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From: LeakyStain <leakstan@erols.com>
Subject: Re: Exposing agreggate members of a class
Date: 1998/12/31
Message-ID: <368B738E.23BC574@erols.com>#1/1
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NNTP-Posting-Date: 31 Dec 1998 12:52:39 GMT
Newsgroups: comp.lang.ada
Date: 1998-12-31T12:52:39+00:00
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David Botton wrote:
> 
> Given:
> 
> with GCanvas_Package; use GCanvas_Package;
> 
> type GWindow_Object is tagged with private;
> type GWindow_Pointer is access all GWindow_Object;
> 
> private:
> 
> type GWindow_Object is tagged
>    record
>         Canvas : GCanvas_Object;
>    end record;
> 
> What is the cleanest way to give access to the Canvas by reference in a way
> that will not require Unchecked Programming

Depends on how much "acess" the clients need. If they can destroy
Canvas, then it should not be private.

Windex (http://www.erols.com/leakstan/stephe/index.html) solves a
similar problem; shared access to a Win32 Device Context. The approach I
take is to declare the basic Device Context operations to take a
class-wide object parameter:

package Windex.Graphics_Devices is

   type Handle_Type is new Root_Handle_Type with private;

   procedure BitBlt
      (Dest_Image      : in Handle_Type'class;
       <other params>);

   ... other operations
end package Windex.Graphics_Devices;

Root_Handle_Type is derived from Ada.Finalization.Limited_Controlled.
Then, in a child package, declare a type that takes a Windex.Windows
object as a discriminant:

package Windex.Graphics_Devices.Windows is

   type Handle_Type (Owner : access Windex.Windows.Handle_Type'class) is
new
      Windex.Graphics_Devices.Handle_Type with null record;
   -- Handle_Type can be used with all Windex.Graphics_Devices
operations.

   procedure Initialize (Handle : in out Handle_Type);
   -- Get the device context for Window. This locks the window for
drawing;
   -- after drawing, Handle must be finalized to allow normal Window
   -- painting.

   procedure Finalize (Handle : in out Handle_Type);
   -- Release window device context.

end Windex.Graphics_Devices.Windows;

This allows any Windex.Windows object to gain temporary exclusive access
to the Device_Context. All Paint operations in a window procedure use a
Windex.Graphics_Devices.Windows object to gain lock the Device_Context
for drawing.

Hope this helps.

-- Stephe