C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  A question related to virtual function          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message babu Guest Posted: Fri May 18, 2007 9:10 am    Post subject: A question related to virtual function class CPrintString { void printName(Person* p) { printf("person"); } void printName(Student * s) { printf("student"); } }; Here, Person is the base class of Student. Now if we write - Person * st = new Student(); CPrintString * pr =new CPrintString(); pr->printName(st); The output is "person". But we need "student" to be printed. Why such thing occurs? Now What is the solution so that the correct function is called at runtime based on the parameter object type, so that "student" can be printed in the previous calling sequence? Is there any solution without changing class CPrintString? If exists please give. Thanks Babu Back to top Invincible Guest Posted: Fri May 18, 2007 9:10 am    Post subject: Re: A question related to virtual function On May 18, 10:38 am, babu <nasif4...@gmail.com> wrote: Quote: On May 18, 11:24 am, Invincible <vinay.jmis...@gmail.com> wrote: Hi, If one function is declared Virtual in Base class and derived class has the implementation for that, then you can call derived version of function as you had shown by using base class pointer ( Person * st ): But in this case Base class pointer(Person * ) would be passed to printName() as calling to printname is through CPrintString* . If you really want to print "Student" then derive CPrintString from Person class and make printName() to be virtual there. ~Vinay But can I find any solution without changing the class CPrintSting? I dont think It would be possible to get solution without changing CPrintSting. You want behavior of polymorphism so u have to abide by the rules of it. Back to top Salt_Peter Guest Posted: Fri May 18, 2007 9:10 am    Post subject: Re: A question related to virtual function On May 18, 1:06 am, babu <nasif4...@gmail.com> wrote: Quote: class CPrintString { void printName(Person* p) { printf("person"); } void printName(Student * s) { printf("student"); } }; Here, Person is the base class of Student. Now if we write - Person * st = new Student(); CPrintString * pr =new CPrintString(); pr->printName(st); The output is "person". But we need "student" to be printed. Why such thing occurs? Now What is the solution so that the correct function is called at runtime based on the parameter object type, so that "student" can be printed in the previous calling sequence? Is there any solution without changing class CPrintString? If exists please give. Thanks Babu CPrintString should not be a class unless its purpose is akin to a printer. Make it a function instead. If its purpose was to be a printer, its purpose in life is not to detect *what* type of Person is being passed to it. Let the Person object determine that. The printer doesn't care. #include <iostream> #include <string> class Person { std::string m_s; public: Person() : m_s("person") { } Person(const std::string s) : m_s(s) { } std::string getString() const { return m_s; } }; class Student : public Person { public: Student() : Person("student") { } }; struct CPrintString { void printName(const Person& r_p) { std::cout << r_p.getString(); std::cout << std::endl; } }; int main() { Student student; CPrintString printer; printer.printName(student); } This way, if you add another type of Person or Student to your project, the printer is unaffected. Note: pointers replaced by const references. Back to top Guest Posted: Fri May 18, 2007 9:11 am    Post subject: Re: A question related to virtual function On May 17, 10:38 pm, babu <nasif4...@gmail.com> wrote: Quote: On May 18, 11:24 am, Invincible <vinay.jmis...@gmail.com> wrote: Hi, If one function is declared Virtual in Base class and derived class has the implementation for that, then you can call derived version of function as you had shown by using base class pointer ( Person * st ): But in this case Base class pointer(Person * ) would be passed to printName() as calling to printname is through CPrintString* . If you really want to print "Student" then derive CPrintString from Person class and make printName() to be virtual there. ~Vinay But can I find any solution without changing the class CPrintSting? I think it does not exist a solution without changing CPrintString. From the OO object, I sugest you make printName as Person's virtual function. Because printName closely belongs to Person class! Back to top babu Guest Posted: Fri May 18, 2007 9:11 am    Post subject: Re: A question related to virtual function On May 18, 11:24 am, Invincible <vinay.jmis...@gmail.com> wrote: Quote: Hi, If one function is declared Virtual in Base class and derived class has the implementation for that, then you can call derived version of function as you had shown by using base class pointer ( Person * st ): But in this case Base class pointer(Person * ) would be passed to printName() as calling to printname is through CPrintString* . If you really want to print "Student" then derive CPrintString from Person class and make printName() to be virtual there. ~Vinay But can I find any solution without changing the class CPrintSting? Back to top Invincible Guest Posted: Fri May 18, 2007 9:11 am    Post subject: Re: A question related to virtual function Hi, If one function is declared Virtual in Base class and derived class has the implementation for that, then you can call derived version of function as you had shown by using base class pointer ( Person * st ): But in this case Base class pointer(Person * ) would be passed to printName() as calling to printname is through CPrintString* . If you really want to print "Student" then derive CPrintString from Person class and make printName() to be virtual there. ~Vinay Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.