Re: dynamic_cast and accessible base classes.

From:
Pavel Minaev <int19h@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Sat, 25 Apr 2009 00:07:15 CST
Message-ID:
<09fc989d-fe81-454e-a45f-4fac879f4c60@d38g2000prn.googlegroups.com>
On Apr 24, 3:03 pm, Paul Bibbings <paul.bibbi...@googlemail.com>
wrote:

In ?15.4.1, "Dynamic_cast," of Stroustrup's TC++PL (Sp. Ed. 2000, 2nd
printing, p. 408), the author gives the following as an illustration
of the application of dynamic_cast, using:

    dynamic_cast<T*>(p);

saying "if p is of type T* or an accessible base class of T, the
result is exactly as if we had simply assigned p to a T*." He then
goes on to give an example where the base class in inaccessible
(protected):

    class BB_ival_slider
         : public Ival_slider, protected BBslider { /* ... */ };

    void f(BB_ival_slider* p) {
        // <snip>
        BBslider* pbb1 = p; // error: BBslider is a protected base
        BBslider* pbb2 = dynamic_cast<BBslider*>(p); // ok: pbb2 = 0
    }

However, when I attempt to convert this into a brief working example,
as for instance here...

    #include <iostream>

    class A {
    public:
       virtual void do_stuff() = 0;
    };

    class B : protected A {
    public:
       void do_stuff() { }
    };

    int main()
    {
       B *b = new B();

       A *a = dynamic_cast<A *>(b); // line 17

       if (!a) {
          std::cout << "A *a = 0\n";
       }

       delete b;

       return 0;
    }

... I get errors that suggest that the dynamic cast, in this
instance, is no more allowed than the attempt at direct
assignement is for the compilers that I have access to.

Only VC++, being lax, provides a warning where the others produced an
error...

Is it possible that I have "lost something in translation" in
my example above?


I don't see anything wrong in your example, nor do I see anything in
the Standard prohibiting using dynamic_cast to cast to a non-public
base (of course, the result is then 0, as the cast fails at run-time).
In fact, it even has an example for such a case, along the lines of
Stroustroup's one, with comment "fails" rather than "ill-formed" - the
former means that result should be 0, the latter means that it
shouldn't compile.

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