C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  using derived class in base class          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Jan Bernatik Guest Posted: Mon Jan 26, 2004 12:06 am    Post subject: using derived class in base class hi. I'm stuck with it for few days, and can't find a way to work it out. I want to make a pointer to object of derived class in base class. ( I use QT + OpenGL, QGLWidget is inherited from qgl.h) So I tried to make it like this: ---------------------------------------------------------------------- class base_object : public QGLWidget {}; class triangle : base_object {}; class base_object : public QGLWidget { Q_OBJECT // makro needed by QT libraries public: base_object( QWidget *parent = 0, const char *name = 0 ); ~base_object(); void SetPtr(triangle *temp) {p_obj = temp; }; public slots: protected: private: triangle *p_obj; }; class triangle : public base_object { public: triangle() { SetPtr(this);}; }; ---------------------------------------------------------------------- compiler says: redefinition of `class base_object' ... I dont know where to look for help, I tried to find answer in inheritance, declaration and definition of classes, in Thinking in C++ book, but I haven't found it. Is it possible to solve it like this, or is this idea completely wrong ?? thanks for help, have a nice day. John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Magnar Nedland Guest Posted: Mon Jan 26, 2004 10:28 am    Post subject: Re: using derived class in base class Quote: class base_object : public QGLWidget {}; class triangle : base_object {}; Your problem lies here, in the alledged forward declarations. These are in fact full definitions of the two classes, thus the compiler is in its right to tell you "redefinition of `class base_object'". What you want is simply: class triangle; Magnar [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top void Guest Posted: Mon Jan 26, 2004 10:35 am    Post subject: Re: using derived class in base class U¿ytkownik Jan Bernatik napisa³, On 2004-01-26 01:06: Quote: hi. I'm stuck with it for few days, and can't find a way to work it out. I want to make a pointer to object of derived class in base class. ( I use QT + OpenGL, QGLWidget is inherited from qgl.h) So I tried to make it like this: ---------------------------------------------------------------------- class base_object : public QGLWidget {}; class triangle : base_object {}; class base_object : public QGLWidget { Q_OBJECT // makro needed by QT libraries public: base_object( QWidget *parent = 0, const char *name = 0 ); ~base_object(); void SetPtr(triangle *temp) {p_obj = temp; }; public slots: protected: private: triangle *p_obj; }; class triangle : public base_object { public: triangle() { SetPtr(this);}; }; ---------------------------------------------------------------------- compiler says: redefinition of `class base_object' ... I dont know where to look for help, I tried to find answer in You should find help in your book. Here we have two declarations of base_object, if really want to have pointer to derived class in base your class you should replace this two lines: Quote: ---------------------------------------------------------------------- class base_object : public QGLWidget {}; class triangle : base_object {}; with this one line and it should work correctly: class triangle; IMHO all you need is pointer to base class and virtual methods and not pointer tpo derived class in base class, IMHO this is very strange constuction. Best Darek Ostolski -- MORE CORE AVAILABLE, BUT NOT FOR YOU [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Mon Jan 26, 2004 10:36 am    Post subject: Re: using derived class in base class Jan Bernatik wrote: Quote: class base_object : public QGLWidget {}; class triangle : base_object {}; class base_object : public QGLWidget { triangle *p_obj; }; class triangle : public base_object { }; Firstly, remove the first two lines. Then, the compiler will only complain about unknown type 'triangle' being used. Since you only need a pointer to it, a declaration will be enough: class trangle; Uli -- Questions ? see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first ! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gordon Marr Guest Posted: Mon Jan 26, 2004 10:44 am    Post subject: Re: using derived class in base class Hi - The problem here isn't inheritance; it's the fact that you've defined both of your classes twice. Perhaps you meant the first definitions to be forward declarations? Try changing these lines: Quote: class base_object : public QGLWidget {}; class triangle : base_object {}; to: class base_object; class triangle; It compiles for me with G++ 3.3.2 then, if I add definitions for the QT stuff (no QT here). Why do you need pointers to derived objects in your base classes? GM. PS. Thanks to those of you who answered my question about C++ loop syntax - much appreciated! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Jan Bernatik Guest Posted: Mon Jan 26, 2004 2:51 pm    Post subject: Re: using derived class in base class Quote: Gordon Marr wrote: Why do you need pointers to derived objects in your base classes? My idea is, that I will make one base_object, with static array of derived objects, and than I will make one method for draw all this objects. So in the end there will be base_object.Draw method which will draw all derived objects. I now it can be done in different way, but I think this is not completely wrong, or is it ? Please tell me. Thank all of you for help. Of course it works mow. have a nice day John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sergei Emantayev Guest Posted: Mon Jan 26, 2004 8:32 pm    Post subject: Re: using derived class in base class [email]bernatik (AT) kn (DOT) vutbr.cz[/email] (Jan Bernatik) wrote in message news:<24f76b1f.0401251029.5eb49cc0 (AT) posting (DOT) google.com>... Quote: hi. I'm stuck with it for few days, and can't find a way to work it out. I want to make a pointer to object of derived class in base class. ( I use QT + OpenGL, QGLWidget is inherited from qgl.h) So I tried to make it like this: ---------------------------------------------------------------------- class base_object : public QGLWidget {}; class triangle : base_object {}; Just put forward declaration, not class definition! class triangle; class base_object : public QGLWidget { ... void SetPtr(triangle *temp) {p_obj = temp; }; ... }; class triangle : public base_object { ... }; Quote: thanks for help, have a nice day. John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Daniel Krügler (nee Spang Guest Posted: Tue Jan 27, 2004 2:44 pm    Post subject: Re: using derived class in base class Good Morning Jan or John, Jan Bernatik schrieb: Quote: ---------------------------------------------------------------------- class base_object : public QGLWidget {}; class triangle : base_object {}; class base_object : public QGLWidget { Q_OBJECT // makro needed by QT libraries public: base_object( QWidget *parent = 0, const char *name = 0 ); ~base_object(); void SetPtr(triangle *temp) {p_obj = temp; }; public slots: protected: private: triangle *p_obj; }; class triangle : public base_object { public: triangle() { SetPtr(this);}; }; ---------------------------------------------------------------------- compiler says: redefinition of `class base_object' ... The compiler expresses quite clearly what is wrong. Both the base_object class and the triangle class are redefined in the same translation unit (Additionally comes: These definitions are violationg the ODR - one definition rule, but that second problem is another one) First you write class base_object : public QGLWidget {}; which **defines** the base_object class. Later on you write class base_object : public QGLWidget { //... } which also defines the base_object class (and additionally this time differently from the first). The very same problem applies to the multiple definitions of the triangle class. What you actually need to do is to **declare** the triangle class by using the syntax class triangle; before the definitions of the 2 mentioned classes. Then everything should be fine. Greetings from Bremen, Daniel [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gordon Marr Guest Posted: Tue Jan 27, 2004 3:02 pm    Post subject: Re: using derived class in base class Jan Bernatik wrote: Quote: Gordon Marr wrote: Why do you need pointers to derived objects in your base classes? My idea is, that I will make one base_object, with static array of derived objects, and than I will make one method for draw all this objects. So in the end there will be base_object.Draw method which will draw all derived objects. I now it can be done in different way, but I think this is not completely wrong, or is it ? Please tell me. Sounds like it would work, but you'd have to have an array for each type of derived object, though. If you have time, try looking up virtual member functions in your C++ book. The FAQ for this group discusses the subject, too. GM. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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