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Date: Sat, 6 Apr 2024 15:09:59 +0200
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Subject: Re: [PHP-DEV] [RFC] [Discussion] Support object type in BCMath
To: Jordan LeDoux <jordan.ledoux@gmail.com>
Cc: Saki Takamachi <saki@sakiot.com>, Lynn <kjarli@gmail.com>,
 Aleksander Machniak <alec@alec.pl>, php internals <internals@lists.php.net>
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From: tim@bastelstu.be (=?UTF-8?Q?Tim_D=C3=BCsterhus?=)

Hi

On 4/5/24 22:04, Jordan LeDoux wrote:
> That is an absurd example. Why would anyone use inheritance for that class
> design? If what you are arguing is "if you look at use cases where
> composition is clearly the correct choice then inheritance causes
> problems", then I'm not sure what the point of the discussion is.
> 

Okay, then please tell me in which use cases inheritance rather than 
composition is the right choice? For the "Money" example, I've already 
showcased how it would be broken and I can't think of any example where 
inheritance would be superior to composition.

Yes, my example is dumb, but nevertheless it showcased that the native 
and unchangeable operations would be unsound for child classes with 
custom properties, because the operations would not be able to correctly 
fill in the custom properties.

And even without custom properties, the behavior would be unsound 
because the class name acts as an implicit unit:

     $twoMeters = new Distance(2);

     $fourSquareMeters = $twoMeters * $twoMeters;

Multiplying two distances does not result in a distance, but rather in 
an area. Making it a Distance(4) would be incorrect.

So, which use case would be enabled by allowing inheritance without 
being subtly broken by being unable to override the native operations?

As I've said yesterday in Stack Overflow chat: "Operator overloads 
through a backdoor are even worse than actual operator overloads".

Best regards
Tim Düsterhus