C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Referring to a base class in a derived class          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Webster Guest Posted: Sat Jan 31, 2004 1:01 am    Post subject: Referring to a base class in a derived class Hello, I have a class Shape and subclasses Circle, Rectangle etc. I also have a function to write them to a file given a filepointer. However, I store the colour info in the Shape class. My question is, is it possible to create a WriteColour function in the Shape class, but when I call the sub class writes (i.e. Circle.Write(...)), it will call the Shape's WriteColour function?? Or is there a way to refer to the base class in a derived class to call it?? i.e. base->function(); Thanks for any help! -- Regards, Webster Back to top Janusz Szpilewski Guest Posted: Sat Jan 31, 2004 1:18 am    Post subject: Re: Referring to a base class in a derived class Webster wrote: Quote: Or is there a way to refer to the base class in a derived class to call it?? i.e. base->function(); You can always use the fully qualified name, in this case: Shape::function(); Regards, Janusz Back to top lilburne Guest Posted: Sat Jan 31, 2004 1:20 am    Post subject: Re: Referring to a base class in a derived class Webster wrote: Quote: Hello, I have a class Shape and subclasses Circle, Rectangle etc. I also have a function to write them to a file given a filepointer. However, I store the colour info in the Shape class. My question is, is it possible to create a WriteColour function in the Shape class, but when I call the sub class writes (i.e. Circle.Write(...)), it will call the Shape's WriteColour function?? Or is there a way to refer to the base class in a derived class to call it?? i.e. base->function(); Unless you have also defined WriteColour in the derived class then: WriteColour(); will do fine. Otherwise you need to use scope resolution: Base::WriteColour() Back to top Webster Guest Posted: Sat Jan 31, 2004 1:26 am    Post subject: Re: Referring to a base class in a derived class "lilburne" <lilburne (AT) godzilla (DOT) net> wrote Quote: Unless you have also defined WriteColour in the derived class then: WriteColour(); will do fine. Otherwise you need to use scope resolution: Base::WriteColour() Right now Write() is defined as Virtual in the Shape class. So is it better to have it non-virtual, and instead write the colour, and then in the subclasses, call the Base::Write() ?? What is the better programming style? -- Regards, Webster Back to top Andrey Tarasevich Guest Posted: Sat Jan 31, 2004 1:29 am    Post subject: Re: Referring to a base class in a derived class Webster wrote: Quote: I have a class Shape and subclasses Circle, Rectangle etc. I also have a function to write them to a file given a filepointer. However, I store the colour info in the Shape class. My question is, is it possible to create a WriteColour function in the Shape class, but when I call the sub class writes (i.e. Circle.Write(...)), it will call the Shape's WriteColour function?? Or is there a way to refer to the base class in a derived class to call it?? i.e. base->function(); ... If you derived classes do not declare their own 'WriteColor' functions then by simply writing 'WriteColor' you'll refer to base class' function. Otherwise you can use scope resolution operator '::' and write 'Shape::WriteColor' to explicitly refer to 'Shape's 'WriteColor' function. -- Best regards, Andrey Tarasevich Back to top Janusz Szpilewski Guest Posted: Sat Jan 31, 2004 1:47 am    Post subject: Re: Referring to a base class in a derived class Webster wrote: Quote: Right now Write() is defined as Virtual in the Shape class. So is it better to have it non-virtual, and instead write the colour, and then in the subclasses, call the Base::Write() ?? What is the better programming style? Having the only functionality of the overriding function limited to calling the overridden one in the base class is definitely not a great idea. In such a case just do not redefine the base class function in the derived class. If a function is virtual it does not mean you have to override it in the derived classes. Nevertheless if the method is very simple, not going to be overriden, like returning the color of a shape you may even leave it as non-virtual. It may be a good idea to read carefully more in your C++ primer about polymorphism. Regards, Janusz Back to top lilburne Guest Posted: Sat Jan 31, 2004 1:52 am    Post subject: Re: Referring to a base class in a derived class Webster wrote: Quote: "lilburne" <lilburne (AT) godzilla (DOT) net> wrote in message news:bvevra$rpij1$1 (AT) ID-203936 (DOT) news.uni-berlin.de... Unless you have also defined WriteColour in the derived class then: WriteColour(); will do fine. Otherwise you need to use scope resolution: Base::WriteColour() Right now Write() is defined as Virtual in the Shape class. So is it better to have it non-virtual, and instead write the colour, and then in the subclasses, call the Base::Write() ?? What is the better programming style? You only need to be concerned if you really, really want to call the base class method. Otherwise just call Write(). You make the function non-virtual if you can't think of a good reason why a derived class would want to redefine it. Be aware that if you make it virtual and *have* to use the Base::Write() syntax then the base method will be called even if you alter the class heirarchy. There is no equivalent of 'super' in C++. Back to top Daniel T. Guest Posted: Sat Jan 31, 2004 6:57 am    Post subject: Re: Referring to a base class in a derived class "Webster" <noone (AT) nowhere (DOT) net> wrote: Quote: "lilburne" <lilburne (AT) godzilla (DOT) net> wrote: Unless you have also defined WriteColour in the derived class then: WriteColour(); will do fine. Otherwise you need to use scope resolution: Base::WriteColour() Right now Write() is defined as Virtual in the Shape class. So is it better to have it non-virtual, and instead write the colour, and then in the subclasses, call the Base::Write() ?? What is the better programming style? Better programming style would be to not have member-variables in your base class, IMO. Back to top Rolf Magnus Guest Posted: Sat Jan 31, 2004 12:43 pm    Post subject: Re: Referring to a base class in a derived class Webster wrote: Quote: Hello, I have a class Shape and subclasses Circle, Rectangle etc. I also have a function to write them to a file given a filepointer. However, I store the colour info in the Shape class. My question is, is it possible to create a WriteColour function in the Shape class, but when I call the sub class writes (i.e. Circle.Write(...)), it will call the Shape's WriteColour function?? You can use something similar to the following: class Shape { public: //... void Write() const; private: void WriteColour() const; virtual void DoWrite() const = 0; //... }; void Shape::WriteColour() const { //... } void Shape::Write() const { WriteColour(); DoWrite(); } class Circle { //... private: void DoWrite() const; }; So when someone calls Write() on the object, WriteColour() will always be called first, then DoWrite() will be called polymorpically. Quote: Or is there a way to refer to the base class in a derived class to call it?? i.e. base->function(); Just: function(); should do fine. But the downside is that you have to remember that in every derived class. And if you decide at some point that you might want to add something else that is the same for all shapes, you need to change every derived class, while with the approach I showed above, you only need to change Shape::Write() to handle it. 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