Programming Structure - help?

I''m having quite a time figuring out how to lay out my classes in an engine I''m developing. I''ll lay out how I think it should work, but some problems which have arisen. One problem in particular eludes me ... It begins with an application class, which contains a variety of other classes ... right now, just a graphics one. See, I''m thinking of adding a "log" class, such that I can post problems to a log file. I want this kind of functionality in the graphics class as well, but not to a different file, to the same file. Does this mean I need to pass a pointer to the same instance of a log class to the graphics class? Or can I use extern in some silly way to accomplish this. Appreciate any help, - Ben