[Openmcl-devel] CLOS make-instance :around behaviour

[Bob Hutchison]

On 19-Oct-08, at 4:46 PM, Pascal Costanza wrote:

>
> On 19 Oct 2008, at 22:40, Bob Hutchison wrote:
>> That example is simplified. What I'm trying to do is implement a  
>> cache like thing. If someone calls, say, (make-instance 'thing :id  
>> "abc") then if there is already a thing with id "abc" return it  
>> rather than a new instance.
>>
>> As far as I know there's no way to change the return value of the  
>> primary method just using an :after method.
>>
>> So... I suppose either I define my own metaclass (a new adventure  
>> for me :-) or stop using make-instance.
>>
>> Better ideas?
>
> Stop using make-instance is not a bad idea. You could define your  
> own create-instance function (or so), or shadow cl:make-instance in  
> your own package to replace it with your own version.

Ohhh... Shadowing is an interesting idea... I'll check into it. Though  
I suppose that it might be a little confusing.

> If the caching functionality is the only functionality you want to  
> add, the CLOS MOP may be too big a weapon for this purpose.

Only the caching. I already had everything else in the initialize- 
instance :after method.

>
>
> Are you sure that you need such caching? Garbage collectors are  
> nowadays pretty good at recycling short-lived objects, so that you  
> shouldn't need this for performance purposes - unless this is about  
> semantics and not performance...

It's semantics I'm afraid. A simple situation is where there is a  
cycle of objects -- only the id of the object is available to identify  
that an object exists twice.

Yes, I agree, the GC is really quite good... and I take full advantage  
of that elsewhere :-)

Cheers,
Bob

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> Pascal
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> -- 
> Lisp50: http://www.lisp50.org
>
> Pascal Costanza, mailto:pc at p-cos.net, http://p-cos.net
> Vrije Universiteit Brussel, Programming Technology Lab
> Pleinlaan 2, B-1050 Brussel, Belgium
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