wxDelegateRendererNative allows reuse of renderers code by forwarding all the wxRendererNative methods to the given object and thus allowing you to only modify some of its methods -- without having to reimplement all of them.
Note that the "normal'', inheritance-based approach, doesn't work with the renderers as it is impossible to derive from a class unknown at compile-time and the renderer is only chosen at run-time. So suppose that you want to only add something to the drawing of the tree control buttons but leave all the other methods unchanged -- the only way to do it, considering that the renderer class which you want to customize might not even be written yet when you write your code (it could be written later and loaded from a DLL during run-time), is by using this class.
Except for the constructor, it has exactly the same methods as wxRendererNative and their implementation is trivial: they are simply forwarded to the real renderer. Note that the "real'' renderer may, in turn, be a wxDelegateRendererNative as well and that there may be arbitrarily many levels like this -- but at the end of the chain there must be a real renderer which does the drawing.
The default constructor does the same thing as the other one except that it uses the generic renderer instead of the user-specified rendererNative.
In any case, this sets up the delegate renderer object to follow all calls to the specified real renderer.
Note that this object does not take ownership of (i.e. won't delete) rendererNative.
This class also provides all the virtual methods of wxRendererNative, please refer to that class documentation for the details.