Re: Another OOP Question

[cosc19z5@Bayou.UH.EDU (Spasmo)]

Jon S Anthony (jsa@alexandria) wrote:
: In article <4v609i$fgj@Masala.CC.UH.EDU> cosc19z5@Bayou.UH.EDU (Spasmo) writes:

: > Ok, here's another OOP question about accessing private members.
: > Right now let's say I have a class called A, and B inherits from
: > A.  Now let's say that B needs to access a private data member
: > in A -- is there any way I can do this without placing B into
: > a Child package?  

: I'm not sure I follow this.  Do you mean "private" in the C++ sense or in
: the Ada sense?  In the Ada sense, if A and B are in the same package, then
: you get this for free:

Well private in the Ada sense.  As usual I'm being way too
vague (I need to get out of this habit!).  I've got a class
defined in one package, and an inherited class in another
package so I'm speaking of 2 classes in 2 different packages.
That's pretty much how I'm trying to commit myself in terms
of doing things.  Ada provides such awesome modular programming
support that there's no reason not to use it.

[Example snipped, it was about another scenario]


: If you want A and B in separate packages, then in order for the impls of
: B's ops to have access to the private definition of A, it needs to go into
: a child package.

Right, that's pretty much the scenario I'm working with.  I'm
using class to mean a tagged datatype and its associated
functions all placed into a package since that's how I'm doing
things, but of course others may do things differently so
forgive the error (and arrogance).


: > The reason I'd like to now is because I'm confronted with such a
: > scenario and Child packages (among other things) will make all of my
: > parents members visible which is the last thing I want to do.

: I don't understand why you make this claim.  Children do not make any of
: the private stuff of the parent visible:

Are you sure about this?  I was able to pick into the private
goodies of my parent when I declared a child class in Gnat3.05
for DOS.  It was something like this:

The package with the parent had a class that was tagged private.
I declared a child package and was able to use the data fields
	of the class in the parent that were private.



: package P is
:     type A is tagged private;
:     ...
: private
:     type A is...
: end P;


: package P.C is
: --
: -- None of P's private definitions are visible here.
:     type B is new A with private;
:     ...
: private
: --
: -- P's private section is visible here and in the body of P.C
:     type B is ...
: end P.C;


: with P.C;
: procedure Proc is
: --
: -- Nothing of P's private section is visible here...
: ...
: end Proc;

Well yes I know of the above, again I'm being vague.  I meant
that the child package could see the parent package's private
goodies, not that the child package would make the parent's
private goodies available to whatever package with'ed it. 


: My guess is that you are not clear on child package visibility rules
: and that you really do want to use a child package and that it will
: work just fine for what you want to achieve.

Well I knew about the above so I believe I'm relatively clear
on the visibility rules.  What was troubling me was that the
way I was doing it was making all of my parent's private stuff
visible to the child, rather than finding a way to just access
the one member I wanted in question.  Of course I could try
returning a pointer to the internal aliased type and indirectly
manipulate it through there, but the thing would be clumsy and
once again it wouldn't change having to deal with the child
package (unless I wanted this to be available to all!).



: Lastly, if you mean "private" in the C++ sense, you have to put the actual
: definitions in the _bodies_ of the packages.  The only way for anything
: else to get at these will be through accessors (public or private) defined
: in the specification.  Nothing outside the body (and any separates) has
: direct access.

Ok, I'll try to be precise in what I am trying to do and use an
example from *gasp* C++.  I should have done this from the start
but to be honest I didn't want to drag C++ in there since
the more I use Ada the more I look down on C++, but it seems
that for example's sake this may be the way to go.

Ok, consider this scenario.  A person is working on a an OO package.
This package defines a class and all the operations to be performed
on this class.  No problemo so far, but there's something the
designer is confronted with.  There's a data item whose direct
access may be of great use to any children.  At this point in
time the designer cannot visualize what the children might
do with it, but he/she realizes that being able to directly
access this data item can be of importance.  Now the designer
wants to make this data item visible without making anything
else visible, but at the same time this person cannot just
make this data item visible to everything -- only children.

The scenario is similar to C++'s protected types.  Here
you have items that cannot be accessed except by inherited
classes, and the thing is you can have protected types
and private types mixed in so that only the stuff you want
to be protected is in fact protected.  Ok so far so good?

Now with Ada you have the private section of a package.
This makes everything private but child packages can see
into this.  The problem is that if you use child packages
the children can see EVERYTHING.

The precise scenario that I'm confronted with is this.
I'm working on some menu classes to get more familiarized
with OOP in Ada.  Anyway I've written some wrappers for
enhanced I/O by accessing C classes and I've made it so
that the output is double buffered.  Naturally to share
the screen items need to be able to access the current
buffer.  Now the box class has an access to the buffer
so it can modify it (via alias), but of course to be able
to write to the same screen children need to be able
to get a hold of this buffer since they want to do more
with it.  Ie: the next child is a textbox so it needs
to put text in the box and it needs to get the buffer
to do this.

Now there is one idea and that's just having a function
that returns an access to this buffer so that we can
get the screen that we're working on.  Of course the
problem is that in order to circumvent this whole
mess such a function would have to be made public
which would give other non-related units access
to this private data member (via an alias).

Messed up isn't it?  Maybe my whole concept of how
I'm doing this is flawed, but still am I right in
assuming that there is nothing equivalent to
C++'s protected type?  

Wow what a mouthfull! :)
 

: /Jon
: -- 
: Jon Anthony
: Organon Motives, Inc.
: 1 Williston Road, Suite 4
: Belmont, MA 02178

: 617.484.3383
: jsa@organon.com


--
Spasmo
"Everyone has secrets, but sometimes you get caught,
 So if it's just between us, my silence can be bought"
	"Blackmail" by Sloppy Seconds