Re: [Swarm-Support] passing information between classes

[Paul Johnson]

The two declarations do basically the same thing, however the compiler 
gets more information if you do

ClassA * aClassA;

This tells it where to look for the right methods.

If you just do the id method, and the compiler finds 2 similarly named 
methods in different classes, then it may complain it is confused about 
the multiple declarations. You see, if you declare

id aClassA;

you have not really declared a ClassA object. You declared a generic thing.

In Swarm, protocols are frequently used, and if there is a protocol 
ClassA, the confusion is avoided if you do

id <ClassA> aClassA;

If

Class * aClassA

will work, I generally like to use it, for simplicity. Sometimes it will 
not.  Here are some instances.

in a header file ClassB.h, doing this may fail becuase "ClassA.h" is not 
included in the headers.  It is not possible to include the header file 
because it would create a circularity, i.e., ClassB.h already includes 
ClassA.h.

Incidentally, there's a class declaration that can be used to fix this. 
WIthout including ClassA.h, you can put an instruction in ClassB.h to 
tell the compiler to allow you to use ClassA * aClassA even though 
ClassA.h is not included.  I think I saw that recently in ArborgamesII.

The other time ClassA * aClassA will not work is when you are building 
an instance of a Swarm class that is designed to block subclassing and 
direct instantiation. I'm out on the edge of my precise understanding 
here.  I know for sure that Swarm collections, such as List, will 
prohibit you from making instances like

List * aList;
aList = [List create: self];

and the only option is

id <List> aList
aList = [List create: self];

I know I've answered this question several times recently, but did not 
formalize a SwarmFaq entry about it.  This is a start

http://www.ku.edu/~pauljohn/SwarmFaq/SwarmOnlineFaq.html#7.6

but I'll do more

Derek Farren wrote:
> Hello everybody.
>
> I have a doubt about passing information between classes. It ends on a
> specific question: Whats the diference between creating an instance of the
> class "ClassA" via id aClassA and ClassA * aClassA ?
>
> Sometimes both ways work, but some other (when the imported class is
> ModelSwarm) just the first choice works.
>
> Thanks, Derek
>
>
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--
Paul E. Johnson                       email: address@hidden
Dept. of Political Science            http://lark.cc.ku.edu/~pauljohn
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