On Mar 15, 12:12 pm, Rolf Magnus <ramag...@t-online.de> wrote:
pietro...@gmail.com 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
{
public:
virtual const int get() const = 0;
};
class B : public A
{
public:
virtual const int get() const { return 0; }
};
class C : public A
{
public:
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);
if(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 (http://www.acm.org/crossroads/xrds3-1/
ovp3-1.html):
"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?
dynamic_cast. It might fail (returning a null pointer or throwing an
exception), but you're allowed to use it.