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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada Procedure Parameters That Use the Plus Sign
Date: Fri, 12 Jun 2015 10:00:47 +0200
Organization: cbb software GmbH
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Date: 2015-06-12T10:00:47+02:00
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On Thu, 11 Jun 2015 10:24:45 -0700 (PDT), NiGHTS wrote:

> I recently stumbled across some Ada code that appends a plus sign in front
> of a parameter name of a function or procedure call. From the context it
> seems like it is doing a forced cast between two incompatible types. 

Compatible types actually.
 
> Here is one example I found in the AdaGtk demo create_tree_view.adb line 243:
> 
> Gtk_New (Tree, +Model);
> 
> My questions are:
> 
> 1. What is this called? Is there a name for this?

In this case it is a downcast, I believe.

> 2. What exactly is happening here?

A concrete tree model is converted to the model interface. There is no
physical conversion, of course, just a reference object is converted to
another reference object. Thus from the language POV it is a conversion,
from the GTK's POV it is not.

> 3. Is it safe? Should it be avoided?

It is safe, though a quite lot irritating.

> 4. Is there another way to accomplish this without the use of '+' ?

In this concrete case there is no other way (I mean without an explicit
call) because Ada lacks the sort of MI/subtyping which would be required
for this.

In GtkAda, I think (too lazy to look), you could also use To_Interface
instead:

   Gtk_New (Tree, To_Interface (Model));

which is just same, but looks better and cleaner.

> 5. Where can I find out more about this?

Ada reference manual, GtkAda reference?

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