Using virtual function in child after casting operation in C++

Question

I have the following code:

class A
{
};

class B : public A
{
    public:
        virtual void f() {}
};

int main()
{
    A* a = new A();
    B* b = static_cast<B*>(a);
    b->f();
}

This program fails with a segmentation fault. There are two solutions to make this program work:

1. declare f non-virtual
2. do not call b->f() (i.e. it fails not because of the cast)

However, both are not an option. I assume that this does not work because of a lookup in the vtable.

(In the real program, A does also have virtual functions. Also, the virtual function is not called in the constructor.)

Is there a way to make this program work?

Answer 1

You can't do that because the object you create is A, not B. Your cast is invalid-- an object of A (created with new) cannot magically become an object of B.

Did you mean the A* a = new A() to actually be A* a = new B()? In that case, I would expect it to work.

Answer 2

In order to do a static_cast, you should be sure that the object can be casted, i.e. is an object of class B.

In this case, I'm sure it isn't.

Answer 3

You can't do that.

In your example, a is a object of class A. Not B. Casting it to B does not make it a B.

If you want to use polymorphic object behaviors, then you can give virtual function f to class A, and you can use code like A* a = new B(); Then you can use the virtual functions through the a pointer to get behavior from class B.

Answer 4

In your code:

    A* a = new A();

You instantiate an A object. Then you try to use static_cast to go from a base type to a derived type:

B* b = static_cast<B*>(a);

If the value in a pointed to an object that actually was of type B, this would be legal and well-formed. But a does not point to an object of type B, it points to an A, so the cast evokes undefined behavior.

The fix is to change how you instantiate the object. Change:

A* a = new A();

...to:

A* a = new B();