returning a derived class object through a pointer of its abstract base class

Question

I must write a program which in one of its function will return a derived class via an abstract base class, so when the class being returned to the main may access the derived class virtual methods.

Please keep in mind that I can't change anything in the main program since I am not the one writing it.

#include<iostream>
using namespace std;
class A
{
private:
public:
    virtual void DoIt(void)=0;
    A(void){};
    ~A(void){};
};
class B:
        public A
{
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 I try to run this the last line breaks the build saying that DoIt is a pure virtual function call. any ideas?

Answer 1

You are returning a reference to a local variable which is destroyed when the call to in returnvalue is complete. Instead try the following:

A &returnValue(void) {
    return *(new B);
}

int main() {
    A& item = returnValue();
    item.DoIt();
}

A better solution is to return a smart pointer and let the people maintaining the main function take responsibility for the lifetime of the object returned by returnvalue:

#include <memory>
...
std::unique_ptr<A> returnValue(void) {
    return std::unique_ptr<A>(new B);
}

int main() {
    auto item = returnValue();
    item->DoIt();
}