C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Stupid function name reuse          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message davidmontgomery Guest Posted: Fri Oct 22, 2004 5:21 pm    Post subject: Stupid function name reuse Hi, I'm trying to untangle some code that looks, in part, like this: class Base { public: void Caller() { Function( "Whence?" ); } void Function( char * szArg = NULL ) {} }; class Derived : public Base { public: void Function( char * szArg = NULL, int nArg = 0 ) {} }; int main() { D d; d.Caller(); return 0; } Will Caller() call Base::Function() or Derived::Function()? Why? Thanks much, David Montgomery [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Victor Bazarov Guest Posted: Sat Oct 23, 2004 2:14 pm    Post subject: Re: Stupid function name reuse davidmontgomery wrote: Quote: I'm trying to untangle some code that looks, in part, like this: class Base { public: void Caller() { Function( "Whence?" ); } void Function( char * szArg = NULL ) {} }; class Derived : public Base { public: void Function( char * szArg = NULL, int nArg = 0 ) {} }; int main() { D d; d.Caller(); return 0; } Will Caller() call Base::Function() or Derived::Function()? Why? The 'Caller' should call Base::Function, unfortunately. I say 'unfortunately' because "Whence?" is an array of _constant_ chars and the argument to Base::Function is a pointer to non-const char. V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top John M. Dlugosz Guest Posted: Sun Oct 24, 2004 4:03 am    Post subject: Re: Stupid function name reuse "davidmontgomery" <davidmontgomery (AT) netzero (DOT) com> wrote Quote: class Derived : public Base { public: void Function( char * szArg = NULL, int nArg = 0 ) {} }; int main() { D d; // assuming you meant ==> Derived d; d.Caller(); Will Caller() call Base::Function() or Derived::Function()? Why? It will call Derived::Function. The type of d is Derived, and there is one function named Function in it. That's all it can possibly call. The Base::Function is hidden by the use of the name in the derived class; they will not overload. --John [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top rishi Guest Posted: Sun Oct 24, 2004 4:11 am    Post subject: Re: Stupid function name reuse Hi, names in derived class hide those in the base class. In this case only derived class "Function" is visible. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Frank Birbacher Guest Posted: Mon Oct 25, 2004 4:41 pm    Post subject: Re: Stupid function name reuse Hi! John M. Dlugosz wrote: Quote: Will Caller() call Base::Function() or Derived::Function()? Why? It will call Derived::Function. The type of d is Derived, and there is one function named Function in it. That's all it can possibly call. The Base::Function is hidden by the use of the name in the derived class; they will not overload. That is not true. The Base class does not know about Derived. So Base::Caller will always call Base::Function, despite of function hiding. If Base::Function was declared 'virtual', then Base::Caller would still call Base::Function through virtual dispatch, because Derived::Function would not override Base::Function due to incompatible parameters lists. Frank [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top davidmontgomery Guest Posted: Mon Oct 25, 2004 7:47 pm    Post subject: Re: Stupid function name reuse Thanks to all for their responses. Both g++ (3.3.3) and MSVC++ 7.1 call Base::Function(), as Victor suggested. (Of course I should have written Derived d, not D d. Thanks John. I should have avoided the distraction of "Whence?"'s const nature by making Function()s take a const char * as the first argument; that wasn't the issue I meant to get at.) John Dlugosz wrote: Quote: It will call Derived::Function. The type of d is Derived, and there is one function named Function in it. That's all it can possibly call. rishi wrote: Quote: names in derived class hide those in the base class. In this case only derived class "Function" is visible. The call to Function is from Base::Caller(). Certainly within Base::Caller() the function Base::Function is visible. I guess what convinces me that g++, msvc and Victor are correct is thinking about separate compilation. Base::Function() isn't virtual. So the call from Base::Caller() to Function() can be (I guess must be) compiled directly without any indirection. The fact that we later link a Base.obj with a Derived.obj shouldn't change what gets called here. John and rishi (or anyone else), if you believe that both gcc and msvc are wrong on this, please post a follow-up. Thanks, David Montgomery [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Tokyo Tomy Guest Posted: Tue Oct 26, 2004 4:17 am    Post subject: Re: Stupid function name reuse "davidmontgomery" <davidmontgomery (AT) netzero (DOT) com> wrote Quote: ... class Base { public: void Caller() { Function( "Whence?" ); } void Function( char * szArg = NULL ) {} }; class Derived : public Base { public: void Function( char * szArg = NULL, int nArg = 0 ) {} }; int main() { D d; d.Caller(); return 0; } Will Caller() call Base::Function() or Derived::Function()? Why? ... IMHO, since Caller() is a member function of Base class, the Caller will call the Function in the base class. If you add a Caller() to Derived class, the Caller call the Function in the derived class. In this case, (1) if you remove void Function( char * szArg = NULL, int nArg = 0 ) {} , the Caller will call Function in the base class, and (2) if you replace void Function( char * szArg = NULL, int nArg = 0 ) {} with void Function() {}, then an error will be issued, because void Function() hide the Function in the base class. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alberto Barbati Guest Posted: Tue Oct 26, 2004 1:29 pm    Post subject: Re: Stupid function name reuse rishi wrote: Quote: Hi, names in derived class hide those in the base class. In this case only derived class "Function" is visible. Quite an imprecise answer. In the scope of class Derived, Derived::Function is visible, Base::Function having been hidden. In the scope of function main, Derived::Function is visible, because Base::Function is hidden and the static type of the variable d is Derived. (notice that Base::Function could still be called in both scopes if explicitly qualified, but that's not our case) However, in the scope of class Base, *only* Base::Function is visible. Such function is not virtual and even if it were virtual Derived::Function would not override Base::Function because the parameter list differs. As the call to Function is made from inside Base::Caller, that is from inside the scope of class Base, Base::Derived will thus be called. Alberto [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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