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

[yuzhouzhiwai]

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.

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

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:aintEngine() const" in qwidget_win.cpp
Can someone help me and explain to me the reason ?
Thanks

[viulskiez]

Is your code inside paintEvent?

[yuzhouzhiwai]

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

[viulskiez]

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.

[yuzhouzhiwai]

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

[Spitfire]

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.