c++ - returning a derived class object through a pointer of its abstract base class -

i must write program in 1 of function return derived class via abstract base class, when class being returned main may access derived class virtual methods.

please keep in mind can't change in main program since not 1 writing it.

#include<iostream> using namespace std; class { private: public:     virtual void doit(void)=0;     a(void){};     ~a(void){}; }; class b:         public { private:     int num; public:     virtual void doit(void){num=7;cout<<"its done";};     b(void){};     ~b(void){}; }; a& returnvalue(void)  {         b item;     return item; } void main() {     a& item=returnvalue();     item.doit(); } 

when try run last line breaks build saying doit pure virtual function call. ideas?

you returning reference local variable destroyed when call in returnvalue complete. instead try following:

a &returnvalue(void) {     return *(new b); }  int main() {     a& item = returnvalue();     item.doit(); } 

a better solution return smart pointer , let people maintaining main function take responsibility lifetime of object returned returnvalue:

#include <memory> ... std::unique_ptr<a> returnvalue(void) {     return std::unique_ptr<a>(new b); }  int main() {     auto item = returnvalue();     item->doit(); }