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Date: Thu, 30 Sep 2010 16:13:40 +0100
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Subject: Re: [PHP-DEV] RFC: Comparable interface
From: glopes@nebm.ist.utl.pt ("Gustavo Lopes")

On Thu, 30 Sep 2010 14:33:05 +0100, Adam Harvey <aharvey@php.net> wrote:=


> I've just written an RFC (with a patch against trunk) to implement a
> Comparable interface similar to that in Java =E2=80=94 in effect, allo=
wing
> object instances to be compared with semantics defined in userspace.
> This is admittedly at the lower end of RFC-worthy proposals, but it's
> a good system, and I'd like to see it used a little more. Plus, it's
> good practice for the more interesting stuff to come. :)
>
> The RFC is at http://wiki.php.net/rfc/comparable, and the patch at
> http://www.adamharvey.name/patches/comparable.diff.txt. Any and all
> feedback welcome, particularly since I'm still learning my way around
> the innards of the Zend Engine, and it's entirely possible (indeed,
> likely) that I've overlooked something obvious.
>
> If there's no great resistance to the concept or implementation, I'm
> happy enough to flip this into the voting stage sooner rather than
> later so we can get it out of the way.
>

I think this would be a good idea if limited to provide a userspace  =

alternative to compare_objects, like Countable provides a userspace  =

alternative to count_elements.

That is, you would only modify zend_std_compare_objects (not  =

compare_function!) and add there the additional logic where you'd check =
if  =

the objects had the same class (or perhaps one was of a subclass of the =
 =

other, in which case you'd use the compare method of the subclass) and  =

call the compare method.

Allowing comparison of objects with non-objects can become very confusin=
g:

* What if the objects are not of the same class? What if one is of a  =

subclass of the other? Is equality not symmetric?
* How do you interact with 'get' and 'cast_object'/'__tostring'?

It's true that there's already overloading of '=3D=3D' for internal clas=
ses,  =

but it's very predictable: both operands must have the same  =

'compare_objects', otherwise it's not used.

For the record, the current rules are relatively simple:

* If both are objects and have the same 'compare_objects' handler, use i=
t.
* If the first operand is an object and has a 'get' (tried first) or  =

'cast_object' handler call it, and repeat with the new value.
* Try the same for the second operand.

-- =

Gustavo Lopes