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From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: The python implementation of the "relationships between classes".
Date: Thu, 10 Nov 2011 14:38:58 -0500
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On 11/10/2011 9:31 AM, Jerry Zhang wrote:

>     Unfortunately there is a difference between composition and
>     aggregation in my real word, and my application really care this
>     since it is trying to simulate this real world model, so my system
>     should track this difference accurately, otherwise the system may
>     not work well.
>
>     For example,
>     a. the Cls_arm and Cls_body may be composition, but not aggregation.
>     My app must ensure that " one arm instance only live with one body
>     instance, if the body instance die, the arm instance must die.

Create the arm as a private member '_arm' of body and make sure that no 
method of body passes out a reference to the arm. (In Python, outside 
code can still grab a reference to the private attribute, but that is a 
coding bug.)

I will point out that in the real world, dead donor transplants are 
based on the fact the parts of the body do NOT have to die when the 
composition does. I will not be surprised if we someday see arm transplants.

-- 
Terry Jan Reedy