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Message-ID: <3A9E77D4.D40FEAA@earthlink.net>
From: "Marc A. Criley" <mcqnet@earthlink.net>
Organization: Quadrus Corporation
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Subject: JGNAT and Null references
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Date: Thu, 01 Mar 2001 17:23:31 GMT
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Date: 2001-03-01T17:23:31+00:00
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Let me just plow right into an annotated example here, rather than
trying to explain it first...

The Java class javax.swing.JTabbedPane provides a method called addTab
that looks like this:

public void addTab(String title,
                   Icon icon,
                   Component component,
                   String tip)

When this class is run through JGNAT's jvm2ada, the Ada equivalent of
that method is:

   procedure addTab (This : access Typ; 
                     P1_String : access java.lang.String.Typ'Class; 
                     P2_Icon : access javax.swing.Icon.Typ'Class; 
                     P3_Component : access java.awt.Component.Typ'Class; 
                     P4_String : access java.lang.String.Typ'Class);

Okay, so far so good.

In Java, if I don't have an icon to accompany this tab pane, I can just
supply that parameter with a null:

   jtp.addTab("My title", null, a_component, "a_tool_tip");

Attempting this in Ada:

   AddTab(Jtp, +"My title", null, A_Component, +"a_tool_tip");

generates an error when compiled:

   null cannot be of an anonymous access type

which is perfectly correct since that P2_Icon parameter requires a
pointer to , or the "'access of" an actual object.

Buuuuuut, I don't have, or want to pass, an Icon to display with the
tab, and specifying "null" is how I would indicate that in Java.  What
do I provide as a "null argument" in JGNAT?  I was hoping to find some
sort of predefined "null_reference" constant, analogous to
System.Null_Address, but no such luck.

Thanks much,

Marc A. Criley