[Qt-interest] qobject_cast<> questions

[Ross Bencina]

Hello Thiago

> dynamic_cast should never crash. But it may fail to work if you are trying
> to cast across DLL boundaries, depending on how the class was declared.
> It's a problem caused by the platform ABI's deviating from the C++
> standard mandated behaviour: it's the case on Windows as well as on
> platforms with hidden visibility.

Thanks for replying. I have been (apparently successfully) using 
dynamic_cast to downcast QWidget ptrs on Windows for 2 years (Qt in DLLs, 
all subclasses in App) so the actual behavior must be more subtle than you 
imply.

Good to know that it shoudln't crash though... it turned out in this 
instance I was passing a ptr to a freed object to dynamic_cast<>.

>>Also, can I rely on qobject_cast<> to work with null pointers? (ie to
>> return null if it is passed null?)
>
>The source code for Qt is available and writing a test for that is a
>couple of seconds.

Testing the actual behavior tells me nothing about the specified, expected 
or required behavior. I hope that you guys do more than "check the source 
code" to determine how such fundamental mechanisms as a type safe dynamic 
cast are supposed to behave.

I'll file a documentation bug.

Thanks!

Ross.