Re: Can someone help me understand this queue package?

["Liddle Feesh" <no_see_reply_address@spam.com>]

"Michal Nowak" wrote:

> "ADA 95: THE CRAFT OF OBJECT-ORIENTED PROGRAMMING" - chapter 11
> (in particular - 11.4)

Okay. In the example given from that page - adding a new "Node" or
"Appoinment" would be done using the procedure (in my example) "Add".

This procedure is listed as follows:

 procedure add (n:  in integer;  q:  in out queue) is
 begin
  null; -- Do nothing
 end add;

Now - in the example from the site:

    New_Item := new Appointment_Record;
    if Current /= null then        -- insert after another appointment
        New_Item.Next := Current.Next;
        Current.Next  := New_Item;
    else                           -- insert at start of list
        New_Item.Next := Diary.First;
        Diary.First   := New_Item;
    end if;


But the problem comes when you think, what is "New_Item"? Is it a function
call? No - it can't be, no parameters. Is it an instance of a record? Maybe,
but my compiler (OE) throws up the error "Direct name, New_Item is not
visable" leading me to believe that I haven't instantiated the name
somewhere. But what does it instantiate to? And surely when setting up
"New_Item", you want to fill it with data - which happens in the example
above. But how can you fill it with data when it doesn't exist...?

Even if I wrote:

New_Item := new Node;

This would still fall over with the same error.


How do I create a new node, and point the 'next' value to the 'value' of the
next node? I know how this is done on paper...

V = Value
N = Next

[V|N]--->[V|N]--->[V|N] -- etc. The last node containing 'null' for the
'next'.

But - HOW do I do this?

Surely:

New_Item := new Node;

would work, then all I would have to do is :
New_Item.Next := Current.Next;
Current.Next := New_Item;

(as from the website)

But the values for New_Item and Current have not been instantiated. In the
example they are not instantiated. In every book I read they are never
instantiated. What am I missing here?

--
Liddle Feesh
 '  O 0 o <"//><  ' o'^
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