C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  accessing virtulal function from constructor          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Kiran Guest Posted: Fri Nov 19, 2004 3:36 pm    Post subject: accessing virtulal function from constructor Hi, I was doing something for which I tried to access the virtual function from constructor which gave me error. can anybody explain why accessing virtual function from constructor is barred Thanks Regards, Kiran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top fabioppp Guest Posted: Sat Nov 20, 2004 10:20 am    Post subject: Re: accessing virtulal function from constructor A good explanation is given by Bruce Eckel in his first volume of Thinking in C++. Chapter 15. virtual functions & constructors. --Fabio Kiran wrote: Quote: Hi, I was doing something for which I tried to access the virtual function from constructor which gave me error. can anybody explain why accessing virtual function from constructor is barred [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Peter Koch Larsen Guest Posted: Sat Nov 20, 2004 10:28 am    Post subject: Re: accessing virtulal function from constructor "Kiran" <kiran.lokhande (AT) gmail (DOT) com> skrev i en meddelelse news:2721ed0c.0411182128.73780258 (AT) posting (DOT) google.com... Quote: Hi, I was doing something for which I tried to access the virtual function from constructor which gave me error. can anybody explain why accessing virtual function from constructor is barred Thanks Regards, Kiran It is not barred, but you call the virtual function for the object that is constructed: struct b { virtual void f() { std::cout << "b"; } b() { f();} }; struct d: b { virtual void f() { std::cout << "d"; } b() { f();} }; void test() { d dd; // should produce "bd"; } The reason is that the constructor of b can not call d's f as d is not constructed yet. Kind regards Peter [ 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 Nov 20, 2004 10:35 am    Post subject: Re: accessing virtulal function from constructor Kiran wrote: Quote: I was doing something for which I tried to access the virtual function from constructor which gave me error. can anybody explain why accessing virtual function from constructor is barred Nobody can explain why it's barred because it is _not_. If you have problems with particular code, post your code and the error message you're getting from your compiler. The only thing that _might_ be bad is calling a _pure_ virtual function from a constructor because that produces _undefined behaviour_. V [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Kurt Krueckeberg Guest Posted: Sat Nov 20, 2004 4:23 pm    Post subject: Re: accessing virtulal function from constructor Quote: Hi, I was doing something for which I tried to access the virtual function from constructor which gave me error. can anybody explain why accessing virtual function from constructor is barred Thanks Regards, Kiran In C++ a derived object doesn't 'come to life' any it's constructor has been run. If you want to be able to create copies of an object using only a base class pointer, you could add a virtual clone method to the base class. class base { public: //. . . virtual base *clone() const = 0; //. . . }; class derived { public: derived *clone() const { return new(*this); } // . . . }; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Sat Nov 20, 2004 6:15 pm    Post subject: Re: accessing virtulal function from constructor Kiran wrote: Quote: Hi, I was doing something for which I tried to access the virtual function from constructor which gave me error. can anybody explain why accessing virtual function from constructor is barred Virtual or pure virtual? -- A. Kanawati [email]NO.antounk.SPAM (AT) comcast (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ulrich Eckhardt Guest Posted: Sat Nov 20, 2004 6:21 pm    Post subject: Re: accessing virtulal function from constructor Kiran wrote: Quote: I was doing something for which I tried to access the virtual function from constructor which gave me error. can anybody explain why accessing virtual function from constructor is barred It isn't, but its semantics are a bit surprising at first glance. The point is, that inside the base-class' ctor, the object IS of type base-class, i.e. both the static and dynamic type are base-class. That means that a call to a virtual function will call the base-class' version thereof, which will make problems when that function is pure virtual. Only once you enter the ctor of the derived class, the type changes to the derived class and calls to virtual functions also reach that class. Note: the same applies to the destructor, just that the order is inverse. Oh, and btw: if you say "gave me error", it would be helpful if you would tell exactly what error and where, I'm just assuming a common case here without knowing if it really applies to you. Uli -- Questions ? see C++-FAQ Lite: http://parashift.com/c++-faq-lite/ first ! [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Thomas Mang Guest Posted: Sat Nov 20, 2004 6:22 pm    Post subject: Re: accessing virtulal function from constructor "Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> schrieb im Newsbeitrag news:Tuond.11175$Ae.3102 (AT) newsread1 (DOT) dllstx09.us.to.verio.net... Quote: Kiran wrote: I was doing something for which I tried to access the virtual function from constructor which gave me error. can anybody explain why accessing virtual function from constructor is barred Nobody can explain why it's barred because it is _not_. If you have problems with particular code, post your code and the error message you're getting from your compiler. The only thing that _might_ be bad is calling a _pure_ virtual function from a