C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Problem with inheritance          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Victor Chew Guest Posted: Thu Jul 31, 2003 3:42 am    Post subject: Problem with inheritance Can someone tell me why the following code doesn't work: Quote: TestClass.cpp ------------- class A { public: virtual void read(wchar_t& ch) { read(&ch, 0, 1); } virtual void read(wchar_t* buf, int off, int len) = 0; }; class B : public virtual A { public: virtual void read(wchar_t* buf, int off, int len) {} }; int main(int argc, char* argv[]) { wchar_t ch; B myclass; myclass.read(ch); } I have tried both gcc 2.96 and gcc 3.2. I get: Quote: TestClass.cpp: In function `int main(int, char**)': TestClass.cpp:21: no matching function for call to `B::read(wchar_t&)' TestClass.cpp:14: candidates are: virtual void B::read(wchar_t*, int, int) Shouldn't B have inherited read(wchar_t& ch) from A? Back to top Josephine Schafer Guest Posted: Thu Jul 31, 2003 3:49 am    Post subject: Re: Problem with inheritance "Victor Chew" <vchew (AT) post1 (DOT) com> wrote Quote: Can someone tell me why the following code doesn't work: TestClass.cpp ------------- class A { public: virtual void read(wchar_t& ch) { read(&ch, 0, 1); } virtual void read(wchar_t* buf, int off, int len) = 0; }; class B : public virtual A { public: virtual void read(wchar_t* buf, int off, int len) {} }; int main(int argc, char* argv[]) { wchar_t ch; B myclass; myclass.read(ch); } I have tried both gcc 2.96 and gcc 3.2. I get: TestClass.cpp: In function `int main(int, char**)': TestClass.cpp:21: no matching function for call to `B::read(wchar_t&)' TestClass.cpp:14: candidates are: virtual void B::read(wchar_t*, int, int) Shouldn't B have inherited read(wchar_t& ch) from A? Your base class function virtual void read(wchar_t& ch) has been hidden by the derived class function virtual void read(wchar_t* buf, int off, int len). Back to top ES Kim Guest Posted: Thu Jul 31, 2003 3:50 am    Post subject: Re: Problem with inheritance "Victor Chew" <vchew (AT) post1 (DOT) com> wrote Quote: Can someone tell me why the following code doesn't work: TestClass.cpp ------------- class A { public: virtual void read(wchar_t& ch) { read(&ch, 0, 1); } virtual void read(wchar_t* buf, int off, int len) = 0; }; class B : public virtual A { public: virtual void read(wchar_t* buf, int off, int len) {} }; int main(int argc, char* argv[]) { wchar_t ch; B myclass; myclass.read(ch); } I have tried both gcc 2.96 and gcc 3.2. I get: TestClass.cpp: In function `int main(int, char**)': TestClass.cpp:21: no matching function for call to `B::read(wchar_t&)' TestClass.cpp:14: candidates are: virtual void B::read(wchar_t*, int, int) Shouldn't B have inherited read(wchar_t& ch) from A? B::read(wchar_t* buf, int off, int len) hides A::read(wchar_t& ch). If you override an overloaded base class functions, redefine full set of the functions. -- ES Kim Back to top Alf P. Steinbach Guest Posted: Thu Jul 31, 2003 4:12 am    Post subject: Re: Problem with inheritance On Thu, 31 Jul 2003 11:42:17 +0800, Victor Chew <vchew (AT) post1 (DOT) com> wrote: Quote: Can someone tell me why the following code doesn't work: TestClass.cpp ------------- class A { public: virtual void read(wchar_t& ch) { read(&ch, 0, 1); } virtual void read(wchar_t* buf, int off, int len) = 0; }; class B : public virtual A { public: using A::read; Quote: virtual void read(wchar_t* buf, int off, int len) {} }; int main(int argc, char* argv[]) { wchar_t ch; B myclass; myclass.read(ch); } I have tried both gcc 2.96 and gcc 3.2. I get: TestClass.cpp: In function `int main(int, char**)': TestClass.cpp:21: no matching function for call to `B::read(wchar_t&)' TestClass.cpp:14: candidates are: virtual