Re: "accessibility check failed" on Node

[Simon Wright <simon@pushface.org>]

Thomas Schmidt <TC.Schmidt@gmx.net> writes:

>   function findNode2
>     (graph : DirectedGraph;
>      aNode : access Node'Class)
>      return  Node_Class_Access is

I spent quite some time making a compilable example. It would have been
kind of you to have done the same!

The call in

     position : constant NodeSets.Cursor :=
       graph.nodes.Find (aNode.all'Access);

seemed a bit magical (a respected colleague said that he found himself
using ".all.Access" as a band-aid for shutting up the compiler's
complaints about type mismatches without bothering to understand
them). So I tried

      N : constant Node_Class_Access := Node_Class_Access (aNode);
      position : constant NodeSets.Cursor := graph.nodes.Find (N);

which moved the "accessibility check failed" to the "N :=" line.

I made a findNode3 with the type of aNode changed to Node_Class_Access:

   function findNode3
     (graph : DirectedGraph;
      aNode : Node_Class_Access)
     return  Node_Class_Access is

      position : constant NodeSets.Cursor := graph.nodes.Find (aNode);

   begin
      if position /= NodeSets.No_Element then
         return Nodesets.Element (position);
      else
         raise No_Element with "failed in Findnode3";
      end if;
   end findNode3;

Calling findNode3 with an aliased local Neuron now requires me to use
'Unchecked_Access, or I get the compilation error "non-local pointer
cannot point to local object".

As to your original question, why "Node (aNode.all)'Access" should be OK
while "aNode.all'Access" isn't .. I expect the answer is something deep,
and I'd go along with Jeff Carter about avoiding the use of anonymous
access types.