C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  virtual member function in constructor          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message Karsten Hochkirch Guest Posted: Fri Jan 30, 2004 8:21 pm    Post subject: virtual member function in constructor Hi, I have just encountered a problem when calling a virtual member function in a constructor. Only the memberfunction of the parent class is called instead of the overloaded function. I have attached a small example. Is that bug or feature. If feature: is there an easy way to get the behaviour as I am looking for? Thanks Karsten #include <iostream> using namespace std; class A { public: virtual A(){init();} virtual ~A(){} virtual void init(){ cout << "class A::initn"; } void test(){init();} }; class B: public A{ public: B():A(){} virtual ~B(){} virtual void init(){ cout << "class B::initn"; } }; int main(int,char**){ // only class A's init is called B b; // works as I expected b.test(); return 0; } Back to top Karsten Hochkirch Guest Posted: Fri Jan 30, 2004 8:23 pm    Post subject: Re: virtual member function in constructor Oops, the 'virtual' in front of the constructor was just desparate try I forgot to remove before attaching the file... Karsten Hochkirch wrote: Quote: Hi, I have just encountered a problem when calling a virtual member function in a constructor. Only the memberfunction of the parent class is called instead of the overloaded function. I have attached a small example. Is that bug or feature. If feature: is there an easy way to get the behaviour as I am looking for? Thanks Karsten ------------------------------------------------------------------------ #include using namespace std; class A { public: virtual A(){init();} virtual ~A(){} virtual void init(){ cout << "class A::initn"; } void test(){init();} }; class B: public A{ public: B():A(){} virtual ~B(){} virtual void init(){ cout << "class B::initn"; } }; int main(int,char**){ // only class A's init is called B b; // works as I expected b.test(); return 0; } Back to top Ron Natalie Guest Posted: Fri Jan 30, 2004 8:31 pm    Post subject: Re: virtual member function in constructor "Karsten Hochkirch" <K.Hochkirch (AT) vbew (DOT) net> wrote Quote: Is that bug or feature. If feature: is there an easy way to get the behaviour as I am looking for? That is the way it is supposed to work. For the duration of the execution of the the constructor body, the dynamic type of the object is considered to be that of the constructor that is running. The primary reason is that the derived class has yet to be initialized (the base class constructor is run first), so you don't want to be roaming around in the derived code. The work around is to place any virtual function based common initialization in a base class member function other than the constructor and then call it from the derived constructor. The same thing happens for destructors (just in the reverse order). Back to top Le Géant Vert Guest Posted: Fri Jan 30, 2004 8:37 pm    Post subject: Re: virtual member function in constructor Karsten Hochkirch wrote: Quote: Oops, the 'virtual' in front of the constructor was just desparate try I forgot to remove before attaching the file... Karsten Hochkirch wrote: Hi, I have just encountered a problem when calling a virtual member function in a constructor. Only the memberfunction of the parent class is called instead of the overloaded function. I have attached a small example. Is that bug or feature. If feature: is there an easy way to get the behaviour as I am looking for? Thanks Karsten this is not really a feature, this is the way things are supposed to happen... When the constructor's called, the object does not exist yet : so the code of your overloaded virtual method is unknown and unreachable ; thus, only the one of the parent class can be called. (a funny thing you can do once you've understood this, is to use it to call code of a pure virtual method) the tip I'd told you is to never call any method in a constructor, and if needed, to provided an init() that should be called by the programmer (not as you're doing in your code sample, where init() is called from the ctor) Back to top Jonathan Turkanis Guest Posted: Fri Jan 30, 2004 8:40 pm    Post subject: Re: virtual member function in constructor "Karsten Hochkirch" <K.Hochkirch (AT) vbew (DOT) net> wrote Quote: Hi, I have just encountered a problem when calling a virtual member function in a constructor. Within a constructor, virtual calls are resolved using the vtable of the class in which the constructor is defined, rather than the vtable of the fully derived type of the object being constructed. Therefore, you must be very careful when calling virtual functions from constructors; in particular, the possibility of calling a pure virtual function exists. Jonathan Back to top Alf P. Steinbach Guest Posted: Fri Jan 30, 2004 8:55 pm    Post subject: Re: virtual member function in constructor On Fri, 30 Jan 2004 15:31:10 -0500, "Ron Natalie" <ron (AT) sensor (DOT) com> wrote: Quote: "Karsten Hochkirch" <K.Hochkirch (AT) vbew (DOT) net> wrote Is that bug or feature. If feature: is there an easy way to get the behaviour as I am looking for? [Correct explanation of why, snipped.] See also the FAQ: <url: http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.3> Quote: The work around Actually there are many different workarounds. The most common workarounds are also in the FAQ (I'm happy to take much credit for their inclusion, although credit for the text goes to Marshall Cline): <url: http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.4> Quote: is to place any virtual function based common initialization in a base class member function other than the constructor and then call it from the derived constructor. This only seems to allow virtuality for one level of class derivation. It's not a commonly used workaround. I don't recommend it, but perhaps it should be included among the others in the FAQ? Back to top David Harmon Guest Posted: Fri Jan 30, 2004 8:56 pm    Post subject: Re: virtual member function in constructor On Fri, 30 Jan 2004 21:21:34 +0100 in comp.lang.c++, Karsten Hochkirch <K.Hochkirch (AT) vbew (DOT) net> was alleged to have written: Quote: I have just encountered a problem when calling a virtual member function in a constructor. Only the memberfunction of the parent class is called instead of the overloaded function. This issue is covered in Marshall Cline's C++ FAQ. See the topic "[23.3] When my base class's constructor calls a virtual function on its this object, why doesn't my derived class's override of that virtual function get invoked?" It is always good to check the FAQ before posting. You can get the FAQ at: http://www.parashift.com/c++-faq-lite/ Back to top Karsten Hochkirch Guest Posted: Fri Jan 30, 2004 11:08 pm    Post subject: Re: virtual member function in constructor Ok, I think I understand by now. Thanks to all of you for the anwers!! 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