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From: dugal@neon.Stanford.EDU (Douglas S. Gray)
Newsgroups: comp.lang.ada
Subject: Re: Difference between inheritance and package use
Keywords: Inheritance, Object Orientation
Message-ID: <1991Jun24.193341.27161@neon.Stanford.EDU>
Date: 24 Jun 91 19:33:41 GMT
References: <1991Jun21.224640.24583@src.honeywell.com>
Distribution: usa
Organization: Computer Science Department, Stanford University, Ca , USA
List-Id: <comp.lang.ada>

In article <1991Jun21.224640.24583@src.honeywell.com> msmiller@aconcagua.src.honeywell.com (Mike Miller) writes:
>
>How does (or more properly, how will) the upcoming addition of
>inheritance and object oriented stuff (for lack of a better word at
>the moment) differ from the definition of something in a package and
>its use later?

   Although there have been a number of good answers covering inheritance 
(notably, Jim Showalter's), I haven't seen one which addressed one very
basic difference between a C++ object and an Ada package.  The object is a
first-class programming construct, where the package is not.  The
encapsulation provided by a package is not as powerful as that in an
object.  You can create an array of objects or pass an object to a
subprogram.  Packages are static where objects are dynamic.

   From what I've read in the Ada 9X reports, it appears that there is no
plan to enhance the power of packages.  Instead, inheritance will be added
to types*.  Does anyone have more complete information than that?

Doug Gray
dgray@sadis02.sa.aflc.af.mil
dugal@cs.stanford.edu

* From Draft Mapping Document (Feb 1991) section 3.4(Derived Types) and
section 7(Ada 9X Packages).  Also, see Draft Mapping Rationale Document
(Feb 1991) section 3(Data Abstraction and Object-Oriented Programming)