David Abrahams Guest Posted: Mon Apr 10, 2006 6:06 pm    Post subject: Re: Simplifying cost-correctness Sylvain Pion <Sylvain.Pion (AT) sophia (DOT) inria.fr> writes: Quote: When one writes const-correct classes, usually a lot of code has to be duplicated between the const and the non-const version of the member functions. I was wondering if there is some ongoing to work on attempting to enhance the language to simplify this task (e.g. some way of factorizing the code between the two versions in a compact manner) ? Not that I know of. I'd like to see cv qualification as a template parameter: template <class T, cvqual CV> struct foo { int CV x; // for example }; That could make these things much easier to handle cleanly. -- Dave Abrahams Boost Consulting www.boost-consulting.com --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Guest Posted: Mon Apr 10, 2006 10:04 pm    Post subject: Re: Simplifying cost-correctness Yes, I do this exact same thing. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Gene Bushuyev Guest Posted: Tue Apr 11, 2006 4:06 pm    Post subject: Re: Simplifying cost-correctness "Sylvain Pion" <Sylvain.Pion (AT) sophia (DOT) inria.fr> wrote in message news:e1e1fd$pkf$1@news-sop.inria.fr... Quote: When one writes const-correct classes, usually a lot of code has to be duplicated between the const and the non-const version of the member functions. I was wondering if there is some ongoing to work on attempting to enhance the language to simplify this task (e.g. some way of factorizing the code between the two versions in a compact manner) ? It was discussed before on std.c++. You can find discussion and some solutions here: http://groups.google.com/group/comp.std.c++/tree/browse_frm/thread/c7f237927b1eb150/81d70ed62abce301?rnum=1&_done=%2Fgroup%2Fcomp.std.c%2B%2B%2Fbrowse_frm%2Fthread%2Fc7f237927b1eb150%2F81d70ed62abce301%3Ftvc%3D1%26#doc_36b01fac9172235a -- Gene Bushuyev (www.gbresearch.com) ---------------------------------------------------------------- There is no greatness where there is no simplicity, goodness and truth. ~ Leo Tolstoy --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Bronek Kozicki Guest Posted: Thu Apr 13, 2006 4:06 pm    Post subject: Re: Simplifying cost-correctness David Abrahams wrote: Quote: I was wondering if there is some ongoing to work on attempting to enhance the language to simplify this task (e.g. some way of factorizing the code between the two versions in a compact manner) ? Not that I know of. I'd like to see cv qualification as a template parameter: and I'd like to see const constructor that can be used only to create const objects. That would allow 1. more flexibility in defining class interface; 2. performance optimizations when creating const objects. struct A { A(); // 1 A() const; // 2 }; class B { B(); // 3 public: B() const; // 4 }; int main() { A a1; // 1 called const A a2; // 2 called B b1; // error, constructor 3 not accessible const B b2; // 4 called } ... but there was not such proposal, thus there's no chance having something like this in C++0x . Oh, and while we are at this; with "rvalue-this" we could extend semantics to have separate constructor for creation of temporary variables and for creation of named variables (direct-initialization) B. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Martin Vejnár Guest Posted: Fri Apr 14, 2006 8:06 am    Post subject: Re: Simplifying cost-correctness Tomás wrote: Quote: Sylvain Pion posted: When one writes const-correct classes, usually a lot of code has to be duplicated between the const and the non-const version of the member functions. I was wondering if there is some ongoing to work on attempting to enhance the language to simplify this task (e.g. some way of factorizing the code between the two versions in a compact manner) ? Do any of you do something akin to the following: class Monkey { const Monkey& Climb() const { //Very complicated function containing //a thousand lines of code return *this; } Monkey& Climb() { const Monkey& untouchable = *this; return const_cast<Monkey&>( untouchable.Climb() ); } }; Seems like a very legitimate use of "const_cast" to me. Why don't you do it this way? It seems cleaner to me... class Monkey { void Climb2() const { //Very complicated function containing //a thousand lines of code } Monkey & Climb() { Climb2(); return *this; } const Monkey & Climb() const { Climb2(); return *this; } }; OP was talking about duplicating "a lot of code" in const-correct classes. I don't think that two lines are that many. Surely not enough to advocate a use of a cast. Although the code you (Tomás) posted is correct _now_, after several modifications by different