How best to test for NULL in Ada?

How best to test for NULL in Ada?

[NiGHTS]

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 ...

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

What is the correct way to test for NULL in Ada?


(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)


Thank you in advance for your help!

Re: How best to test for NULL in Ada?

[Jeffrey R. Carter]

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

Re: How best to test for NULL in Ada?

[Randy Brukardt]

"Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org> wrote in message 
news:mn4gr7$97r$2@dont-email.me...
> 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.

Right. You often get an error that the operator is not visible, which means 
that a suitable "use type" clause is needed.

In Ada 2012 there is a hack available to avoid the use clause. (This comes 
from Tucker Taft, who admitted to doing this during our recent ARG meeting 
in Madrid. He got suitable groans for this usage...)

    if Some_Object not in null then

This membership uses the predefined equality for the type of Some_Object, 
and it need not be visible. But this is a hack (it harms readability), and 
it's dangerous if "=" has been redefined (as it always uses the predefined 
"=", not the overridden one). Use at your own risk.

                                      Randy. 

Re: How best to test for NULL in Ada?

[Shark8]

On Thursday, July 2, 2015 at 6:12:53 PM UTC-6, Randy Brukardt wrote:
>
> In Ada 2012 there is a hack available to avoid the use clause. (This comes 
> from Tucker Taft, who admitted to doing this during our recent ARG meeting 
> in Madrid. He got suitable groans for this usage...)
> 
>     if Some_Object not in null then
> 
> This membership uses the predefined equality for the type of Some_Object, 
> and it need not be visible. But this is a hack (it harms readability), and 
> it's dangerous if "=" has been redefined (as it always uses the predefined 
> "=", not the overridden one). Use at your own risk.

Hm I can see where, if you were writing your own "=" operator for Some_Object's type and it was (a) an access type, and (b) you needed to test for null that could be useful.

Re: How best to test for NULL in Ada?

[Dmitry A. Kazakov]

On Thu, 02 Jul 2015 16:19:22 -0700, Jeffrey R. Carter wrote:

> 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.

Yes. There is the Null_Gtk_Tree_Path constant for that.

Gtk_Tree_Path is a wrapped pointer. One thing Ada should have is the
abstract access type to have such wrappers derived from, and thus making
comparison to null legal (instead of the implicit dereference hack).

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: How best to test for NULL in Ada?

[NiGHTS]

This helped me out a lot. Thank you all for your help.