Re: What the hell is dynamic_cast for?

James Kanze <>
Thu, 29 May 2008 14:09:24 -0700 (PDT)
On May 29, 4:16 pm, ""
<> wrote:

On May 29, 4:13 pm, James Kanze <> wrote:

On May 29, 3:49 am, ""

<> wrote:

On May 29, 1:46 am, Juha Nieminen <nos...@thanks.invalid> wrote:

dynamic cast actually checks that the object is of that
type, and if it isn't, it returns a null pointer. You can
check the returned pointer to see if the cast was
successful. (With static_cast you probably just get a
segmentation fault.)

``is of that type'' means that
(*pointer) is declared as T or T's derived classes, right?
This cast fails if (*pointer) is declared as T's parent
classes or other un-related classes. Am I right?

I'm not too sure I understand. If pointer is of type Derived*,
dynamic_cast< Base* >( pointer ) always succeeds, of course,
returning a pointer to the Base sub-object of the Derived. If
pointer is of type Base*, dynamic_cast< Derived* >( pointer )
will return a null pointer if the actual object doesn't contain
an unambiguous Derived in its inheritance hierarchy, and a
pointer to the Derived if it does.

Suppose there are three classes: Base, Child, GrandChild are
polymorphic classes. Child is derived from Base, and
GrandChild from Child.

Public derivation, or private? And does Base have at least one
virtual function?

I think only the dynamic_cast in ``Case 1'' fails, am I right?

    Base *pB, *p;
    Child *pC;
    GrandChild *pG;

    /* pB, pC, pG, proper initialization .. */

    // Case 1
    p = pB; // legal up-cast
    pC = dynamic_cast<Child*>(p); // illegal dynamic-cast

    // Case 2
    p = pC; // legal up-cast
    pC = dynamic_cast<Child*>(p); // legal dynamic-cast

    // Case 3
    p = pG; // legal up-cast
    pC = dynamic_cast<Child*>(p); // legal dynamic-cast

If I suppose public derivation in all cases, and that Base has
at least one virtual function, all of the casts are "legal".
The first will result in a null pointer, and the others to a
pointer to the correct (sub-)object.

James Kanze (GABI Software)
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