Type of NULL

Some compilers (e.g. the native IRIX cc) define NULL to be 0L so that no conversion to pointers is allowed. Because of that, all these occurrences of NULL in the GTK+ port use an explicit conversion such as

  wxWindow *my_window = (wxWindow*) NULL;

It is recommended to adhere to this in all code using wxWidgets as this make the code (a bit) more portable.