C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  how can i know weather Object A is an instance of class B. Goto page 1, 2  Next          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message lfeiman888@gmail.com Guest Posted: Mon Feb 27, 2006 11:06 am    Post subject: how can i know weather Object A is an instance of class B. Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Kodt Guest Posted: Mon Feb 27, 2006 3:06 pm    Post subject: Re: how can i know weather Object A is an instance of class lfeiman888 (AT) gmail (DOT) com wrote: Quote: Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. If the class B has virtual functions, and RTTI is on (see options of your compiler; for VC it is /GR), then use 1) dynamic_cast if( dynamic_cast<B*>(pA) != NULL ) cout << "Yes, it is B (or its descendant)"; Note that dynamic_cast failure on pointers returns NULL but on references throws std::bad_cast. 2) typeid if( typeid(*pA) == typeid(B) ) cout << "Yes, it is exactly B"; ------------- And if you need to check it in compile time, use boost::is_base_and_derived<B,A> [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Eugene Alterman Guest Posted: Mon Feb 27, 2006 3:06 pm    Post subject: Re: how can i know weather Object A is an instance of class <lfeiman888 (AT) gmail (DOT) com> wrote in message news:1141032114.073823.136700 (AT) i40g2000cwc (DOT) googlegroups.com... Quote: Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. If you have a pointer or a reference to an object of a polymorphic type (a class that has at least one virtual member function) you can use dynamic_cast: class B { public: .... virtual ~D(); .... }; class D: public B { .... }; void foo(B* pb) { D* pd = dynamic_cast<D*>(pb); if(pd != 0) // pb points to an instance of D (or a class publicly derived from D) .... }; Note: if you use Microsoft VC make sure you have "Enabled Run Time Type Info" flag (/GR) on (it is off by default). [ 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: Mon Feb 27, 2006 3:06 pm    Post subject: Re: how can i know weather Object A is an instance of class * lfeiman888 (AT) gmail (DOT) com: Quote: Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. I'm surprised the moderators let this posting through; perhaps there's mainly one very overworked moderator doing the screening now? First, the question is platform specific, not concerning standard C++, and it's platform specific in the way which caused clc++m to be created. And second, it smells like a homework problem or just trolling (the names mentioned do indeed appear in that library, if memory serves me right). -- 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 Dietmar Kuehl Guest Posted: Mon Feb 27, 2006 3:06 pm    Post subject: Re: how can i know weather Object A is an instance of class lfeiman888 (AT) gmail (DOT) com wrote: Quote: Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. You can test the result of 'dynamic_cast<>()': if (B* b_ptr = dynamic_cast<B*>(&A)) std::cout << "the object 'A' is of class 'B'\n"; assuming 'A' refers to a reference to the object. If 'A' is actually a pointer to the object, you would, of course, just omit taking the address (i.e. drop the '&'). -- <mailto:dietmar_kuehl (AT) yahoo (DOT) com> <http://www.dietmar-kuehl.de/> <http://www.eai-systems.com> - Efficient Artificial Intelligence [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top benben Guest Posted: Mon Feb 27, 2006 3:06 pm    Post subject: Re: how can i know weather Object A is an instance of class lfeiman888 (AT) gmail (DOT) com wrote: Quote: Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. Well if the object is of a polymorphic class then the answer is yes, just use dynamic_cast. However if the object is of a non-polymorphic type then you will have to do some round about way...take a look at boost::any Regards, Ben [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Eugene Alterman Guest Posted: Mon Feb 27, 2006 5:06 pm    Post subject: Re: how can i know weather Object A is an instance of class "Alf P. Steinbach" <alfps (AT) start (DOT) no> wrote in message news:46gc08FauneeU1 (AT) individual (DOT) net... Quote: * lfeiman888 (AT) gmail (DOT) com: Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. I'm surprised the moderators let this posting through; perhaps there's mainly one very overworked moderator doing the screening now? First, the question is platform specific, not concerning standard C++, and it's platform specific in the way which caused clc++m to be created. And second, it smells like a homework problem or just trolling (the names mentioned do indeed appear in that library, if memory serves me right). The confusion seems to have been caused by the ambiguous parsing of the question :-) The OP meant: "Is there some methods (in the standard C++) like <IsKindOf ,RUNTIME_CLASS in MFC>?" , and not: "Is there some methods like <IsKindOf ,RUNTIME_CLASS> in MFC?" MFC provides its own RTTI implementation, and many MFC programmers are not used to the standard C++ RTTI facilities. [ 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: Mon Feb 27, 2006 5:06 pm    Post subject: Re: how can i know weather Object A is an instance of class Alf P. Steinbach wrote: Quote: * lfeiman888 (AT) gmail (DOT) com: Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. I'm surprised the moderators let this posting through; perhaps there's mainly one very overworked moderator doing the screening now? First, the question is platform specific, not concerning standard C++, and it's platform specific in the way which caused clc++m to be created. There is nothing platform specific about the question "How can I know whether Object A is an instance of class B". I suspect that you have been confused because the main question is only in the title of the post, and the body of the post just contains supplementary information (which is poor netiquette, but not bad enough to provoke rejection). Quote: And second, it smells like a homework problem or just trolling (the names mentioned do indeed appear in that library, if memory serves me right). I think a question of the form "is there a platform independant way to achieve this functionality (the equivalent of this platform specific code)" is entirely on-topic. I don't think it is homework, because IsKindOf is a moderately advanced MFC feature - not one I would expect students to be introduced to. