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Date: Sat, 23 Nov 2024 15:32:32 +0000
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Subject: Re: [PHP-DEV] [RFC] Data Classes
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")

On 23/11/2024 15:02, Eugene Sidelnyk wrote:
> If I remember correctly, the whole concept of "value" is fully 
> described in DDD book by Eric Evans. If that's the point of the RFC, I 
> wonder of there's any point in not making such classes immutable by 
> default, and to keep only one instance of value object unique per 
> given set of properties in memory, thereby eliminating cloning 
> altogether and optimizing the memory usage.


As I mentioned on the "Records" thread, guaranteeing that every 
combination of values will exist exactly once in memory could create 
more overhead than it saves.

Certainly if you write this, sharing memory makes a lot of sense:

$arr = []; $i = 0;
while ( $i++ < 100 ) {
    $arr[] = new Point(0,0);
}

But if you instead write this, maintaining the cache will end up more 
expensive than just allocating each object/record/struct directly:

$arr = []; $i = 0;
while ( $i++ < 100 ) {
    $arr[] = new Point($i, $i);
}

If the guarantee is copy-on-write, caching could be a compile-time 
optimisation; e.g. an OpCache pass might rewrite the first loop to the 
equivalent of this:

$arr = []; $i = 0;
$__cachedPoint = new Point(0,0);
while ( $i++ < 100 ) {
    $arr[] = $__cachedPoint;
}


The main thing that would prevent this optimisation is a custom 
constructor which might make the number of "new" calls observable. 
Either the optimiser would have to detect a custom constructor, or data 
classes / structs / records would have to prohibit defining one.


-- 
Rowan Tommins
[IMSoP]