[swift-users] object.self?

[Rick Mann]

> On Jul 8, 2016, at 09:45 , Austin Zheng <austinzheng at gmail.com> wrote:
> 
> 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.

Thanks, Austin. I'm familiar with all this in Swift. What threw me was that "subclassObject" was an instance, not a class.

> 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.

I would have thought so, but the response to this answer was something along the lines of "I never know when to use the object or its self."

To me, for an instance, foo an foo.self should be equivalent in all respects (shouldn't it?).

>  
> 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
> 
> 
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> https://lists.swift.org/mailman/listinfo/swift-users
> 


-- 
Rick Mann
rmann at latencyzero.com