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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Ada Interfaces and the Liskov Substitution Principle
Newsgroups: comp.lang.ada
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Date: Fri, 25 May 2007 10:21:24 +0200
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Date: 2007-05-25T10:19:17+02:00
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On 25 May 2007 00:52:21 -0700, Maciej Sobczak wrote:

> On 25 Maj, 00:58, "Randy Brukardt" <r...@rrsoftware.com> wrote:
> 
>> Given the current Ada rules, what classwide assignment is good for is
>> *re*construction:
>>
>>      procedure Do_Something(Y : out Object'Class) is
>>      begin
>>          Y :=  Constructor_Func (<some details of Y>);
>>      end Do_Something;
>>
>> that is, the replacement of an object with different one of the same type.
>> The Constructor_Func is dispatching on the real type of the LHS object
> 
> Interesting. In most of the cases, the factory function creates the
> object of whatever type *it* decides (based on its parameters or some
> other input values) and initializes the class-wide type accordingly.

Not necessarily. In one case, I generate nodes of a graph. The nodes can be
of different types regarding their persistence. For example, there are
nodes resident in the database and nodes resident in the memory etc. Now,
when creating a new node a factory moves along two axes: the standard
root/branch/leaf hierarchy and persistence stuff. For the latter the
factory receives an already existing node, to create "a new node like
this." This is not copying. Ideally it should double dispatching along both
axes. I have implemented it as:

   New_Node := Create (Get_Factory (Some_Existing_Node), ...);

> There is a very simple rule of thumb for checking whether any given OO
> pattern makes sense - try to stick it to the classical geometry
> example.
> If it falls off, beware. :-)

You certainly mean the circle-ellipse LSP controversy... (:-))

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de