在C ++中进行强制转换操作后，在子级中使用虚函数

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() (ie 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

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 3

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();

Answer 4

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

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