Re: Private declaration question

[Stephen Leake <Stephen.Leake@gsfc.nasa.gov>]

Jerry van Dijk wrote:
> 
> Why has the completion of a private declaration to be a full view ?
> 
>      1. package Oops is
>      2.
>      3.    type A_Type is limited private;
>      4.
>      5. private
>      6.
>      7.    type A_Type is array (Positive range <>) of Integer;
>                 |
>         >>> full view of type must be definite subtype
> 
> After browsing the RM I think the error message comes from rule
> 7.3(4). If that is true, why is this ? It seems a logical construction
> to protect A_Type objects from user interference.

Actually, the error is due to RM 7.3(12); the partial view of A_Type has
no discriminants, so the full view must be a definite subtype. A
definite subtype has a known size; an unconstrained array is not a
definite subtype (as the error message says).

The reason is that the compiler must know how much memory to allocate
when a user of A_Type declares an object:

	My_Object : Oops.A_Type;

Oops provides no way to set the size of the array. The only way to fix
this, while keeping A_Type limited, is to provide a discriminant:

   package Oops is
      type A_Type (Size : Positive) is limited private;

   private
      type Array_Type is array (Positive range <>) of Integer;

      type A_Type (Size : Positive) is record
         Contents : Array_Type (1 .. Size);
      end record;
   end Oops;

Now the user can declare an object:

   A : Oops.A_Type (2);

If A_Type does not need to be limited, you can set the size by returning
a value from an initialization function:

   package Oops is
      type A_Type (<>) is private;

      function Init (Size : in Positive) return A_Type;
   private
      type A_Type is array (Positive range <>) of Integer;

   end Oops;

   package body Oops is
      function Init (Size : in Positive) return A_Type
      is begin
         return (1 .. Size => 0);
      end Init;
   end Oops;

   A : Oops.A_Type := Oops.Init (2);

-- 
- Stephe