void B::read(wchar_t*, int, int) Shouldn't B have inherited read(wchar_t& ch) from A? It has, but without the 'using' it's hidden by the other read. Now don't ask me _why_ someone thought that would be sensible. Back to top Victor Chew Guest Posted: Thu Jul 31, 2003 5:45 am    Post subject: Re: Problem with inheritance I don't get it. Isn't overriding based on method signatures? There is clearly a difference between read(wchar_t&) and read(wchar_t*, int, int)! Why can't I selectively override one of the methods from the base class? What is the workaround? Alf P. Steinbach wrote: Quote: On Thu, 31 Jul 2003 11:42:17 +0800, Victor Chew <vchew (AT) post1 (DOT) com> wrote: Can someone tell me why the following code doesn't work: TestClass.cpp ------------- class A { public: virtual void read(wchar_t& ch) { read(&ch, 0, 1); } virtual void read(wchar_t* buf, int off, int len) = 0; }; class B : public virtual A { public: using A::read; virtual void read(wchar_t* buf, int off, int len) {} }; int main(int argc, char* argv[]) { wchar_t ch; B myclass; myclass.read(ch); } I have tried both gcc 2.96 and gcc 3.2. I get: TestClass.cpp: In function `int main(int, char**)': TestClass.cpp:21: no matching function for call to `B::read(wchar_t&)' TestClass.cpp:14: candidates are: virtual void B::read(wchar_t*, int, int) Shouldn't B have inherited read(wchar_t& ch) from A? It has, but without the 'using' it's hidden by the other read. Now don't ask me _why_ someone thought that would be sensible. Back to top Kevin Goodsell Guest Posted: Thu Jul 31, 2003 5:58 am    Post subject: Re: Problem with inheritance ES Kim wrote: Quote: B::read(wchar_t* buf, int off, int len) hides A::read(wchar_t& ch). If you override an overloaded base class functions, redefine full set of the functions. There is no overriding in this case. Also, redefining the entire set of functions would be a pointless waste of time. A using declaration brings the base class's overloads into scope nicely. -Kevin Back to top Victor Chew Guest Posted: Thu Jul 31, 2003 6:06 am    Post subject: Re: Problem with inheritance Do you mind posting a short code segment showing me how to do this? Thanks! Kevin Goodsell wrote: Quote: ES Kim wrote: B::read(wchar_t* buf, int off, int len) hides A::read(wchar_t& ch). If you override an overloaded base class functions, redefine full set of the functions. There is no overriding in this case. Also, redefining the entire set of functions would be a pointless waste of time. A using declaration brings the base class's overloads into scope nicely. -Kevin Back to top John Harrison Guest Posted: Thu Jul 31, 2003 6:15 am    Post subject: Re: Problem with inheritance "Victor Chew" <vchew (AT) post1 (DOT) com> wrote Quote: Do you mind posting a short code segment showing me how to do this? Thanks! class B : public virtual A { public: virtual void read(wchar_t& ch) { A::read(ch); } virtual void read(wchar_t* buf, int off, int len) {} }; john Back to top Makis Papapanagiotou Guest Posted: Thu Jul 31, 2003 6:28 am    Post subject: Re: Problem with inheritance Hello, You can use polymorphism in order to have the workaround solution. Even though I wouldn't call it workaround, anyway... A* myclass = new B; myclass.read(ch); Then everything will work properly, since late binding will take part, and it will resolve the correct functions. "Victor Chew" <vchew (AT) post1 (DOT) com> wrote Quote: I don't get it. Isn't overriding based on method signatures? There is clearly a difference between read(wchar_t&) and read(wchar_t*, int, int)! Why can't I selectively override one of the methods from the base class? What is the workaround? Alf P. Steinbach wrote: On Thu, 31 Jul 2003 11:42:17 +0800, Victor Chew <vchew (AT) post1 (DOT) com> wrote: