Why we are not creating Derived Class Reference in case of virtual

Question

I am new in C++. I don't know why we cant able to create Derived Class Reference.In derived class all the features of base will get in this case, then also why..Please help me with exact reason.If this is a duplicate one please share me the exact link.

#include<iostream>  
using namespace std;  
struct A  
{  
  virtual void get()  
  {   
    cout<<"I am in Base"<<endl;  
  }  
};

struct B:A  
{
  virtual void get()
  {
    cout<<"I am in Derived"<<endl;
  }  
};

int main()  
{  
  B*ptr = new A();  // virtual.cpp:21: error: invalid conversion from A* to B*  
  ptr->get();
  return 0;
}

You can't make implicit downcasts. Pretending an `A` is a `B` is a bad idea. It's avoided even when the `A` started as a `B`. — chris, Jun 07 '14 at 12:46
A * ptr = new B() is possible right. Will you please explain me the exact difference — user2390140, Jun 07 '14 at 12:51
@user2390140, Try with an example that isn't `A` and `B`. Make `A` `Animal` and `B` `Cat`. Now which makes sense, creating a cat and pretending it's an animal, or creating an animal and pretending it's a cat? The latter doesn't work. It won't have any cat-specific features, but you're saying it's a cat. — chris, Jun 07 '14 at 13:00

score 0 · Answer 1 · answered Jun 07 '14 at 12:53

What you want to assign to is not a reference but a pointer.
B is derived from A, so B is an A. But A is not a B.
Thus A*ptr=new B; is allowed, but not what you want.
Suppose B*ptr=new A; was possible. If B contains data members which A doesn't have, interpreting A as B makes no sense and trying to access those members will at best cause a crash.

Btw, this is not a C++ tutorial site. I recommend to read a beginner's book.

Why we are not creating Derived Class Reference in case of virtual

1 Answers1