[swift-users] object.self?

[Austin Zheng]

Hi Rick,

If you have a type (let's call it "T"), you can use it two ways:

* As a type, or part of a type, like such: "let x : T = blah()"
* As a value, just like any other variable, function argument, property,
etc.

In the second case (type-as-value), you need to append ".self" to the type
name according to the grammar:

"let x : Any.Type = T.self"

There was a "bug" in Swift 2.x where you could sometimes use just "T",
without the ".self", in certain cases (in particular, when you were passing
in a type-as-value to a function with one unlabeled argument). That bug has
since been fixed.

As for types-as-values: Swift allows you to treat a type as a normal value,
which means you can do whatever you want with it: pass it to functions and
return it from functions, store it in properties or variables, etc. If you
have one of these types-as-values (called 'metatypes'), you can do certain
things like call static methods or initializers on them, use them to
parameterize generic functions, etc.

However, to get back to your original question, the `.self` in that switch
statement actually isn't necessary and you should really just be switching
on the value of subclassObject itself, not the value of its type.

Best,
Austin


On Fri, Jul 8, 2016 at 9:38 AM, Rick Mann via swift-users <
swift-users at swift.org> wrote:

> I just saw a question which brought up something I didn't know about.
> Apparently sometimes you have to call object.self in a place that looks
> like you should just use "object." What does this usage mean?
>
> for subclassObject in objects {
>     switch subclassObject.self {        <--- Here, why not
> "subclassObject" alone?
>     case is Subclass1:
>         doSomethingWith(subclassObject as! Subclass1)
>
>     case is Subclass2:
>         doSomethingWith(subclassObject as! Subclass2)
>
>     case is Subclass3:
>         doSomethingWith(subclassObject as! Subclass3)
>
>     default:
>         break
>     }
> }
>
> Thanks,
> Rick
>
> > On Jul 8, 2016, at 08:15 , Dan Loewenherz via swift-users <
> swift-users at swift.org> wrote:
> >
> > To my knowledge, you can’t do exactly what you’re trying to do, but this
> is close:
> >
> > for subclassObject in objects {
> >     switch subclassObject.self {
> >     case is Subclass1:
> >         doSomethingWith(subclassObject as! Subclass1)
> >
> >     case is Subclass2:
> >         doSomethingWith(subclassObject as! Subclass2)
> >
> >     case is Subclass3:
> >         doSomethingWith(subclassObject as! Subclass3)
> >
> >     default:
> >         break
> >     }
> > }
> >
> > On Fri, Jul 8, 2016 at 10:11 AM, Nate Birkholz via swift-users <
> swift-users at swift.org> wrote:
> > This looks like it doesn't work (swift 2.x), but wanted to be sure it's
> not supported:
> > class Superclass {}
> > class Subclass1 : Superclass {}
> > class Subclass2 : Superclass {}
> > class Subclass3 : Superclass {}
> >
> > let sc1 = Subclass1()
> > let sc2 = Subclass2()
> > let sc3 = Subclass3()
> >
> > let objects : [Superclass] = [sc1, sc2, sc3]
> >
> > for subclassObject in objects {
> >     switch subclassObject {
> >     case let object = subclassObject as? Subclass1:
> >         doSomethingWith(object)
> >     case let object = subclassObject as? Subclass2:
> >         doSomethingWith(object)
> >     case let object = subclassObject as? Subclass3:
> >         doSomethingWith(object)
> >     default:
> >         return
> >     }
> > }
> >
> > This gives an error, expecting a colon (:) after object on every case.
> >
> > I wanted to be sure I wasn't missing something in my syntax (nor some
> obvious-to-others reason this isn't supported) before going to swift
> evolution.
> >
> >
> > --
> > Nate Birkholz
> >
> > _______________________________________________
> > swift-users mailing list
> > swift-users at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-users
> >
> >
> > _______________________________________________
> > swift-users mailing list
> > swift-users at swift.org
> > https://lists.swift.org/mailman/listinfo/swift-users
>
>
> --
> Rick Mann
> rmann at latencyzero.com
>
>
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>
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