Re: why dynamic_cast fail here?

Rolf Magnus <>
Thu, 15 Mar 2007 13:39:15 +0100
<etbepi$kps$01$> wrote:

On Mar 15, 12:12 pm, Rolf Magnus <> wrote: wrote:

In the example below, why does the dynamic_cast fail (return NULL)?

Because a B is not a C.

It should be able to cast between sibling classes ...

No, it shouldn't. An instance of class B only provides the A interface
and the B interface. So why would you want to access it through the C
interface? What would you expect to happen if C had another member
function and you'd try to call that on your B?

#include <iostream>

class A
        virtual const int get() const = 0;

class B : public A
        virtual const int get() const { return 0; }

class C : public A
        virtual const int get() const { return 1; }

int main()
    A *a;

    a = new B();
    std::cout << a->get() << std::endl;

    a = dynamic_cast<C*>(a);
        std::cout << a->get() << std::endl;

    return 0;

Yes, I see how that could case problems. So does that mean that the
following is not true (

"The dynamic_cast Operator

The dynamic_cast operator takes the form

   dynamic_cast<T> (expr)

and can be used only for pointer or reference types to navigate a
class hierarchy. The dynamic_cast operator can be used to cast from a
derived class pointer to a base class pointer, cast a derived class
pointer to another derived (sibling) class pointer, or cast a base
class pointer to a derived class pointer. Each of these conversions
may also be applied to references. In addition, any pointer may also
be cast to a void*."

Or else, what are the conditions for casting between sibling classes?

I think it means just that those are the cases where you may use a
dynamic_cast. It might fail (returning a null pointer or throwing an
exception), but you're allowed to use it.
And there is also multiple inheritance:

class Base
    virtual ~Base() {}

class A : public Base

class B : public Base

class C: public A, public B

int main()
    A* a = new C;
    B* b = dynamic_cast<B*>(a); // shoulnd't fail
    delete b;

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