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From: Martin Krischik <martin@krischik.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2005?
Date: Sat, 18 Dec 2004 09:51:48 +0100
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Date: 2004-12-18T09:51:48+01:00
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conradwt@runbox.com wrote:

> Hi, will Ada support keyword class for designing and implementing
> classes?  For example, I'm forced to convert the following C++ class in
> Ada as follows:
> 
> // C++
> 
> class A_Device {
> 
> public:
> 
> A_Device( char*, int, int );
> 
> char* Name( void );
> 
> int Major( void );
> 
> int Minor( void );
> 
> protected:
> 
> char* name;
> 
> int major;
> 
> int minor;
> };
> 
> // Now, if I need to interact with the, this class I can do the
> // following:
> 
> void main(void) {
> 
> A_Device aDevice = new A_Device( "Test", 1, 1 );
> 
> cout << aDevice.Name() << endl;
> cout << aDevice.Major() << endl;
> cout << aDevice.Minor() << endl;
> 
> }
> 
> // Ada
> 
> package Devices is
> 
> type Device is tagged private;
> 
> type Device_Type is access private;

You do to much pointer work! Ada does not need that much pointer work! Read:

http://en.wikibooks.org/wiki/Programming:Ada:OO#The_class-wide_type

> function Create( Name : String,
> Major : Integer,
> Minor : Integer ) return Device_Type;
> 
> function Name( this : Device_Type ) return String;

You did the equivalent of:

class Device
   {
   static char*
   Name(Device* this);
   }

A "static" class member will not dispatch! You should have done:

function Name( this : in Device) return String;

or

function Name( this : in out Device) return String;

Read:

http://en.wikibooks.org/wiki/Programming:Ada:OO#Primitive_Operations

> 
> function Major( this : Device_Type ) return Integer;
> 
> function Minor( this : Device_Type ) return Integer;
> 
> private
> 
> type Device is tagged
> 
> record
> 
> name : String(1..20);
> major: Integer;
> minor: Integer;
> 
> end record;
> 
> end Devices;
> 
> Now, interact with Ada package I would need to do the following:
> 
> procedure main
> 
> aDevice : Device_Type := Devices.Create( "Test", 1, 1 );
> 
> begin
> 
> Put_Line( Name( aDevice ) );
> Put_Line( Major( aDevice ) );
> Put_line( Minor( aDevice ) );
> end main;
> 
> It seems that I'm trying to mimic the behavior of a OO language in a
> procedural language when converting C++ to Ada.  Is this correct?

No! Ada has OO - but you have not read enough tutorials to know how Ada's OO
works. Keep going and you get there!

> If 
> so, why doesn't Ada have OO contructs similar to C++,
> Java, Eiffel, and Smalltalk to name a few where one passes a message to
> an instance of a class?

Ada does have these features! The difference is that Ada is package
(namespace) centric and not record (struct) centric. In fact a Ada class
need to be defined inside a package (namespace)! 

It is not uncomme to declare a classes like this:

package Parent_Class
is

  type Object is tagged
     record
        .....
     end record;

end Class_Name;

with Parent_Class;

package Child_Class
is

  type Object
  is new  
     Parent_Class.Object
   with  record
        .....
   end record;

end Class_Name;


> Will this be something in Ada 2005 because I 
> have been able to find a good overview of the language to date?

Only interfaces are added to Ada 2005 - everything else is allready there.

With Rergards

Martin
-- 
mailto://krischik@users.sourceforge.net
http://www.ada.krischik.com