Re: Generic Embedded List Nodes

[Warren <ve3wwg@gmail.com>]

On Sunday, 19 June 2016 00:45:54 UTC-4, Simon Wright  wrote:
> Warren <ve3wwg@gmail.com> writes:
> 
>    type Node_Type(Obj: access Object_Type) is new Emb_Node
>      with null record;

I've come to the conclusion that my original idea in Ada terms is too half baked. 

What it will come down to in the end is doing an offset calculation from the Node back to the Object when its access is required (through a function). 

Even in C/C++ this is a bit of a problem. While there is an offsetof(type,member) macro, this is only usable on non-dynamic types. Calculating an offset of a struct is trivial when you have it allocated. But when the class has a constructor, then you may not want to invoke that overhead only to create one for offset calculations.

In Ada for some record Type R.M where R is the record and M is the member, you cannot do R.M'Position when R is just a type. But you can cheat this, similar to how I have done it in C/C++:

with System;
with Ada.Integer_Text_IO;

procedure T is
   use Ada.Integer_Text_IO;

   type R_Type is
      record
         I : Integer;
         F : Float;
      end record;

   A:    System.Address;

   R:    R_Type;
   for R'Address use A;
   
begin
   Put(R.I'Position);
   Put(R.F'Position);
end;

This never allocates the object, but assumes it already exists at address A. Since the record and its members are never actually used, it is possible to have the compiler do the necessary calculations. This will complain "warning: variable "A" is read but never assigned", however.

I suppose a tagged R.Initialize could also establish an 'Access value in all Emb_Node members, using some other procedure call for Emb_Node (perhaps Set_Object). But it is desireable not to have to carry the object reference in the Emb_Node at all.

Warren