QPainter ouside of paintEvent with a pixmap

[bitChanger]

I'm porting a Qt 3.3.2 application to Qt 4.1.1

In Qt 3 you can create a QPainter p(&pixmap) and paint to the pixmap anywhere in the code.

In Qt 4 it says that all painting must take place inside the paintEvent.

Why is that the following will work in Qt 4 outside of paintEvent

void someFunc()
{
  Pixmap pix(size());
  pix.fill(this, 0, 0);
  QPainter p(&pix);   // no error
  // paint away
}

void someFunc()
{
  Pixmap pix(size());
  pix.fill(this, 0, 0);
  QPainter p(&pix);   // no error
  // paint away
}

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but, the following will not.

// here ptrPix was filled by another function
void someFunc()
{
  QPainter p(ptrPix);   // QPainter::begin(), paintdevice returned engine == 0, type: 2
  // paint away
}

// here ptrPix was filled by another function
void someFunc()
{
  QPainter p(ptrPix);   // QPainter::begin(), paintdevice returned engine == 0, type: 2
  // paint away
}

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am i missing a concept here?

I will move all of my painting to the paintEvent so that I can take advantage of the double buffering built in to Qt 4, I just want to know what is going on with the above statements.

Thanks.