Error Message "Paint device returned engine == 0, type:1"

hello ,
I wrote a class named CEditorWin, and inherits QFrame class. I set the Attribute of CEditorWin with "setAttribute(Qt::WA_PaintOutsidePaintEvent);" . And I know what the Qt Assitant say
"Warning: When the paintdevice is a widget, QPainter can only be used inside a paintEvent() function or in a function called by paintEvent(); that is unless the Qt::WA_PaintOutsidePaintEvent widget attribute is set. On Mac OS X and Windows, you can only paint in a paintEvent() function regardless of this attribute's setting."

When I run the code below, it will return the Error Message "Paint device returned engine == 0, type:1" and the variable "ret" == FALSE.

Code:

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CEditorWin *m_pCanvas = new CEditorWin();
QPainter *m_pPainter= new QPainter();
assert(m_pPainter != NULL);
bool ret = m_pPainter->begin(m_pCanvas);
assert(ret != FALSE);

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I think I write the code Strictly in accordance with what Qt says, but I also get the error. Why? I need help . Thanks very much.

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Added after 1 47 minutes:

Sorry, My English is not good. I don't know what i say is clear.
But I really want to solve it. I follow the source code to qpainter.cpp and qwidget_win.cpp. And I found the difference between the source code of Qt 4.7 and the source code of Qt 4.3 is huge. For example : the function "QPaintEngine *QWidget::paintEngine() const" in qwidget_win.cpp
Can someone help me and explain to me the reason ?
Thanks

Is your code inside paintEvent?

No, But I set the Attribute:
m_pCanvas->setAttribute(Qt::WA_PaintOutsidePaintEvent);

Based on Qt documentation, Qt::WA_PaintOutsidePaintEvent makes it possible to use QPainter to paint on the widget outside paintEvent(). This flag is not supported on Windows, Mac OS X or Embedded Linux.

哈哈 是我弄错了。� �。。
Oh，I know. My English is really bad. I have a wrong interpretation about "On Mac OS X and Windows, you can only paint in a paintEvent() function regardless of this attribute's setting".

thanks

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Added after 15 minutes:

Ok. But If I still user QPainter to paint on the widget outside paintEvent(), and I ignore the error message("Paint device returned engine == 0, type:1"), what is the result?

I do use it like that, And I get right result up to now.

thanks

That message means that the painter wasn't initialized, so you can't use it for paining.

If you try you should get "Painter not active" error and nothing should be drawn.