wxMemoryDCs are used to draw to wxBitmap 's in memory. When creating, you may specify zero or one parameters. The parameter must be a pointer to a wxBitmap. After creation, the specified bitmap will be selected into the wxMemoryDC.
You must either delete the wxMemoryDC or call select_object( wxNullBitmap ) before selecting the bitmap into another wxMemoryDC.
Selects a bitmap into the wxMemoryDC. If obj is wxNullBitmap, then the bitmap currently selected into the DC is deselected.