wxBufferedPaintDC GetGraphicsContext return NULL pointer - wxwidgets

Question

To get rid of flicker, I use wxBufferedPaintDC in place of wxPaintDC. But there comes a problem. In my paint function, in order to draw a cubic beizer curve I must use GetGraphicsContext to create a path. My question is why GetGraphicsContext returns a NULL pointer when using wxBufferedPaintDC.
void DotGraphView::OnPaint(wxPaintEvent & WXUNUSED(evt))
{
wxBufferedPaintDC dc(this);
PrepareDC(dc);
PaintBackground(dc);
wxGCDC &gdc = (wxGCDC&)dc;
wxGraphicsContext * gc = gdc.GetGraphicsContext(); /* here gc = NULL */
wxGraphicsPath path = gc->CreatePath(); /* program collapses here */
...
}

Answer 1

The cast that you have from wxBufferedPaintDC to wxGCDC looks a little suspicious to me, the wxGraphicsContext pages suggests doing it like this:
wxGraphicsContext *gc = wxGraphicsContext::Create( dc );
if (gc)
{
//drawing code here
delete gc;
}

Answer 2

I feel suspicious too, the suggestion page does not work.
Now I use the following codes and the program run ok.
wxBufferedPaintDC pdc(this);
wxGCDC gdc;
wxGraphicsRenderer * const renderer = wxGraphicsRenderer::GetDefaultRenderer();
wxGraphicsContext * context = renderer->CreateContext(pdc);
gdc.SetGraphicsContext(context);
wxDC & dc = (wxDC &)gdc;
PrepareDC(dc);
Draw(dc);
...

Answer 3

You should use wxAutoBufferedPaintDC.
There is no need to buffer on some platform (like GTK).
Then, you can use the constructor
wxGCDC gcdc(dc);
This works at least on GTK and MSW.