[Tutor] Communication between classes

[Alan Gauld]

"Greg Perry" <gregp at liveammo.com> wrote 

> I am still in the process of learning OOP concepts and 
> reasons why classes should be used instead of 
> functions etc.

That's OK, many folks find the transition hard at first.
It is a new way of looking at problems.

> One thing that is not apparent to me is the best way 
> for classes to communicate with each other.  

Classes don't.
Its *object* oriented programming and its objects 
which communicate. Objects pass messages 
between each other and the parameters are other 
objects. Classes are just the mechanisms for defining 
and creating objects. And thats a very important distinction.

> For example, I have created an Args class that sets 
> a variety of internal variables (__filename, __outputdir etc) 
> by parsing the argv array from th command line.  

OK, What is the responsibility of an Args object?
What kind of things would you do with/to an Args 
instance? You might want to get the vatrious values 
from it - treating it somewhat like a dictionary maybe?
You might want to store it so the same set of args 
can be used over and over. What else do you want 
the args to do? Focus on the responsibility of the object 
as a whole.

> What would be the preferred mechanism for returning 
> or passing along those variables to another class?  

You pass an instance of the args class as an object to 
another object via a message.

> Maybe by a function method that returns all of those variables?

You might have a method of the Args class that can get 
the values for a given instance but you wouldn't want to 
extract the values from the object and manage them elsewhere. 
That would be to defeat the point of the object, which is to 
encapsulate both the data and the operations on the data 
in one entity, the object, which can be passed around in 
its entirety.

For example when you drive your car(an object) you don't 
dismantle it and then reassemble just the bits you need.
Let's see its only me so only one seat and its a warm day 
so I don't need the heater... You just jump into the complete 
car and drive it. Same with your args class(*). You create 
an instance and use it whichever way you need. If somebody 
else needs your args to do something you give them the args
(just like if somebody borrows your car you don't give them 
bits of it you lend them the whole car)


(*)And same with the car, you don't lend someone the factory 
blueprint (the class) and tell them to build their own car, you 
lend them your specific instance of the car.

HTH,

-- 
Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld