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From: Maciej Sobczak <see.my.homepage@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada Interfaces and the Liskov Substitution Principle
Date: 25 May 2007 00:52:21 -0700
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On 25 Maj, 00:58, "Randy Brukardt" <r...@rrsoftware.com> wrote:

> > I was talking about real assignment - and this assumes that two
> > objects already exist and we make one to be equal to the other. There
> > is no way to do it right.
>
> (Using the *correct* terminology as Bob noted)

Yes, thank you both for explaining it - I was using the C++
terminology.

> 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.
The one above means "remake me, please", but it's applicability must
be greatly limited - you neatly write <some details of Y>, but it
should be really <some details of whatever-Y-happens-to-be which we
don't know here>.
That complicates this scheme a bit.

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. :-)

--
Maciej Sobczak
http://www.msobczak.com/