CODEWITHSUNDEEP
swiftswift-protocolsdynamic-dispatchmember-hidingswift-modules
Sweeper
Sweeper

Reputation: 273860

Redeclaring members in an extension hides the original member *sometimes*. Why?

By chance, I discovered that you can do this without the compiler complaining:

extension Date {
    var timeIntervalSinceNow: TimeInterval {
        return 1000
    }
}

What's weirder, is that this actually evaluates to 1000:

Date().timeIntervalSinceNow

So I tried to do this with my own class:

class A {
    var a: String {
        return "A"
    }
}

extension A {
    var a: String {
        return "a"
    }
}

I observed that this does not affect the usage of the original member through a protocol, which is expected behaviour of hiding:

extension Date {
    var description: String {
        return "XXXX"
    }
}

let date: CustomStringConvertible = Date()
date.description // normal date

Date().description // "XXXX"

Can you explain why does the bullet pointed phenomena occur?

Upvotes: 7

Views: 537

Answers (1)

Infinity James
Infinity James

Reputation: 4735

This works because you are declaring this extension in a separate module from the original variable declaration.

Across modules a variable name can be overloaded but in my mind this has been a shortcoming of Swift as there is currently no way to explicitly state which module declaration it is that you want.

Upvotes: 3

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