C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Inheritance and virtual functions          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Colin Guest Posted: Thu Dec 08, 2005 11:53 am    Post subject: Inheritance and virtual functions I'm coding a game that has different types of animations. I want to have a parent class called Animation, and then inherit classes from it called things like AnimFoo, AnimBar, etc. that do different types of animations. I'm thinking the basic skeleton will look like this: class Animation { virtual draw(); virtual step(); }; class AnimFoo : public Animation { draw(); step(); }; etc... I will have a vector of Animation, and I will put the inherited classes into it. This way, I can just iterate through one vector calling draw() and step() to handle all the different types of animations. I have implemented this: for (vector< Animation >::iterator i = animations.begin(); i != animations.end(); ++i) { (*i).draw(); (*i).step(); } But the functions for Animation are being called rather than the different inherited classes. I tried making the virtual functions pure virtual, but then it complained about making a vector of abstract classes. Any ideas? Thanks, Colin Kern [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Heinz Ozwirk Guest Posted: Thu Dec 08, 2005 12:57 pm    Post subject: Re: Inheritance and virtual functions "Colin" <razael1 (AT) gmail (DOT) com> schrieb im Newsbeitrag news:1134018950.888486.318290 (AT) f14g2000cwb (DOT) googlegroups.com... Quote: I'm coding a game that has different types of animations. I want to have a parent class called Animation, and then inherit classes from it called things like AnimFoo, AnimBar, etc. that do different types of animations. I'm thinking the basic skeleton will look like this: class Animation { virtual draw(); virtual step(); }; class AnimFoo : public Animation { draw(); step(); }; etc... I will have a vector of Animation, and I will put the inherited classes into it. This way, I can just iterate through one vector calling draw() and step() to handle all the different types of animations. I have implemented this: for (vector< Animation >::iterator i = animations.begin(); i != animations.end(); ++i) { (*i).draw(); (*i).step(); } But the functions for Animation are being called rather than the different inherited classes. I tried making the virtual functions pure virtual, but then it complained about making a vector of abstract classes. Any ideas? When you put instances of derived classes into a vector of base class objects, only the base class part will be put into your array (slicing). Polymorphism only works with pointers or references to objects, not with the objects themselves. What you need is a vector of pointers, like std::vector<Animation*> HTH Heinz [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Hrayr BABAJANYAN Guest Posted: Thu Dec 08, 2005 4:17 pm    Post subject: Re: Inheritance and virtual functions Hi! To use the polymorphic mechanism you should use vector of pointers or references (e.g. vector<Animation*>). In your case the instances of derived classes (e.g. AnimFoo) are being cut during the insertion into the vector<Animation>, so the functions of the base class Animation are being called. And that is why if you make Animation abstract, the vector<Animation> contains only "instances" (cannot instantiate abstract class) of abstract class, and the compiler complains. Cheers [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Tutone Guest Posted: Thu Dec 08, 2005 4:18 pm    Post subject: Re: Inheritance and virtual functions Heinz Ozwirk wrote: Quote: When you put instances of derived classes into a vector of base class objects, only the base class part will be put into your array (slicing). Polymorphism only works with pointers or references to objects, not with the objects themselves. What you need is a vector of pointers, like std::vector<Animation* Or better yet, a vector of smart pointers, like std::vector< std::tr1::shared_ptr Best regards, Tom [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Martin Bonner Guest Posted: Thu Dec 08, 2005 4:20 pm    Post subject: Re: Inheritance and virtual functions Heinz Ozwirk wrote: Quote: "Colin" <razael1 (AT) gmail (DOT) com> schrieb im Newsbeitrag news:1134018950.888486.318290 (AT) f14g2000cwb (DOT) googlegroups.com... I'm coding a game that has different types of animations. I want to have a parent class called Animation, and then inherit classes from it called things like AnimFoo, AnimBar, etc. that do different types of animations. [snip] Quote: When you put instances of derived classes into a vector of base class objects, only the base class part will be put into your array (slicing). Polymorphism only works with pointers or references to objects, not with the objects themselves. What you need is a vector of pointers, like std::vector<Animation* Or std::vector< boost::shared_ptr if you aren't using garbage collection (google for boost::shared_ptr) [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Fri Dec 09, 2005 3:07 pm    Post subject: Re: Inheritance and virtual functions Martin Bonner wrote: Quote: Heinz Ozwirk wrote: "Colin" <razael1 (AT) gmail (DOT) com> schrieb im Newsbeitrag news:1134018950.888486.318290 (AT) f14g2000cwb (DOT) googlegroups.com... I'm coding a game that has different types of animations. I want to have a parent class called Animation, and then inherit classes from it called things like AnimFoo, AnimBar, etc. that do different types of animations. [snip] When you put instances of derived classes into a vector of base class objects, only the base class part will be put into your array (slicing). Polymorphism only works with pointers or references to objects, not with the objects themselves. What you need is a vector of pointers, like std::vector<Animation* Or std::vector< boost::shared_ptr if you aren't using garbage collection (google for boost::shared_ptr) You mean there are people who don't use a garbage collector with C++? :-) That's http://www.hpl.hp.com/personal/Hans_Boehm/gc/ for the garbage collector, and http://www.boost.org for Boost (which you'll need for other things, even if you are using the garbage collector). -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Colin Guest Posted: Mon Dec 12, 2005 12:08 am    Post subject: Re: Inheritance and virtual functions I don't like the extra overhead. It's not that hard to make sure you destroy anything you've dynamically created. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Joshua Lehrer Guest Posted: Mon Dec 12, 2005 12:07 pm    Post subject: Re: Inheritance and virtual functions Heinz Ozwirk wrote: Quote: "Colin" <razael1 (AT) gmail (DOT) com> schrieb im Newsbeitrag When you put instances of derived classes into a vector of base class objects, only the base class part will be put into your array (slicing). Polymorphism only works with pointers or references to objects, not with the objects themselves. What you need is a vector of pointers, like std::vector This is a benefit of using type-erasure over inheritence to implement polymorphism. Your Animation class could contain a pointer to a function that implements 'move', 'draw', and any other polymorphic behavior. You can then store a vector of Animation objects and not have to worry about slicing. Google for 'type erasure' for a more detailed explanation. joshua lehrer http://www.lehrerfamily.com/ [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Mon Dec 12, 2005 3:12 pm    Post subject: Re: Inheritance and virtual functions Colin wrote: [Talking about garbage collection, I think...] Quote: I don't like the extra overhead. What extra overhead? Quote: It's not that hard to make sure you destroy anything you've dynamically created. But it's more overhead -- both for me and for the machine, but especially for me. -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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