Re: Problem trying to implement generics.

["Ayende Rahien" <Dont@spam.me>]

"chris.danx" <chris.danx@ntlworld.com> wrote in message
news:0JBB6.10484$FD1.1197250@news6-win.server.ntlworld.com...
>
> > >
> > > >  private
> > > >   type Binary_Tree is new Ada.Finalization.controlled with record
> > > >    root: Node_access := null;
> > > >   end record;
> > >
> > > the error comes because
> > >
> > > > package body binTree is
> > > [snip]
> > > >  procedure insert(insert_this : in data_type; Sort : in sort_by) is
> > > >   New_node : Node_Access := new node;
> > > >   current : Node_access := root; -- This is the line it compline
> about!!
> > >
> > > you've forgot to that Root is in Binary_Tree.
> >
> > Okay, so how do I fix it? And what is wrong, anyway?
>
> What you do is pass in a Binary_Tree type with appropriate mode.  In C++
you
> have objects that store the methods with the data in the class.  This
means
> you don't need to refer to the object name and you can just use "root".
In
> Ada methods are simply procedures and functions that work with objects
> passed in.
>
> In C++ you'd have
>
>     BinTree.insert(insert_this, sort);
>
> but in Ada
>
>     insert(BinTree, insert_this, sort);
>
> You use the methods with the object type.  So you'd have to pass in an
item
> of type Binary_Tree with appropriate mode and use the "root" field in
this.
>
> e.g.
>     procedure insert (BT            :     in out Binary_Tree;
>                                 insert_this : ...
>                                 sort          : ...) is
>         New_node : Node_Access := new node;
>         current : Node_access := BT.root; -- This is what you need to do!
>     begin
>         .........................
>         .........................
>     end insert;
>
> If your interested i have a generic doubly linked list ADT written in Ada.
> It's part of the Heather modules i'm writing.  I know it isn't based on
> tagged types(Ada objects), but it does show how to build an ADT.  Your
> implementation of Binary_Tree is "Limited controlled" which means it's a
> tagged type, but i haven't added this property to it yet.  I may soon, or
at
> least make it controlled.  It really doesn't matter that much since it's
> just an example.
>
> I will put it up on my site if you let me know you want it.

Maybe later, right now I want to implement those myself, it's a good
practice, and make for excellent tools afterward.


> > > I have a question.  From your code it doesn't look like an ADT.  Is
this
> > > what you intended?  I can't remember the term for what this looks
like,
> > but
> > > I think it's to do with storing the Tree as a global variable in the
> > package
> > > not as an ADT.
> >
> > Okay, major cocept problem here.
> > I'm coming here with C/C++ background, and I understand that Ada has a
> > different OO paradigm.
> > I assumed that for each Binary_Tree spesific type that I create, there
> would
> > be its own Root.
> > I mean:
> > package Int_BinTree is new Binary_Tree(data_type=>
> > Integer,Sort_by=>Integer);
> >
> > Bin_Tree_First, Bin_Tree_Second : Int_BinTree;
> >
> > Does Bin_Tree_First.Root = Bin_Tree_Second.Root ?
> >
> >
> > > If you want an ADT you need to pass in a Binary_Tree type as
necessary.
> I
> > > didn't see it any where in your code so i'm assuming it's based on a
> > global
> > > variable.
> >
> > I think that you are talking about ADO vs ADT , right?
> > generic abstract data type & generic abstract data object
>
> Yes, that's it.  ADO -- i must remember that!
>
>
> > > There's nothing wrong doing it this way!
> >
> > Then why doesn't it compile?
>
> It doesn't compile because Ada's OO is based on extensible records.  You
> have to pass the record in even though it's an Object.  In C++ the object
> methods are held inside the class which means they have access to the
> variables inside.  This is not the same as Ada.
>
> Maybe, this is a better explanation, it's just like C records.  Just pass
in
> the record and use it as you wish.  Remember to get the correct mode, and
> use the record like you would in C.
>
> There's more to Ada's OO but you don't need it for your Binary_Tree and
I'm
> still learning about it myself.

Thanks, that helped, it now compiles.
BTW, is there any reason why I can't use out parameter with a function?