Can someone tell me why the following code doesn't work: TestClass.cpp ------------- class A { public: virtual void read(wchar_t& ch) { read(&ch, 0, 1); } virtual void read(wchar_t* buf, int off, int len) = 0; }; class B : public virtual A { public: using A::read; virtual void read(wchar_t* buf, int off, int len) {} }; int main(int argc, char* argv[]) { wchar_t ch; B myclass; myclass.read(ch); } I have tried both gcc 2.96 and gcc 3.2. I get: TestClass.cpp: In function `int main(int, char**)': TestClass.cpp:21: no matching function for call to `B::read(wchar_t&)' TestClass.cpp:14: candidates are: virtual void B::read(wchar_t*, int, int) Shouldn't B have inherited read(wchar_t& ch) from A? It has, but without the 'using' it's hidden by the other read. Now don't ask me _why_ someone thought that would be sensible. Back to top Kevin Goodsell Guest Posted: Thu Jul 31, 2003 6:41 am    Post subject: Re: Problem with inheritance Victor Chew wrote: Quote: Do you mind posting a short code segment showing me how to do this? Thanks! Please do not top-post. Re-read section 5 of the FAQ for posting guidelines: http://www.parashift.com/c++-faq-lite/how-to-post.html See Alf's reply for the example you requested. -Kevin Back to top David White Guest Posted: Thu Jul 31, 2003 9:44 am    Post subject: Re: Problem with inheritance "Victor Chew" <vchew (AT) post1 (DOT) com> wrote Quote: I don't get it. Isn't overriding based on method signatures? There is clearly a difference between read(wchar_t&) and read(wchar_t*, int, int)! Please read all replies, such as my other one, if you see it there. It has a link to a thread that explains the rule and why. DW Back to top Alf P. Steinbach Guest Posted: Thu Jul 31, 2003 5:13 pm    Post subject: Re: Problem with inheritance On Thu, 31 Jul 2003 13:45:46 +0800, Victor Chew <vchew (AT) post1 (DOT) com> wrote: Quote: I don't get it. Isn't overriding based on method signatures? There is clearly a difference between read(wchar_t&) and read(wchar_t*, int, int)! Why can't I selectively override one of the methods from the base class? You can, and you just did. It is just as static type B that one 'read' method is hidden. The other is still there. E.g., you can cast it to A& and access the other 'read'. David White here provided a URL to an earlier discussion where Russel Hanneken provided a URL to an even earlier discussion where Chris Newton tried to explain the original thinking, see [http://tinyurl.com/hlts]. Quote: What is the workaround? 'using', as shown in my first reply. Back to top Victor Chew Guest Posted: Fri Aug 01, 2003 1:40 am    Post subject: Re: Problem with inheritance Thanks for all your replies. I support Chris' comment that "I personally regard this decision as unfortunate". If there is a perfect method signature match in the base class which is not overridden in the subclass, then it should simply be used. It's the principle of least surprise. Alf P. Steinbach wrote: Quote: On Thu, 31 Jul 2003 13:45:46 +0800, Victor Chew <vchew (AT) post1 (DOT) com> wrote: I don't get it. Isn't overriding based on method signatures? There is clearly a difference between read(wchar_t&) and read(wchar_t*, int, int)! Why can't I selectively override one of the methods from the base class? You can, and you just did. It is just as static type B that one 'read' method is hidden. The other is still there. E.g., you can cast it to A& and access the other 'read'. David White here provided a URL to an earlier discussion where Russel Hanneken provided a URL to an even earlier discussion where Chris Newton tried to explain the original thinking, see [http://tinyurl.com/hlts]. What is the workaround? 'using', as shown in my first reply. 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.