wrong virtual function called at some events on MacOS?

[akos.maroy]

I'm experiencing a strange phenomenon: it seems that on some occasions, the wrong virtual function is called at certain events on MacOS X, while all works fine on Linux and Windows.

consider the following:

class A : public QGLWidget {
protected:
    virtual void foo() {}
private:
    virtual void paintEvent(QPaintEvent *event);
};
 
void A::paintEvent(QPaintEvent *event) {
    foo();
}
 
class B : public A {
protected:
    virtual void foo();
};
 
void B::foo() {
    std::cout << "hello from B::foo!" << std::endl;
}

class A : public QGLWidget {
protected:
    virtual void foo() {}
private:
    virtual void paintEvent(QPaintEvent *event);
};

void A::paintEvent(QPaintEvent *event) {
    foo();
}

class B : public A {
protected:
    virtual void foo();
};

void B::foo() {
    std::cout << "hello from B::foo!" << std::endl;
}

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when I use the above in the following manner:

B   b;
b.show()

B   b;
b.show()

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then, it seems that on the first occasion the paintEvent() is called on b, it is in fact an object of class A, and thus A::foo() is called from paintEvent(). interestingly , on the second or so occasion onwards, the object will act as being of class B, and thus B::foo() will be called. on both occasions, the value of the this pointer is the same.

the expectation would be that in all occasions B::foo() is called.

the above works fine on Linux and Windows, but works in the strange way as explained above on MacOS X.

This is with Qt 4.5 (2009.03) and the latest XCode.

I wonder if anyone has experienced this issue...