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Date: Wed, 10 Jul 2024 19:56:30 +0200
To: "Gina P. Banyard" <internals@gpb.moe>
Cc: internals@lists.php.net
Subject: Re: [PHP-DEV] [low priority] WeakMaps with scalar keys
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From: rob@bottled.codes ("Rob Landers")

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On Wed, Jul 10, 2024, at 19:48, Gina P. Banyard wrote:
> On Wednesday, 10 July 2024 at 18:34, Rob Landers <rob@bottled.codes> w=
rote:
>> Hello internals,
>>=20
>> I know now that some of you are swamped getting ready for 8.4, so I w=
ant to be clear that this will be an 8.5+ thing. I am simply sending it =
to the list in case anyone wants to discuss it, burn it with fire, give =
it a gold star, or ignore until later.
>>=20
>> Currently, a WeakMap may only have object keys, and this logically ma=
kes a lot of sense. However, there are cases where you want to reference=
 an object by a value but if the value is no longer referenced, no longe=
r need it. A good example of this might be used in dependency injection =
containers, lookup tables for value objects, etc.
>>=20
>> I'd like to propose a ReverseWeakMap where the keys must be scalar, a=
nd given an object as the value. Once the value is no longer referenced,=
 the key and value is removed from the ReverseWeakMap.
>=20
> Isn't this use case already solved with WeakReferences?
> https://www.php.net/manual/en/class.weakreference.php
>=20
>=20
> Best regards,
>=20
> Gina P. Banyard
>=20
>>=20

Hey Gina,

The answer is: it depends. If you don=E2=80=99t need the array to clean =
up after itself, you can indeed use an array of WeakReference to get mos=
t of the way there. If you want it to clean up after an object gets remo=
ved, you either need to add support to the stored object=E2=80=99s destr=
uctor (which isn=E2=80=99t always possible for built-in or final types),=
 or create your own garbage collector that scans the array.=20

Now that I think about it, it might be simpler to add an =E2=80=9ConRemo=
ve()=E2=80=9D method that takes a callback for the WeakReference class.=20

=E2=80=94 Rob
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<!DOCTYPE html><html><head><title></title><style type=3D"text/css">p.Mso=
Normal,p.MsoNoSpacing{margin:0}</style></head><body><div>On Wed, Jul 10,=
 2024, at 19:48, Gina P. Banyard wrote:<br></div><blockquote type=3D"cit=
e" id=3D"qt" style=3D""><div class=3D"qt-protonmail_quote"><div>On Wedne=
sday, 10 July 2024 at 18:34, Rob Landers &lt;rob@bottled.codes&gt; wrote=
:<br></div><blockquote class=3D"qt-protonmail_quote" type=3D"cite"><div>=
Hello internals,<br></div><div><br></div><div>I know now that some of yo=
u are swamped getting ready for 8.4, so I want to be clear that this wil=
l be an 8.5+ thing. I am simply sending it to the list in case anyone wa=
nts to discuss it, burn it with fire, give it a gold star, or ignore unt=
il later.<br></div><div><br></div><div>Currently, a WeakMap may only hav=
e object keys, and this logically makes a lot of sense. However, there a=
re cases where you want to reference an object by a value but if the val=
ue is no longer referenced, no longer need it. A good example of this mi=
ght be used in dependency injection containers, lookup tables for value =
objects, etc.<br></div><div><br></div><div>I'd like to propose a Reverse=
WeakMap where the keys must be scalar, and given an object as the value.=
 Once the value is no longer referenced, the key and value is removed fr=
om the ReverseWeakMap.<br></div></blockquote><div style=3D"font-family:A=
rial, sans-serif;font-size:14px;color:rgb(0, 0, 0);background-color:rgb(=
255, 255, 255);"><br></div><div style=3D"font-family:Arial, sans-serif;f=
ont-size:14px;color:rgb(0, 0, 0);background-color:rgb(255, 255, 255);">I=
sn't this use case already solved with WeakReferences?<br></div><div sty=
le=3D"font-family:Arial, sans-serif;font-size:14px;color:rgb(0, 0, 0);ba=
ckground-color:rgb(255, 255, 255);"><span><a target=3D"_blank" rel=3D"no=
referrer nofollow noopener" href=3D"https://www.php.net/manual/en/class.=
weakreference.php" class=3D"">https://www.php.net/manual/en/class.weakre=
ference.php</a></span><br></div><div style=3D"font-family:Arial, sans-se=
rif;font-size:14px;color:rgb(0, 0, 0);background-color:rgb(255, 255, 255=
);"><div><span></span><br></div><div><br></div><div><div><div style=3D"b=
ackground-color:rgb(255, 255, 255);"><span class=3D"font" style=3D"font-=
family:Verdana, sans-serif;">Best regards,</span><br></div><div style=3D=
"background-color:rgb(255, 255, 255);"><br></div><div><span style=3D"bac=
kground-color:rgb(255, 255, 255);"><span class=3D"font" style=3D"font-fa=
mily:Verdana, sans-serif;">Gina P. Banyard</span></span><br></div></div>=
</div></div><div style=3D"font-family:Arial, sans-serif;font-size:14px;c=
olor:rgb(0, 0, 0);background-color:rgb(255, 255, 255);"><br></div><block=
quote class=3D"qt-protonmail_quote" type=3D"cite"><br></blockquote></div=
></blockquote><div><br></div><div>Hey Gina,<br></div><div><br></div><div=
>The answer is: it depends. If you don=E2=80=99t need the array to clean=
 up after itself, you can indeed use an array of WeakReference to get mo=
st of the way there. If you want it to clean up after an object gets rem=
oved, you either need to add support to the stored object=E2=80=99s dest=
ructor (which isn=E2=80=99t always possible for built-in or final types)=
, or create your own garbage collector that scans the array.&nbsp;<br></=
div><div><br></div><div>Now that I think about it, it might be simpler t=
o add an =E2=80=9ConRemove()=E2=80=9D method that takes a callback for t=
he WeakReference class.&nbsp;</div><div><br></div><div id=3D"sig12122915=
2">=E2=80=94 Rob<br></div></body></html>
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