0 or 0L for empty pointers?

[Thiago Macieira]

Lubos Lunak wrote:
>On Friday 23 June 2006 14:03, Stefan Teleman wrote:
>> On Friday 23 June 2006 07:42, Frans Englich wrote:
>> > Yes, some features should be avoided
>> > in any case like dynamic_cast, but perhaps there's some template
>> > features or the like that should be avoided.
>>
>> Just out of curiosity, why is dynamic_cast<> a Bad Thing(TM) ?
>
> It is not, in general. But one needs to be careful when using
> dynamic_cast across dlopen boundaries, because under some conditions
> g++ emits multiple rtti info instances which are not merged without
> using RTLD_GLOBAL and that may make dynamic_cast fail. It should be
> fixable though.

In other words, you can only dynamic_cast to type T from type T2 provided 
that:

- T is defined in a library you link to
  (you'd get a linker error if this isn't the case, since it won't find 
the vtable or RTTI info)

- T is "well-anchored" in that library
  By "well-anchored" I mean that the vtable is not a COMMON symbol subject 
to merging at run-time by the dynamic linker. In other words, the first 
virtual member in the class definition must exist and not be inlined: it 
must be in a .cpp file.

- T and T2 are exported

We've seen some hard-to-track problems in non-KDE C++ code we're linking 
with (I think NMM) because of that. It happened that:
  libphonon loads the NMM plugin
  NMM plugin links to NMM
  NMM loads its own plugins
  NMM's own plugins link to NMM

Some classes in the NMM library did not have well-anchored vtables, so 
dynamic_casting failed inside the Phonon NMM plugin for objects created 
in the NMM's own plugins.

-- 
Thiago Macieira  -  thiago (AT) macieira.info - thiago (AT) kde.org
  thiago.macieira (AT) trolltech.com     Trolltech ASA
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