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Date: Tue, 3 Mar 2015 14:36:59 -0800 (PST)
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Subject: Why does that work?
From: Laurent <daemon2@internet.lu>
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Xref: number.nntp.giganews.com comp.lang.ada:192405
Date: 2015-03-03T14:36:59-08:00
List-Id: <comp.lang.ada>

Hi

Why does It.L.Head or Tail work?

It is of type Iterator defined as:

   type Iterator (L : access List) is
      record
         This :  List_Ptr:= null;
      end record;

If It points to the first node in the List L then It.This.Previous = null
and if It points to the last node in the List L then It.This.Next = null.

The pointers are working only in one direction I think? So if I am the pointer I will never see the head or the tail of the list. Probably a too simple/naive view. 

So how can It "know" something about the Head or Tail of the List? Is that caused by L: access List?

In my test program all the nodes are known at compile time. Didn't have the time to write a test to see what happens if I add one node to the list at runtime. Wouldn't that somehow cause a runtime error or simply not work as intended?

Just a small example:

   procedure Add_To_End (L : access List; Element : in Element_Type) is
      It : Iterator (L);

      begin -- Add_To_End
      
         if It.L.Head = null then -- List L is empty
            It.L.Head := new List_Node'(Element, null, null);
            It.L.Tail := It.L.Head;

         else
            It.L.Tail.Next := new List_Node'(Element, null, It.L.Tail);
            It.L.Tail := It.L.Tail.Next;
         end if;
      end Add_To_End;

Hm the first time I whine about something working and I didn't expect it :)

Thanks

Laurent