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[Caml-list] Delegation based OO
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Date: 2004-03-24 (10:43)
From: Alex Baretta <alex@b...>
Subject: Re: [Caml-list] Delegation based OO
Kenneth Knowles wrote:
> On Wed, Mar 24, 2004 at 08:43:00AM +0900, Yamagata Yoriyuki wrote:

> Good point, I think.
> Let's see if I understand; I think you mean the following imaginary situation:
> # class type delegate :
>   object
>     method b : int
>   end;;
> # class x =
>   object(self)
>     method a = 3
>     method b = self # a
>   end;;
> # class y (del : delegate) =
>   object
>     delegate del
>     method a = "hello"
>   end;;

Delegation is no relative of inheritance. Inheritance is a syntactic 
property (essentially classes inherit code), whereas delegation is a 
semantic property: once one method is delegated by a proxy object to an 
executor object, the the visibility of the proxy is lost. The method is 
executed as if it were invoked directly on the executor.

As far as I'm concerned delegation serves specifically to implement 
efficiently (loc-wise) the proxy object pattern. Consider a DB access 
library where you define classes to access different DBMSs. The proxy 
object is backend neutral and dispatches depending on a configuration 
string or equivalent data structure. This can be done now, but each 
method in the proxy must be delegated explicitly to the backend object. 
Delegation passes control from the proxy to the backend in one line of code.


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