Re: Access idiom

[Gene <gene.ressler@gmail.com>]

On Jan 21, 1:15 pm, "Jeffrey R. Carter"
<spam.jrcarter....@acm.nospam.org> wrote:
> Gene wrote:
>
> > I'm at an impasse developing a graph data structure.  There are many
> > node types derived from a "most general" one.  Most of the node types
> > contain fields that are classwide access to child nodes.  Various
> > primitive procedures operate on nodes, dispatching on unnamed node
> > access types.  In many cases, the operations return such an access
> > child value.  Other "identity" ops just return the dispatching
> > parameter as classwide access.
>
> There should be no need for public access types or values in such a data
> structure. Hiding them should make the package easier to use, easier to
> implement, and safer.

First, I think the collective wisdom has put me straight. Thanks.

Another, more succinct explanation...  The graph has many node types,
all with common ancestor.  Edges are pointers--named or anonymous
access Node_Type'Class; don't care which.  Required is a primitive
"transform" procedure that dispatches on node type and must return a
pointer to an _arbitrary node type_ (i.e. again of type access
Node_Type'Class).  Frequently the transform should just return its
parameter without modification.  Using primitive functions (which
would enable return of anonymous access Node_Type'Class) is not good
due to need for multiple return values.

Jeff, in view of all the above, a named "access all Node_Type'Class"
type seems necessary for the return values.

I failed earlier because the out parameter wouldn't convert
automatically from named to unnamed access, and I didn't realize type
conversion would work on L-values.

Further comments very welcome if there is a better way.  Again,
thanks.

This compiles and dispatches as expected:

---- hoo.ads
package Hoo is

   type Node_Type is abstract tagged null record;
   type Node_Ptr_Type is access all Node_Type'Class;
   procedure XForm(P : access Node_Type;
                   Rtn : out Node_Ptr_Type) is abstract;

   type Binary_Type is new Node_Type with record
      Left, Right : access Node_Type'Class;
   end record;

   overriding
   procedure XForm(P : access Binary_Type;
                   Rtn : out Node_Ptr_Type);
end Hoo;

---- hoo.adb
package body Hoo is

   overriding
   procedure XForm
     (P : access Binary_Type;
      Rtn : out Node_Ptr_Type)
   is
   begin
      Rtn := Node_Ptr_Type(P);
   end XForm;

end Hoo;

---- foo.adb
procedure Foo is
   P : access Binary_Type;
begin
   P := new Binary_Type;
   Xform(P.Left, Node_Ptr_Type(P.Left));
end Foo;