Re: Access types as parameters

[Adam Beneschan <adam@irvine.com>]

On Jul 17, 1:39 am, Rick <rickdu...@gmail.com> wrote:
> Given:
>
>    type My_Type is  ...;
> and
>    type My_Access_Type is access all My_Type'Class;
>
> what is the practical difference between:
>
>    function My_Function (Thing : access My_Type'Class) return
> Positive;
> and
>    function My_Function (Thing : My Access_Type return Positive;

The accessibility level rules say that a value of type My_Access_Type
cannot point to an object that might go away before My_Access_Type
does.  (Unless you use 'Unchecked_Access to create the access value.)
This is to prevent dangling references.  The rules apply to parameters
also, so if you call the second My_Function, you can't give it the
'Access of an object that is deeper than My_Access_Type.  If
My_Access_Type is declared at library level (i.e. in a library
package, not inside a procedure or function), then you can't say

   procedure Some_Procedure is
        X : aliased My_Type;  --or some type derived from My_Type
   begin
        My_Function (X'access);  -- i.e. the second My_Function

But this restriction doesn't apply to the first My_Function.  Since
that My_Function has an access parameter, you can pass the 'Access of
any object of the right type, no matter how deeply nested inside
procedures the object is.

                                    -- Adam