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From: Mark Lundquist <mark@rational.com>
Subject: Re: Checking Access Parameters to Procedures
Date: 2000/03/07
Message-ID: <38C56EF1.3057F00D@rational.com>#1/1
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To: Andy Askey <ajaskey@mindspring.com>
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Date: 2000-03-07T00:00:00+00:00
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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).