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Re: Encapsulation

daemon@ATHENA.MIT.EDU (Fri Mar 25 17:51:10 1994 )

To: rayn@q.crossaccess.com (Ray Nawara jr.)
Cc: coldstuff@MIT.EDU
In-Reply-To: Your message of Fri, 25 Mar 94 13:16:34 -0800.
Date: Fri, 25 Mar 94 17:47:38 EST
From: Greg Hudson <ghudson@MIT.EDU>

> It is true that one object shouldnt directly modify another objects
> variable, but shouldnt the first object be able to ask the other
> object to change its variable? or rather, can an object change its
> own variables....


> Should accessors exist which enable the data to be modified or
> viewed, and can be called from other objects? the concept of
> protected data could be enabled by placing a check in the accessor
> method if the var were to be private or protected, most likely.

What you're essentially asking for is the existence of an "automatic"
method which exists whenever a variable exists.  That is, if I have a
variable 'foo' on an object, you're proposing that the object should
also have a method foo() which can be used to retrieve or set the
value of foo.

Coldmud has no such functionality; if you want your object to allow
other objects to get and set variables, you have to explicitly provide
the interface for that.  Other people have asked for this
functionality, and I don't plan to include it in Coldmud for the
following reasons:

First, it adds complexity.  I am averse to doing lots of things behind
the programmer's back, and writing part of an object's interface for
the programmer is not a good thing.

Second, it encourages programmers to think of other objects in terms
of their state variables rather than their operations.  There's no
pragmatic difference between "setting a variable on an object" and
"asking an object to change its variable"; both of these are what
encapsulation is trying to avoid.  Usually, you should think about
abstractions in higher terms.  You are not setting the contents
variable of a container to a list containing an extra item; you are
adding something to a container's contents.

Third, it doesn't fit in well with the encapsulation barriers across
ancestors.  Objects don't have just one set of variables; they have a
set of variables for the parameters of each of their ancestors.

Eric Weber asked what I meant when I said that MOO and Coldmud are
object-oriented at the database level rather than at the language
level.  By that, I meant that inheritance, and in the case of Coldmud,
encapsulation, apply to these big monstrous things called "objects".
Objects have to be explicitly created and destroyed, and they reside
on disk quite a bit of the time.  These are not really part of the
language; they are entries in a database.  If MOO and Coldmud were
object-oriented at the language level, you would be able to apply the
OO constructs to data types.  Associative arrays (dictionaries, in
Coldmud) wouldn't have to be built-in data types for convenience
(although possibly for speed), because you could implement them in the
language just as easily.  C++, Clu, Smalltalk, and the like, are
object-oriented in this way.  (C++ layers it all on top of C, and Clu
doesn't have inheritance, so they aren't perhaps the best examples.)