Checking Access Parameters to Procedures

Checking Access Parameters to Procedures

[Andy Askey]

Another, probably simple, question....

I want to check if a value passed into a procedure is NULL.  For
example:

package tst_pkg is
    type my_ttype is tagged private;
    procedure tst_proc(this : access my_ttype'class);
private
    type my_ttype is tagged
      record
         i : integer;
      end record;
end tst_pkg;

package body tst_pkg is

   procedure tst_proc(this : access my_ttype'class) is
   begin
      if (this = NULL) then
        ....
     end if;
   end tst_proc;
end tst_pkg;

This does not compile so I tried every combination I can think of and
nothing worked so far.  How can I test if "this" is a NULL pointer?

Thanx.

--
Andy Askey
ajaskey@mindspring.com

Re: Checking Access Parameters to Procedures

[David Botton]

"This" can not be NULL as it is passed in by "access". In the event that you
would like to pass in a NULL value, you would have to redefine your
procedure to have an in paramter of an access type.

For example:

package tst_pkg is
    type my_ttype is tagged private;
    type My_ttype_Access is access all My_ttype'Class;
    procedure tst_proc(this : My_Ttype_Access);

Andy Askey wrote in message ...
>I want to check if a value passed into a procedure is NULL.  For
>example:


>    procedure tst_proc(this : access my_ttype'class);

Re: Checking Access Parameters to Procedures

[Mark Lundquist]

Andy Askey wrote:

>    procedure tst_proc(this : access my_ttype'class) is
>    begin
>       if (this = NULL) then
>         ....
>      end if;
>    end tst_proc;
> end tst_pkg;
>
> This does not compile so I tried every combination I can think of and
> nothing worked so far.  How can I test if "this" is a NULL pointer?

The bad news: you can't.
The good news: you don't have to.

When you say "this : access my_ttype'class", you're defining an
anonymous general access-to-variable type, and anonymous access types do
not have the value Null.  Null is only a value of named access types.
That's why the compiler doesn't know what you mean.  At run-time, the
only way you could get a null value to try to pass in "this" would be
through an implicit type conversion from a named access type, and in
that case the conversion will raise Constraint_Error (like any other
conversion where there's no corresponding value of the target type).