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class A: public QObject
{ Q_OBJECT };
class B: public A {};

A *aa = new B;
qobject_cast<B*>(aa);//... ошибка: invalid application of 'sizeof' to incomplete type 'QStaticAssertFailure<false>'....

class A: public QObject {};
class B: public A {Q_OBJECT}; //... ошибка: undefined reference to `vtable for B' 

class A: public QObject {Q_OBJECT};//...ошибка: undefined reference to `vtable for A'
class B: public A {Q_OBJECT};//...ошибка: undefined reference to `vtable for B'