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Date: Thu, 14 Nov 2013 20:37:06 -0500
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To: Chris London <me@chrislondon.co>
Subject: Re: [PHP-DEV] Proposal: Type Casting User Classes
From: will@fitch.so (William Fitch)


On Nov 14, 2013, at 4:13 PM, Chris London <me@chrislondon.co> wrote:

> On Thu, Nov 14, 2013 at 12:20 PM, Pierre du Plessis
> <pierre@pcservice.co.za>wrote:
>=20
>>=20
>> Do you have any use cases why this would be useful?
>>=20
>=20
> Basically the reason I want it is to make sure the function I just =
called
> returned the object I expected. Kind of like how we can specify what =
object
> we're expecting for parameters like this:
>=20
> function foo (User $user) {
>    // do stuff here
> }
>=20
> I want to be able to make sure the object I have is the right type. In =
this
> case, I wouldn't want it to actually change the object type but =
instead
> through an exception I could catch. (obviously, there are ways to do =
that
> now by writing my own functions and throwing my own exceptions). The
> company I'm working for right now uses Zend Framework 2 and in Zend
> Framework 2 they have factories which could return just about anything =
and
> I want to make sure I got the right object.

I=92m confused - on one hand, you=92re referring to the already existing =
type hinting of parameters. On the other, you=92re making the case for =
instanceof. =20

if ($someZendObject instanceof What\I\Expect)

This really isn=92t a use case for object type casting. =20

>=20
> Another minor use case would be that the IDE would be able to do
> function/parameter hints.  (I do know some IDEs will let you do /* =
@var
> $foo User */ )

If you=92re using a relatively modern IDE and using parameter type =
hinting, this already exists.

>=20
> Another situation we ran across is we're using Propel and we would =
pull
> database records that Propel turns into User objects. If some of those
> users are admins then we would want them to be AdminUser objects. =
Obviously
> you could write work arounds like :
>=20
> $admin =3D AdminUser::createFromUser($user);
>=20
> or
>=20
> $admin =3D new AdminUser($user);
>=20
> but if AdminUser extends User I could see how type casting could be
> appropriate.

There are two types of object type casts: upcast and downcast.  Upcast =
is when you have a class that refers to an object that inherits from a =
root class.  Downcast is casting an object from the root towards a child =
or subclass.  What you=92re referring to is downcasting. (User -> =
AdminUser).  This is the only valid use case provided thus far, but even =
then, there are other methodological approaches that can be taken to =
mitigate this scenario.

>=20
> Also what would happen if you cast objects to invalid types?
>>=20
> E.G if you convert a person entity to a product entity, which doesn't =
have
>> the same properties or methods?
>>=20
>=20
> I imagine throwing an exception would be appropriate
>=20
> Thanks!
> Chris London