kurt

Let's say I have a large system. Then, I have a class (A) that does one particular functionality.

If A operates on a data outside of the class, say a structure that is shared by the whole system -- is that a good design, or generally do we have to restrict class operations to member variables and member functions?

Jessehk

Well, I'm not too familiar with design patterns and such, but what you're describing doesn't sound like a typical solution.

Without you actually posting code, my question would be: Why are you writing a class to operate on the data instead of just writing a function that takes the data as a parameter (or accesses a global variable)?

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Jimbo

It depends on your design. If you're modeling a lawnmower repair shop, odds are that the Mechanic class might have to operate on an object of type Lawnmower. I suppose "good" design would involve passing the Lawnmower object to the Mechanic's method of choice, but you'll rarely be able to construct a fully decoupled system.

Personally, I try to design my classes to take any external data as parameters rather than relying on global objects or obscure data sources. I find that it makes for a more adaptable design, more intuitive API, and less troublesome implementation when something goes awry.

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kurt

Thanks, Jessehk and Jimbo.

I totally agree, which is why I asked. Thanks for the input. Anyway, here's an example of what i'm doing.

My class "A" has an API which basically sets a bunch of hardware registers, by writing to the register address. This is the first coupling, but I guess it's not really a big deal, since it's just making some calculations and making writes to registers. All it needs is the main pointer to the first register (the rest are just address additions) which can be passed to the constructor.

The second coupling is, the calculation requires a set of factors, which other subsystems will share the use of. It's a global data, therefore I initially thought of just using it directly in "A"s member function. But since that factor is not frequently set (only set once by other subsystem), I figured I can make a clone of the data as A's member variable. Is that a good idea?
Since the member function that calls the global data is a private member, passing as an argument seems not possible.


Sorry for the vague and wide question, it's the best that I can come out with. It's more of a design style question.

Thanks in advance.
Kurt

Sorrofix

I'd recommend making a global object whose data is read-only (accessor functions only), and (be?)friends any classes which have to make modifications to the factors. Cloning data between objects is a bad idea and could quickly get messy and buggy with the updating of two or more copies. If you really must have external data as a member of the class, at least use a pointer. Unless, of course, the factors as a whole pertain mainly to one class, in which case you should add the data as members of said class (and in that case, they should be exclusively members of the class).

...if that made any sense.