[Issue 1563] dynamic cast is not always performed

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=1563





------- Comment #2 from wbaxter at gmail.com  2007-10-09 22:02 -------
(In reply to comment #1)
> Reply to d-bugmail at puremagic.com,
> 
> > http://d.puremagic.com/issues/show_bug.cgi?id=1563
> > 
> > Summary: dynamic cast is not always performed
> > Product: D
> > Version: 1.022
> > Platform: PC
> > OS/Version: Windows
> > Status: NEW
> > Severity: normal
> > Priority: P2
> > Component: DMD
> > AssignedTo: bugzilla at digitalmars.com
> > ReportedBy: davidl at 126.com
> > class A{}
> > class B:A{}
> > class C:B{}
> > void main()
> > {
> > A a = new B;
> > C c = cast(C)cast(void*)a;
> > c = cast(C)c;
> > assert(c is null);  // assertion failure
> > c = cast(C)cast(A)cast(void*)c;       // workaround
> > assert(c is null);
> > }
> >
> 
> 
> it looks to me like the bug is that "cast(ClassType)voidPt" doesn't check 
> the type. I think that assuming that something of type C actually is of type 
> C should be allowed. The guards should be at the original assignment.
> 
> OTOH that makes things with unions fun.

If you make cast(ClassType)voidPt check the type then D will have no way at all
to do a reinterpret_cast.  And I also agree that assuming something of type C
is actually type C should be allowed.  So I think the example is just bad code.
 If you do a reinterpret_cast then you're taking responsibility into your own
hands.

But the danger involved would definitely be more apparent if there were a more
obvious construct for reinterpret casts like reinterpret_cast!(T).


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