Access conversions

[Simon Wright <simon@pogner.demon.co.uk>]

I need to write a function

  function Is_Member (G : Graph'Class; V : Vertex'Class) return Boolean;

where there is a type

  type Graph_Ptr is access all Graph'Class;

and a Vertex has a component Rep with a component Enclosing of type
Graph_Ptr which accesses its enclosing Graph.

In GNAT I can say in Is_Member

      return V.Rep.Enclosing = G'Unrestricted_Access;

but of course this isn't portable.

If I say

  function To_Ptr is new Ada.Unchecked_Conversion (System.Address,
                                                   Graph_Ptr);

and then in Is_Member

      return V.Rep.Enclosing = To_Ptr (G'Address);

which appears to work as expected in GNAT and ObjectAda 7.1, am I in
fact riding for a fall?

-Simon