CODEWITHSUNDEEP
swiftdynamictypesswift2xcode7
BaseZen
BaseZen

Reputation: 8718

Apple's own Swift documentation seems wrong about dynamicType vs compile-time type

Referencing:

https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Types.html

We have:

class SomeBaseClass {
    class func printClassName() {
        print("SomeBaseClass")
    }
}
class SomeSubClass: SomeBaseClass {
    override class func printClassName() {
        print("SomeSubClass")
    }
}
let someInstance: SomeBaseClass = SomeSubClass()
// The compile-time type of someInstance is SomeBaseClass,
// and the runtime type of someInstance is SomeBaseClass
someInstance.dynamicType.printClassName()
// prints "SomeSubClass"

Use the identity operators (=== and !==) to test whether an instance’s runtime type is the same as its compile-time type.

if someInstance.dynamicType === someInstance.self {
    print("The dynamic type of someInstance is SomeBaseCass") 
} 
else {
    print("The dynamic type of someInstance isn't SomeBaseClass") 
}

but someInstance.self appears to refer to the object, not the object's compile-time type, as claimed by the documentation. In fact in Xcode 7.2, the test does not evaluate to true even when we initialize someInstance as SomeBaseClass.

It does not inspire confidence that the docs have a typo (SomeBaseCass).

The only way I can find to make the "true" clause fire is the mutation:

if someInstance.dynamicType == SomeBaseClass.self { ... }

which is interesting but misses the fully dynamic run-time type-checking capability that the flawed Apple docs are trying to show.

Who's wrong and how to resolve?

Upvotes: 0

Views: 127

Answers (2)

andy Xu
andy Xu

Reputation: 11

i have doubts about

let someInstance: SomeBaseClass = SomeSubClass()
// The compile-time type of someInstance is SomeBaseClass,
// and the runtime type of someInstance is SomeBaseClass
someInstance.dynamicType.printClassName()
// prints "SomeSubClass"

and the runtime type of someInstance should BE [SomeSubClass],

Upvotes: 0

piskov
piskov

Reputation: 46

It’s a typo — kudos to you. That section is about Metatype type and they use type_name.self throughout the section except that very line.

Other than that dynamicType works as advertised — nothing wrong here.

Upvotes: 1

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