Q: Access subprogram with class type parameters

Q: Access subprogram with class type parameters

[Rush Kester]

This is my first attempt at using subprogram access types and class
programming.

Why doesn't Default_Get_Procedure match Get_Procedure in the following?

with text_io;
procedure test is
   type Values is
      tagged record
         null;
      end record;
   type Bool_Values          is new Values
      with record
         Bool_Value : Boolean;
      end record;
   package bool_io is new text_io.enumeration_io(boolean);

   type Get_Procedure is access procedure (
     File : in text_io.file_type;
     Value : out values'class);

   procedure Default_Get_Procedure(
     File : in text_io.file_type;
     Value : out Bool_Values) is

   begin
     bool_io.get(File, Value.Bool_Value);
   end Default_Get_Procedure;

 Get_Message : Get_Procedure := Default_Get_Procedure'Access; -- what's
wrong?
-- ...
 File : text_io.file_type;
begin
  null;
end test;

Re: Q: Access subprogram with class type parameters

[Ted Dennison]

In article <38C6F379.173A92AF@jhuapl.edu>,
  Rush Kester <Rush.Kester@jhuapl.edu> wrote:
> This is my first attempt at using subprogram access types and class
> programming.
>
> Why doesn't Default_Get_Procedure match Get_Procedure in the
following?
>

>    type Get_Procedure is access procedure (
>      File : in text_io.file_type;
>      Value : out values'class);
>
>    procedure Default_Get_Procedure(
>      File : in text_io.file_type;
>      Value : out Bool_Values) is

Because Values'Class and Bool_Values aren't the same type. You can't
define a procedure type that will take a procedure with a parameter of a
different type than is in the declaration, even if that different type
does happen to have been derived from the one specified.

I made this exact mistake about 3 months ago when I was trying to write
the parsing support for OpenToken.


--
T.E.D.

http://www.telepath.com/~dennison/Ted/TED.html


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