[Qt-interest] qobject_cast<> questions

[Thiago Macieira]

Ross Bencina wrote:
>Hi everyone
>
>Does anyone know exactly when qobject_cast is needed and why (ie where
>dynamic_cast will crash)?

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.

>Is it only when using dynamically loaded
> plugins, or can it happen with any use of Qt as a shared library such
> as using the standard Qt DLLs and Frameworks from Qt Software (i'm on
> Win32 and OSX)?

See above. If you're using Windows, you should know how that platform's 
ABI breaks the C++ standard. I don't have all the details either: I'm 
afraid you'll have to learn by experience.

>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.

-- 
Thiago Macieira - thiago.macieira (AT) nokia.com
  Senior Product Manager - Nokia, Qt Software
      Sandakerveien 116, NO-0402 Oslo, Norway
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