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ivan Vecerina Guest Posted: Mon Feb 27, 2006 5:06 pm    Post subject: Re: how can i know weather Object A is an instance of class <lfeiman888 (AT) gmail (DOT) com> wrote in message news:1141032114.073823.136700 (AT) i40g2000cwc (DOT) googlegroups.com... : Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. : thanks to all that helps. Look for dynamic_cast and typeid/type_info in your favorite C++ reference (or web search engine). Ivan -- http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Alex Beluga Guest Posted: Mon Feb 27, 2006 5:06 pm    Post subject: Re: how can i know weather Object A is an instance of class lfeiman888 (AT) gmail (DOT) com wrote: Quote: Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. If I quite right understood you, you have to throw object you want to check and try to catch it as a class you want to check it for. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top john Guest Posted: Mon Feb 27, 2006 9:06 pm    Post subject: Re: how can i know weather Object A is an instance of class Eugene Alterman wrote: Quote: lfeiman888 (AT) gmail (DOT) com> wrote in message news:1141032114.073823.136700 (AT) i40g2000cwc (DOT) googlegroups.com... Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. If you have a pointer or a reference to an object of a polymorphic type (a class that has at least one virtual member function) you can use dynamic_cast: Could someone explain why the class has to be polymorphic for dynamic_cast to work ? Thanks. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Ivan Vecerina Guest Posted: Tue Feb 28, 2006 9:06 am    Post subject: Re: how can i know weather Object A is an instance of class "john" <john.tylo (AT) nospam (DOT) teleme.com> wrote in message news:44033aa0$0$20139$8fcfb975 (AT) news (DOT) wanadoo.fr... : Eugene Alterman wrote: : > <lfeiman888 (AT) gmail (DOT) com> wrote in message : > news:1141032114.073823.136700 (AT) i40g2000cwc (DOT) googlegroups.com... : >> Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. : >> thanks to all that helps. : > : > If you have a pointer or a reference to an object of a polymorphic type (a : > class that has at least one virtual member function) you can use : > dynamic_cast: : > : Could someone explain why the class has to be polymorphic for : dynamic_cast to work ? If type identification is to be performed for an object at run time, some information needs to be associated with the object - which implies some overhead. The default language design choice in C++ is "you don't pay for what you don't use", so this overhead is not present by default. This additional type information actually comes for free if the object already contains a virtual function pointer to support polymorphic calls. And polymorphic objects for which run-time type identification is of interest typically need to have a virtual destructor anyway. So it simply makes sense to combine these two features. Ivan -- http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Markus Moll Guest Posted: Tue Feb 28, 2006 9:06 am    Post subject: Re: how can i know weather Object A is an instance of class Hi john wrote: Quote: If you have a pointer or a reference to an object of a polymorphic type (a class that has at least one virtual member function) you can use dynamic_cast: Could someone explain why the class has to be polymorphic for dynamic_cast to work ? Most probably because the virtual table is used for type identification. If you only have struct Base { int x; }; struct Derived : public Base { void f() { x = 5; } }; the layout of Derived/Base will probably just be "one int". Given a pointer to Base, how would you find out if the object stored at that location has some other dynamic type? C++ was designed so that you need not pay for features that you do not use. This means that in the simple scenario shown above there won't be any extra information from which the dynamic type could be determined. On the other hand, if you have virtual functions, the dynamic type of an object must be determined at runtime anyway. Markus [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Bob Bell Guest Posted: Tue Feb 28, 2006 9:06 am    Post subject: Re: how can i know weather Object A is an instance of class john wrote: Quote: Eugene Alterman wrote: lfeiman888 (AT) gmail (DOT) com> wrote in message news:1141032114.073823.136700 (AT) i40g2000cwc (DOT) googlegroups.com... Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. If you have a pointer or a reference to an object of a polymorphic type (a class that has at least one virtual member function) you can use dynamic_cast: Could someone explain why the class has to be polymorphic for dynamic_cast to work ? The short answer is "because the standard says so." I imaging a longer answer would be something along the lines of "otherwise there's no reasonable way to implement it." A typical implementation of virtual functions puts a pointer to a data structure containing function pointers into each polymorphic object; to make RTTI work, an implementation would put RTTI data into the structure as well. If there are no virtual functions, there's no virtual function data structure and no RTTI data. Bob [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top fei.liu@noaa.gov Guest Posted: Tue Feb 28, 2006 9:06 am    Post subject: Re: how can i know weather Object A is an instance of class john wrote: Quote: Eugene Alterman wrote: lfeiman888 (AT) gmail (DOT) com> wrote in message news:1141032114.073823.136700 (AT) i40g2000cwc (DOT) googlegroups.com... Is there some methods like IsKindOf ,RUNTIME_CLASS in MFC. thanks to all that helps. If you have a pointer or a reference to an object of a polymorphic type (a class that has at least one virtual member function) you can use dynamic_cast: Could someone explain why the class has to be polymorphic for dynamic_cast to work ? Thanks. The standard requires a polymorphic object to contain RTII. It's impossible to apply dynamic_cast on an object without RTII. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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