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Yes, the superfluous D is quite annoying when using -Wall. But for the SceneNode it is alright. It's just a generic SceneNode so what wouls you want to return? All other SceneNodes will handle this through the virtual method.
if that method was pure virtual, then it would require every node to define the function. If your node happens to have no materials, then the default in ISceneNode is fine and you don't have to overload it.