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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: How best to test for NULL in Ada?
Date: Thu, 02 Jul 2015 16:19:22 -0700
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Date: 2015-07-02T16:19:22-07:00
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On 07/02/2015 03:15 PM, NiGHTS wrote:
> I have encountered several situations where I needed to test an access type
> for NULL or not. My instinct is to do something like this:
> 
> if Some_Object /= null then ...

That is correct.

> But many times I get this error:
> 
> invalid operand type for operator "/=" left operand has type
> "Some_Object_Type" defined at ... right operand has an access type

This says that Some_Object is not a value of an access type, so you can't
compare it to null.

> (If you want a specific example, I am trying to test if the Selected Path
> returned by Gtk.Tree_View.Get_Cursor in the GTK Library is valid or NULL. But
> situations like these occur many times in distinct situations so I'm not
> looking for a fix to this specific problem)

The path returned has type Gtk.Tree_Model.Gtk_Tree_Path, which is a tagged
record type. Since it's not an access type, you can't compare it to null. You
have to compare it to a value of type Gtk.Tree_Model.Gtk_Tree_Path.

-- 
Jeff Carter
"What lazy lout left these wires all over the lawn?"
Poppy
98