constructor because that produces _undefined behaviour_. I know what you mean, but for clarity you should add it is undefined behaviour when the call is virtual. You can call it using a qualified-id-syntax (which, of course, requires the pure virtual function to be defined). Thomas [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Howard Lee Harkness Guest Posted: Sun Nov 21, 2004 4:51 am    Post subject: Re: accessing virtulal function from constructor [email]kiran.lokhande (AT) gmail (DOT) com[/email] (Kiran) wrote: Quote: can anybody explain why accessing virtual function from constructor is barred Think about what is happening inside the ctor, and the purpose of the ctor. The object is not fully constructed until the ctor exits successfully. That includes things like vtbl entries. -- Howard Lee Harkness, Licensed Insurance Agent http://www.hlhins.com [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Michael Loritsch Guest Posted: Sun Nov 21, 2004 6:54 am    Post subject: Re: accessing virtulal function from constructor [email]kiran.lokhande (AT) gmail (DOT) com[/email] (Kiran) wrote in message news:<2721ed0c.0411182128.73780258 (AT) posting (DOT) google.com>... Quote: Hi, I was doing something for which I tried to access the virtual function from constructor which gave me error. can anybody explain why accessing virtual function from constructor is barred Thanks Regards, Kiran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] The basic answer lies in what a constructor does. Depending on the compiler you are using, when an object is created something like the following steps happens: - Base class constructors and member constructors are called. - A v-table is set up to allow correct binding of virtual function calls. - The body of the constructor is executed. For example, consider the following (non-valid) code: #include <iostream> class Foo { public: Foo() { this->bar(); } ~Foo() {} virtual void bar() = 0; }; class SpecialFoo : public Foo { public: SpecialFoo() : Foo() {} virtual void bar() { std::cout << "I am a SpecialFoon"; } }; SpecialFoo specialFoo; //Create SpecialFoo object Now, when a SpecialFoo object is constructed (as above), by looking at the SpecialFoo() constructor, and the guidelines I outlined, you can tell that the call to bar() doesn't make sense in this context. The process is as follows: When SpecialFoo's constructor is called, the first thing that happens is that Foo's constructor is called. During the execution of Foo's constructor, it doesn't know that it really ISA SpecialFoo. That would require parent classes to have knowledge of their children. For this reason, during the execution of Foo's constructor, the SpecialFoo object does not yet exist. This means that the v-table of the SpecialFoo object does not yet exist either, therefore the call to bar() does not make any sense. I hope this helps. Michael Loritsch [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Antoun Kanawati Guest Posted: Sun Nov 21, 2004 11:39 am    Post subject: Re: accessing virtulal function from constructor Howard Lee Harkness wrote: Quote: kiran.lokhande (AT) gmail (DOT) com (Kiran) wrote: can anybody explain why accessing virtual function from constructor is barred Think about what is happening inside the ctor, and the purpose of the ctor. The object is not fully constructed until the ctor exits successfully. That includes things like vtbl entries. You can call member functions labeled 'virtual' in the class constructor, and you can tell exactly which functions they are at compile time. This is different from what happens after the object is fully constructed, but it is not illegal nor forbidden. -- A. Kanawati [email]NO.antounk.SPAM (AT) comcast (DOT) net[/email] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alf P. Steinbach Guest Posted: Sun Nov 21, 2004 11:42 am    Post subject: Re: accessing virtulal function from constructor * Howard Lee Harkness: Quote: kiran.lokhande (AT) gmail (DOT) com (Kiran) wrote: can anybody explain why accessing virtual function from constructor is barred Think about what is happening inside the ctor, and the purpose of the ctor. The object is not fully constructed until the ctor exits successfully. Depending on the constructor in question an object _may_ not be fully constructed until the constructor exits successfully, but it also may be fully constructed before that; as a simple example, take any constructor that already effects full construction, and add an output statement. Quote: That includes things like vtbl entries. That is incorrect. As others have pointed out, C++ does not bar you from calling virtual functions from constructors. It is permitted. The effect of doing so is a FAQ, and I direct readers to the FAQ on that. It's always a good idea to read (or at least, skim) the FAQ before posting. -- A: Because it messes up the order in which people normally read text. Q: Why is it such a bad thing? A: Top-posting. Q: What is the most annoying thing on usenet and in e-mail? [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Allan W Guest Posted: Tue Nov 23, 2004 10:58 am    Post subject: Re: accessing virtulal function from constructor [email]kiran.lokhande (AT) gmail (DOT) com[/email] (Kiran) wrote Quote: can anybody explain why accessing virtual function from constructor is barred Start at http://www.gotw.ca/resources/clcm.htm: Quote: [ See http://www.gotw.ca/resources/clcm.htm for info about ] Then click on "The FAQ" Look for "Inheritance — what your mother never told you" Read that whole page (especially item 23.3: http://www.parashift.com/c++-faq-lite/strange-inheritance.html#faq-23.3 Good luck. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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