Re: Object with zero bits

[Georg Bauhaus <sb463ba@l1-hrz.uni-duisburg.de>]

Nick Roberts <nick.roberts@acm.org> wrote:

:> With suitable rep clauses, Boolean discriminants don't seem to consume
:> space in some variant records, when the compiler can find out :-)
: 
: Which compiler please?  Would you be kind enough to post test code.

This "works" with ObjectAda:

package Test_02 is

  type Representation is (As_S, As_V);
  for Representation'Size use 1;

  type String_Access is access String;

  type Component (The_Representation : Representation) is
    record
      case The_Representation is
        when As_S => The_S : String_Access;
        when As_V  => The_V  : Natural;
      end case;
    end record;

  for Component use record
     --The_Representation at 4 range 0 .. 1;  -- adds more bits
     The_S at 0 range 0 .. 31;
     The_V at 0 range 0..31;
  end record;

  OA_Component_Size: constant := 32;
  pragma assert (Component'Size = OA_Component_Size);

  generic
     The_Representation: Representation;
  package Nested is
    type Item  is
       record
          The_Component: Component(The_Representation);
       end record;
  end Nested;

  package One_Or_Other is new Nested(As_V);

  type V_Item is new One_Or_Other.Item;
  pragma Assert(V_Item'Size = Component'Size);
  pragma Assert(V_Item'Max_Size_In_Storage_Elements = 4);

  Test: V_Item;
  pragma Assert(Test.The_Component'Position = 0);

end Test_02;

(The original problem was to specify represenations for a record with
a component of type Component, which isn't possible because objects
of type component vary in size (by definition?). Deriving a new type,
type D is new T(value);  where T has one discriminant
does not seem to "fix" the size.


-- georg