Re: dynamic_cast does not work as specified

"Doug Harrison [MVP]" <>
Mon, 12 Jan 2009 07:42:14 -0600
On Mon, 12 Jan 2009 04:59:06 -0500, Joseph M. Newcomer
<> wrote:

It is not at all clear from the documentation that expression is required to be in the
inheritance chain of type-id

Was that a rimshot I heard in the distance? :)

yes, I see now that I can cast to a void*; I'd read it the
other way...)

Consider if I did
    SubThing * p = (SubThing*)pHint;
then p is a pointer to an object (ideally) of type SubThing.

Indeed, pHint had better be of a type that can be legally converted to
SubThing* via a C-style cast.

But I wanted the additional
typecheck to be done to tell me if the thing that is pointed to is of type (SubThing).

If you want to use dynamic_cast, but you're coming from void*, or T* where
T is a lie, as it in your example where MFC makes you use CObject* as a
generic pointer type, you first have to cast the pointer to a known type
that has RTTI information attached, i.e. a type that has a vtbl, i.e a type
that has a virtual function, the latter being the real requirement. That
initial cast cannot be a dynamic cast. Instead, it must be a /valid/
reinterpret_cast. Then you can dynamic_cast the result of the
reinterpret_cast to another type.

What is interesting is that casting to LPVOID makes it work; this points out once again
the horror of OnUpdate/UpdateAllViews using a CObject* instead of an LPVOID.

It is rather presumptuous.

My suspicion
was that since CObject* was not part of the hierarchy it was failing, but the
documentation does not at all make this clear. The phrase at the end

If type-id is a pointer to an unambiguous accessible direct or indirect base class of
expression, a pointer to the unique subobject of type-id is the result

is not stated as a limitation, but what will happen if typeid is a superclass of
expression's type; it does not suggest what happens if typeid is or is not a subclass of
expression's type.

You can also perform a cross-cast, e.g. cast between X and Y in this MI

#include <stdio.h>

struct X { virtual void f() {} };
struct Y {};
struct Z : X, Y {};

int main()
   Z z;
   X* p = &z;
   Y* q = dynamic_cast<Y*>(p); // q points to z's Y.
   X x;
   X* r = &x;
   Y* s = dynamic_cast<Y*>(r); // s is set to NULL.

   printf("%p, %p, %p\n",
         (void*) static_cast<Y*>(&z),
         (void*) q,
         (void*) s);

X>cl -W4 a.cpp
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.30729.01 for

0012FF30, 0012FF30, 00000000

That's why I spoke in terms of the "inheritance graph" instead of base and
derived classes.

Doug Harrison
Visual C++ MVP

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