Imagine you have a computer game. What are your classes like? Often you have a top-level "program" class. And maybe a "current game" class and "current level" class. And a whole bunch of stuff for each object in the level (whether those are separate class types, or just structs with an enum for type, or whatever).
You often have some data or functionality which is specific to your program, but should be accessible in many parts of the program. Or specific to the current game, or current level. Eg. many different events may add a score. Everything might write to a log file. Commonly functions on objects want to look at other "nearby" objects.
Currently, you basically have a choice of two scopes of values which are available to a function. Global variables, and variables in the current class. What I'm suggesting is that two is the wrong number of scopes to have. If you have two, one or two more is likely to be useful.
Some data naturally lives in a level neither global, nor in in the current class, but in the "program" class. And is visible to all classes "in" the program class.
What does "in" mean? Possibly "declared with a special syntax referencing the program class". But it might be better to treat it like a namespace or module, that says "everything in here can only be used by this class (or compatible classes like mocks)", and all member functions get *two* hidden parameters, one for the program, and one for the "this" pointer. Or four, if you have "program", "current game", and "level".
It's easy to imagine your program and say, "but there'll only be one current game at once". But once you SAY that, you can imagine why there wouldn't be. And then any values associated with that need to not be just shoved in the program class or in global scope, but managed properly.
And you CAN provide them to the "children" classes by giving the child class a pointer to the correct parent . You definitely should decide what should be visible everywhere and what only needs to be some places. But I'm suggesting it would be *clearer* not to do that, but explicitly choose what should be shared.
ETA: Simon points me to a post of gerald-duck's I read ages go but seem to have partially re-invented: http://gerald-duck.livejournal.com/7103
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