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Newsgroups: comp.lang.ada
Date: Sat, 18 Jun 2016 19:50:32 -0700 (PDT)
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Subject: Re: Generic Embedded List Nodes
From: Warren <ve3wwg@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:30804
Date: 2016-06-18T19:50:32-07:00
List-Id: <comp.lang.ada>

On Saturday, 18 June 2016 22:21:18 UTC-4, Warren  wrote:
> On Saturday, 18 June 2016 22:14:22 UTC-4, Jeremiah  wrote:
> > On Saturday, June 18, 2016 at 6:52:06 PM UTC-4, Warren wrote:
> > > Getting back to Ada after a hiatus, I currently have a need to build a 
> > > generic package to implement "embedded list node" lists. The advantage 
> > > is high performance in a server setting to avoid underlying malloc/free
> > > calls.
> > > 
> > > The idea is that the list node resides within the structure/tagged type,
> > > and acts as a doubly linked list node when in a list. The Emb_Node can
> > > be used as a list head when itself (or as part of another structure). This
> > > kind of thing is done in the Linux kernel, for example.
> > >    
> > > The Emb_Node and its operations are trivial. The problem occurs when
> > > you traverse a linked list of Emb_Nodes (or its derived type). With
> > > a given node, I need to then access the object that _contains_ it. In
> > > C/C++ you do some offset calculation from the node address back to
> > > the owning struct/class.
> > > 
> > > My idea (in Ada) was to save some kind of access value in a type
> > > derived from Emb_List. But that is where the trouble starts.
> > > 
> > > package Emb_List is
> > >          
> > >    type Emb_Node is tagged private;
> > >              
> > >    procedure Insert_Head(Head: access Emb_Node; Node: access Emb_Node);
> > >    procedure Unlink(Node: access Emb_Node);
> > > 
> > > private
> > > 
> > >    type Emb_Node is tagged
> > >       record
> > >          Next:    access Emb_Node;  -- Ptr to next node in list (if any)
> > >          Prev:    access Emb_Node;  -- Ptr to prev node in list (if any)
> > >       end record;
> > >    
> > > end Emb_List;
> > >    
> > > The generic extension (below) was intended to hold the reference to the
> > > containing object, which is where the trouble starts. I was hoping this
> > > would work, but running into "missing full declaration for private
> > > extension".
> > > 
> > > generic
> > >    type Object_Type is tagged private;
> > > package Emb_List.Nodes is
> > > 
> > >    type Node_Type(Obj: access Object_Type) is new Emb_Node;
> > >    
> > >    function Object(Node: Node_Type) return Object_Type;
> > >    
> > > end Emb_List.Nodes;
> > 
> > Change Node_Type to:
> > type Node_Type(Obj: access Object_Type) is new Emb_Node with null record;
> > 
> > You need an extension, even if it is a null record. 
> 
> I had clearly forgotten about "with null record". I will try that.
> 
> Warren

generic
   type Object_Type is tagged private;
package Emb_List.Nodes is

   type Node_Type(Obj: access Object_Type := Obj'Access) is new Emb_Node  
      with null record;  
   
   function Object(Node: Node_Type) return Object_Type;
   
end Emb_List.Nodes;

The problem with this now is "discriminant "Obj" cannot be used before end of discriminant part".

This is intended to allow extension of My_Recd with a Node that knows My_Recd'Access.

declare
         type My_Recd is tagged
            record
               Id:         Natural := 99;
            end record;
   
         package EMB is new Emb_List.Nodes(Object_Type => My_Recd);

Is there another way that I could do this: i.e. determine My_Recd'Access from some list node object, embedded in My_Recd?

Warren