people, the cast may not be valid anymore. By using that explicit cast, you deliberately and unnecessarily give up type checking and expose yourself to unexpected behaviors. -- Martin --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Ivan Kolev Guest Posted: Fri Apr 14, 2006 8:06 am    Post subject: Re: Simplifying cost-correctness kevin.hall (AT) motioneng (DOT) com wrote: Quote: Yes, I do this exact same thing. I found that the opposite is usually more convenient: class Monkey { const Monkey& Climb() const { return const_cast<Monkey&>(*this).Climb(); } Monkey& Climb() { //Very complicated function containing //a thousand lines of code return *this; } }; And I think this idiom is obligatory even for simple functions (and even for the most performance-sensitive one-liners, which should be inline anyway and an additional one-line call would surely be inlined too), to avoid the possible modification of just one of the two methods in the future. Quote: Seems like a very legitimate use of "const_cast" to me. Indeed, I hope so, too. Regards, Ivan --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Tomás Guest Posted: Fri Apr 14, 2006 4:06 pm    Post subject: Re: Simplifying cost-correctness Martin Vejnár posted: Quote: class Monkey { void Climb2() const { //Very complicated function containing //a thousand lines of code } Monkey & Climb() { Climb2(); return *this; } const Monkey & Climb() const { Climb2(); return *this; } }; Good solution. I simply hadn't thought of that at the time of posting. Once again, this proves that there's many solutions to any particular problem, and that some solutions are better than others. Best thing is just to sit down and think, think, think... -Tomás --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Bronek Kozicki Guest Posted: Fri Apr 14, 2006 5:06 pm    Post subject: Re: Simplifying cost-correctness Ivan Kolev wrote: Quote: I found that the opposite is usually more convenient: const Monkey& Climb() const { return const_cast<Monkey&>(*this).Climb(); } this is wrong. See what happens here: const Monkey doda; // . . . doda.Climb(); here you effectively cast-away constness of doda object, which is UB. You could notice funny symptoms if doda had static lifetime - such const objects may live in immutable regions of memory and any member variable modification in body of non-const Climb would result in memory access violation. B. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Ivan Kolev Guest Posted: Sun Apr 16, 2006 5:06 am    Post subject: Re: Simplifying cost-correctness "Tomás" wrote: Quote: Martin Vejnár posted: class Monkey { void Climb2() const { //Very complicated function containing //a thousand lines of code } Monkey & Climb() { Climb2(); return *this; } const Monkey & Climb() const { Climb2(); return *this; } }; Good solution. However, I don't see this working in the case when the two methods must return a part of the object, which is not trivially retrieved: class Monkey { Finger& getFinger( int i ); const Finger& getFinger( int i ) const; } Duplication or a const_cast seems inevitable to me. And I also can't see the dangers of the const_cast which Martin Vejnár mentioned. But I agree with Bronek Kozicki that it's better to write the common implementation inside the const method - not exactly because of the reasons he gave (if the two methods are actually exactly the same method with two faces, then that method *cannot* modify any member variable), but because by writing the implementation inside the const method the compiler will point out if you modify a member variable by mistake. Regards, Ivan --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Earl Purple Guest Posted: Wed Apr 26, 2006 2:06 am    Post subject: Re: Simplifying cost-correctness Ivan Kolev wrote: Quote: class Monkey { Finger& getFinger( int i ); const Finger& getFinger( int i ) const; } Duplication or a const_cast seems inevitable to me. And I also can't see the dangers of the const_cast which Martin Vejnár mentioned. template < typename M, typename F > F & getFinger( M& m ) { // complex code } Finger& Monkey::getFinger( int i ) { return getFinger< Monkey, Finger >( i); } const Finger & Monkey::getFinger( int i ) const { return getFinger< const Monkey, const Finger >( i ); } Of course the getFinger template is a free-function and not a friend plus you want to hide this implementation detail completely in the header so it probably gets out of hand just to avoid a const_cast. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] 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, library and standards All times are GMT Page 1 of 1   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.