C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Pointer to virtual object on stack          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message cyrusNew Guest Posted: Wed Dec 29, 2004 10:15 pm    Post subject: Pointer to virtual object on stack Hi, I need help to resolve confusion on following example : class A { public: virtual void GetName() = 0; }; class B : public A { public: void GetName() { cout << "B"; } }; B b; A* pA = &b; pA->GetName(); I think this is correct code, but I dont quite understand whats happening behind it. I mean is pointer pA just equal to B's vptr and if so then does it require some special sorting of elements of vtbl, like first comes A virtual methods then B's virtual methods. Thanks, Back to top Victor Bazarov Guest Posted: Wed Dec 29, 2004 10:33 pm    Post subject: Re: Pointer to virtual object on stack cyrusNew wrote: Quote: I need help to resolve confusion on following example : class A { public: virtual void GetName() = 0; }; class B : public A { public: void GetName() { cout << "B"; } }; B b; A* pA = &b; pA->GetName(); I think this is correct code, but I dont quite understand whats happening behind it. Do you really need to? Compile it and look at the assembly listing. Quote: I mean is pointer pA just equal to B's vptr and if so then does it require some special sorting of elements of vtbl, like first comes A virtual methods then B's virtual methods. pA just points to the A subobject of 'b'. They share the vtbl. Whenever a virtual function call is to be made, it's resolved by indirection with a corresponding entry in the vtbl, the compiler takes care of that. When 'b' is fully constructed, the vtbl contains addresses of all 'A' functions that are not overridden by 'B' and those that override 'A's ones. Until 'b' is fully constructed, the vtbl probably contains only the addresses of 'A's functions. This is not really defined by the language, BTW. So, whatever ideas you get about the implementation of the virtual functions mechanism, it does not have to be the same everywhere. V Back to top cyrusNew Guest Posted: Wed Dec 29, 2004 10:50 pm    Post subject: Re: Pointer to virtual object on stack Uzytkownik "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> napisal w wiadomosci news:NgGAd.34183$NC6.16450 (AT) newsread1 (DOT) mlpsca01.us.to.verio.net... Quote: cyrusNew wrote: I need help to resolve confusion on following example : class A { public: virtual void GetName() = 0; }; class B : public A { public: void GetName() { cout << "B"; } }; B b; A* pA = &b; pA->GetName(); I think this is correct code, but I dont quite understand whats happening behind it. Do you really need to? Compile it and look at the assembly listing. I mean is pointer pA just equal to B's vptr and if so then does it require some special sorting of elements of vtbl, like first comes A virtual methods then B's virtual methods. pA just points to the A subobject of 'b'. They share the vtbl. Whenever a virtual function call is to be made, it's resolved by indirection with a corresponding entry in the vtbl, the compiler takes care of that. When 'b' is fully constructed, the vtbl contains addresses of all 'A' functions that are not overridden by 'B' and those that override 'A's ones. Until 'b' is fully constructed, the vtbl probably contains only the addresses of 'A's functions. This is not really defined by the language, BTW. So, whatever ideas you get about the implementation of the virtual functions mechanism, it does not have to be the same everywhere. V Thank you for answer, I just started using vectors like vector<B> and getting pointers of its elements of type A*. Until now I was mostly using dynamic allocation for creation of objects of polymorphic types so I wanted to be clear if all is working correctly. Now I will just look into "Inside the C++ Object Model" by Stanley Lippman for further info. Back to top cpptutor2000@yahoo.com Guest Posted: Wed Dec 29, 2004 10:51 pm    Post subject: Re: Pointer to virtual object on stack It is a very simple case of inheritance and dynamic binding. Class B is derived from clas A, which means that B 'isa' A, anytime an instance of B is generated, an instance of A has to be. The pure virtual function of A is being overridden in B. By inheritance, the statement 'A* pA = &b;' is perfectly valid, and when you have 'pA->GetName();' the dynamic binding uses B's implementation of 'GetName()'. Try out the following: #include <iostream> using namespace std; class A{ public: A(){} virtual void doit() = 0; }; class B : public A { public: B():A(){} ~B(){} void doit(){ cout<<" This is B"< }; int main(){ B b; A *ap = &b; ap->doit(); return 0; } Hope that helps. cyrusNew wrote: Quote: Hi, I need help to resolve confusion on following example : class A { public: virtual void GetName() = 0; }; class B : public A { public: void GetName() { cout << "B"; } }; B b; A* pA = &b; pA->GetName(); I think this is correct code, but I dont quite understand whats happening behind it. I mean is pointer pA just equal to B's vptr and if so then does it require some special sorting of elements of vtbl, like first comes A virtual methods then B's virtual methods. 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