C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  virtual methods, virtual bases bug<?> in G++ 3.3          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Jim Fischer Guest Posted: Wed Jul 30, 2003 10:33 pm    Post subject: virtual methods, virtual bases bug<?> in G++ 3.3 Paragraph 10.3/2 in the C++ standard [ISO/IEC 14882:1998] has the following code example: <quote> struct A { virtual void f(); }; struct B : virtual A { virtual void f(); }; struct C : B , virtual A { using A::f; }; void foo() { C c; c.f(); // calls B::f, the final overrider c.C::f(); // calls A::f because of the usingdeclaration } </quote> It seems a little odd (to me) that the method call 'c.f()' in function foo() should invoke B::f. I thought it would invoke A::f due to the using declaration 'using A::f;' in class C's declaration block. And in fact, when I use G++ 3.3 to build the code sample shown below, the method call 'c.f()' invokes A::f (which is what I expected), and not B::f as stated in the C++ standard. <example> <code main.cpp> #include <iostream> struct A { virtual void f() { std::cout << "A::f()n"; } }; struct B : virtual A { virtual void f() { std::cout << "B::f()n"; } }; struct C : B, virtual A { using A::f; }; int main() { C c; c.f(); // g++ 3.3 invokes A::f, not B::f c.C::f(); } <build> g++ main.cpp </build> <output> A::f() A::f() </output> </example> FWIW, I looked around on the "JTC1/SC22/WG21 - C++" web site, http://anubis.dkuug.dk/jtc1/sc22/wg21/ but I didn't find anything about this particular issue (e.g., in the "Core Language Issues" pages). So I'd like some verification that c.f() is indeed supposed to invoke B::f, as stated in the standard, and not A::f. -- Jim To reply by email, remove "link" and change "now.here" to "yahoo" jfischer_link5809{at}now.here.com Back to top Janusz Szpilewski Guest Posted: Thu Jul 31, 2003 12:19 am    Post subject: Re: virtual methods, virtual bases bug<?> in G++ 3.3 Jim Fischer wrote: Quote: void foo() { C c; c.f(); // calls B::f, the final overrider c.C::f(); // calls A::f because of the usingdeclaration } /quote It seems a little odd (to me) that the method call 'c.f()' in function foo() should invoke B::f. I thought it would invoke A::f due to the using declaration 'using A::f;' in class C's declaration block. And in fact, when I use G++ 3.3 to build the code sample shown below, the method call 'c.f()' invokes A::f (which is what I expected), and not B::f as stated in the C++ standard. I think there is some rational explanation. If the member function name is not (scope) qualified (like c.f()) then the final overrider is called. If it is, the name used in the qualified scope is chosen. As shows the example member function name does not have to be overridden, it may be referred by using directive. But in this case such a name is not considered when looking for overriders so the last overrider down in the hierarchy is chosen. Btw, Borland C++ compiles the example according to the standard. Regards, Janusz Back to top Victor Bazarov Guest Posted: Thu Jul 31, 2003 12:31 am    Post subject: Re: virtual methods, virtual bases bug<?> in G++ 3.3 "Jim Fischer" <jfischer_link5809 (AT) now (DOT) here.com> wrote... Quote: Paragraph 10.3/2 in the C++ standard [ISO/IEC 14882:1998] has the following code example: quote struct A { virtual void f(); }; struct B : virtual A { virtual void f(); }; struct C : B , virtual A { using A::f; }; void foo() { C c; c.f(); // calls B::f, the final overrider c.C::f(); // calls A::f because of the usingdeclaration } /quote It seems a little odd (to me) that the method call 'c.f()' in function foo() should invoke B::f. I thought it would invoke A::f due to the using declaration 'using A::f;' in class C's declaration block. And in fact, when I use G++ 3.3 to build the code sample shown below, the method call 'c.f()' invokes A::f (which is what I expected), and not B::f as stated in the C++ standard. Did you actually read the paragraph preceding the example? Name lookup for the call c.f(); ignores names introduced by using declarations. That is, C::f is ignored, and B::f is found. So, G++ v3.3 is buggy. Name resolution is a tricky thing, not all of the implementations do it right. Victor Back to top Jim Fischer Guest Posted: Thu Jul 31, 2003 8:28 pm    Post subject: Re: virtual methods, virtual bases bug<?> in G++ 3.3 Janusz Szpilewski wrote: Quote: Jim Fischer wrote: void foo() { C c; c.f(); // calls B::f, the final overrider c.C::f(); // calls A::f because of the usingdeclaration } /quote It seems a little odd (to me) that the method call 'c.f()' in function foo() should invoke B::f. I thought it would invoke A::f due to the using declaration 'using A::f;' in class C's declaration block. And in fact, when I use G++ 3.3 to build the code sample shown below, the method call 'c.f()' invokes A::f (which is what I expected), and not B::f as stated in the C++ standard. I think there is some rational explanation. If the member function name is not (scope) qualified (like c.f()) then the final overrider is called. If it is, the name used in the qualified scope is chosen. As shows the example member function name does not have to be overridden, it may be referred by using directive. But in this case such a name is not considered when looking for overriders so the last overrider down in the hierarchy is chosen. Btw, Borland C++ compiles the example according to the standard. Thanks for the reply. I understand the meaning of paragraph 10.3/2 (although the description given in the standard is different from what I originally expected, but of course that's irrelevant). I was basically looking for confirmation that 10.3/2 still applies -- that there have not been any official changes to that particular paragraph -- before submitting a bug report to the GNU g++ maintainers. FWIW, this afternoon I submitted a G++ bugzilla report on this issue. -- Jim To reply by email, remove "link" and change "now.here" to "yahoo" jfischer_link5809{at}now.here.com 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 (comp.lang.c++) 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.