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Date: Thu, 11 Aug 2011 13:16:32 +0200
From: Luc Maisonobe <Luc.Maisonobe@free.fr>
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To: Commons Developers List <dev@commons.apache.org>
Subject: Re: [math] Read-only RealVector
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Le 11/08/2011 12:24, S�bastien Brisard a �crit :
>>
>> Well, in fact I would very much like to have immutable vectors too.
>> Immutability is really a way to simplify implementations. Surprisingly it
>> sometimes also decrease time and memory consumption, because defensive
>> copies littering user code can be avoided.
>>
> Luc, I have a silly question. Why do you think immutable vectors would
> prevent defensive copy ? Creating an immutable vector from an existing
> mutable vector would require a defensive copy, wouldn't it?

Yes, of course. What I meant is that when users don't have the choice 
and are forced to use only mutable objects, then in many places they do 
defensive copying, just because they do not even know if some other code 
will attempt to modify the object in place after they have retrieved it. 
So you end up with "B b = a.getB().clone()" to protect the B instance.

When instances are known to be immutable, you don't copy them (and you 
often neither implement Cloneable nor set up copy constructors).

> Thats the reason why I'm talking about read-only vectors, and not
> immutable vectors.
> The solver would keep a reference to the underlying (mutable) vector,
> so would be able to modify (indirectly) the read-only vector.
> The observer would only have a reference to the read-only vector, so
> would not be able to modify the underlying vector.

The problem with this approach is that the observer could not preserve 
the instance across several calls (for example to compare the instance 
from one previous call to the current one). If a specific observer wants 
to do that, then it needs to be able to copy the instance. If the 
observer knows the object is immutable, it will only preserve the reference.

Luc

>
> S�bastien
>
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