[Qt-interest] QMetaObject::className() in constructors returns classname of baseclass

[Thiago Macieira]

Em Sexta-feira 28 Maio 2010, às 19:36:21, Niklas Wulf escreveu:
> Hi Qties,
> 
> I have some class Foo, which inherits QObject and QDomDocument, and some
> other classes Bar, which inherit Foo. Calling metaObject()->className() in
> Foo's constructor returns "Foo" even it's a instance of Bar. Am I wrong?

No, that is proper C++ behaviour.

> Is this wanted behavior?

Yes, it is.

> Much more important: Shouldn't this be documented?

It is documented in the C++ standard.

During construction and destruction, the current object *is* of the type in 
question. So when you say "it's an instance of Bar", that is technically 
wrong. It will be an instance of Bar once the Bar constructor is reached. For 
the moment, it's just a Foo. And the inverse in destruction: the object ceases 
to be of type Bar once its destructor ends.

If you make virtual calls from inside the constructor, they are resolved to 
the overrides that exist in Foo, not those in Bar. That's why metaObject()-
>className() returns "Foo" -- it's because the virtual call to metaObject() 
was resolved to Foo::metaObject().

You can try any trick in the book (and off the books) to determine what type 
the current object is: they will all say it's Foo.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
  Senior Product Manager - Nokia, Qt Development Frameworks
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358
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