dynamic_cast does not work as specified

Here's a problem I hit that is inconsistent with the documentation.

I'm handling an OnUpdate notification, where the CObject* can be one of several types
depending on the value of lHint.

So the code is basically

void CMyView::OnUpdate(CView * pSender, LPARAM lHint, CObject *pHint)
{
if(lHint == 0 && pHint == NULL)
{ /* normal update */
CView::OnUpdate(pSender, lHint, pHint);
return;
} /* normal update */

if(lHint == WHATEVER)
{ /* update the whatever */
SubThing * p = dynamic_cast<SubThing *>pHint;
ASSERT(p != NULL);
if(p != NULL)
DoSomething(p);
} /* update the whatever */
} // CMyView::OnUpdate

where I have defined

class Thing { ... };
class SubThing : public Thing { ... };

If I try to single-step into it, it tries to trace a source file along some bizarre path
for rtti.cpp, but this source is not included in the distribution. Bummer.

So, I thought, it might have to do with the fact that (due to exceptionally poor design
decisions in the original MFC design) the pHint is declared to be a CObject*, unrelated to
a Thing or SubThing (in a rational world, it would have been an LPVOID). The line

SubThing * p = dynamic_cast<SubThing *>pHint;

generates the compile-time diagnostic:
C2681: 'CObject *': invalid expression type for dynamic_cast

I have no idea why this is an invalid expression for dynamic_cast. But it may deal with
some obscure, undocumented rule.

But dynamic_cast is defined to work on, among other expressions, a pointer-to-void, so I
tried

LPVOID ptr = (LPVOID)pHint;
SubThing * p = dynamic_cast<SubThing *>p;

but that produces an error
C2681: 'LPVOID': invalid expression type for dynamic_cast

in spite of the documetnation of dynamic_cast, which *clearly* states:

The type-id must be a pointer or a reference to a previously defined class type or
a "pointer to void".

It turns out I can get around this by doing

Thing * ptr = (Thing *)pHint;
SubThing * p = dynamic_cast<SubThing *>ptr;

So does this represent an error in the documentation, or an error in the implementation?
joe
Joseph M. Newcomer [MVP]
email: newcomer@xxxxxxxxxxxx
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm
.

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