Re: class wide iterable (and indexable)

[Simon Wright <simon@pushface.org>]

George Shapovalov <gshapovalov@gmail.com> writes:

> I would be thankful for any pointers, as I am at a loss what that
> class-wide for loop is getting confused about. Or why it even expects
> the Index type in the "of" loop?

Compiling (a subsection of) your code with -gnatG, which lists the
generated expanded code in an Ada-like syntax, we get

      C2220b : test_list_iface__pl__cursor := T2232b!(
        test_list_iface__pl__list_iterator_interfaces__reversible_iteratorH!
        (I2224b).all (1)).all (I2224b);
      L_5 : while test_list_iface__pl__has_element (C2220b) loop
         n : test_list_iface__pl__element_type renames
           test_list_iface__pl__list_reference (lc, C2220b);
         ada__text_io__put__4 (n'img);
         C2220b :=
           $test_list_iface__pl__list_iterator_interfaces__next__2 (
           I2224b, C2220b);
      end loop L_5;

which shows that part of the code (correctly) expects an integer, while
another part provides a cursor; so you're right, the compiler is
confused, this is a compiler bug.

However, I think the issue may be that you've only defined one function
List_Reference: if you look at Ada.Containers.Vectors you'll see

   function Reference
     (Container : aliased in out Vector;
      Position  : Cursor) return Reference_Type;
   pragma Inline (Reference);
...
   function Reference
     (Container : aliased in out Vector;
      Index     : Index_Type) return Reference_Type;
   pragma Inline (Reference);

so GNAT's "bug" is that it doesn't understand how to cope with your
error. AdaCore are always interested in improving error messages, but
this error does seem rather out of the ordinary!

I had a quick-and-dirty go at fixing this, & got a build, but ended with

testing List_Interface'Class ..
assignin values .. done;  values: 
Catchpoint 1, CONSTRAINT_ERROR (Ada.Tags.Displace: invalid interface conversion) at 0x00000001000210da in test_list_iface.pld.iterate (container=..., 
    <iterateBIPalloc>=2, <iterateBIPstoragepool>=0x0, 
    <iterateBIPfinalizationmaster>=0x0, <iterateBIPaccess>=0x0)
    at /Users/simon/tmp/cla/ada_gems/list_iface/src/lists-dynamic.adb:46
46	        return List_Iterator_Interfaces.Reversible_Iterator'Class(ACV.Vector(Container).Iterate);
(gdb) l
41	--     end;
42	
43	    overriding
44	    function Iterate (Container : in List) return List_Iterator_Interfaces.Reversible_Iterator'Class is
45	    begin
46	        return List_Iterator_Interfaces.Reversible_Iterator'Class(ACV.Vector(Container).Iterate);
47	    end;
48	
49	end Lists.dynamic;

Hope this helps ...