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iOS

Posted by Darcy Phillips
Darcy Phillips
4,087 Points

protocol extension question

Here's a multiple choice question:

protocol SomeProtocol {
    func someMethod() -> Int
}

extension SomeProtocol {
    func someMethod() -> Int {
        return 1
    }
}

struct SomeStruct: SomeProtocol {
    func someMethod() -> Int {
        return 2
    }
}

let b: SomeProtocol = SomeStruct()
b.someMethod()

What is the value returned from the someMethod() call

My answer, 1, is apparently wrong. Which I find confusing, cause I thought that this was the exact point of the lesson. Here's a quote from the video, with the type name changed to refer to this example:

"even though this is an instance of [SomeStruct], because the type is specified as the protocol type, it defaults to the implementation provided in the extension, and it ignores the implementation defined in the type."

By that logic the answer should be 1, the implementation in the extension. No?

3 Answers

Greg Kaleka
Greg Kaleka
39,021 Points
Greg Kaleka
Greg Kaleka
39,021 Points

Hi Darcy,

This is a bit tricky, but here's what's happening:

  • The protocol SomeProtocol requires someMethod().
  • The extension provides a default implementation for someMethod(), which means when conforming to SomeProtocol, you don't need to implement someMethod() if you want to simply accept this default implementation
  • Instead, though, when declaring SomeStruct, the default implementation was overridden. If the default implementation is overridden, that behavior is what you'll get.

I'm not sure what the quote is referencing - it will depend a bit on what the code was. Hopefully the above makes sense, though!

Cheers :beers:

-Greg

Darcy Phillips
Darcy Phillips
4,087 Points
Darcy Phillips
Darcy Phillips
4,087 Points

Hi Greg, thanks for that- yes, I understand those points.

This multiple choice question goes one step further though (i think?), asking which implementation will be chosen when the variable is specified as the protocol type. The quote is from the video preceding this quiz, which I transcribed.

Shouldn't I properly conclude that, in this case, the protocol's default implementation will be chosen, because "b" is specified as being of type "SomeProtocol"?

Thank you!

Greg Kaleka
Greg Kaleka
39,021 Points

Can you link me to the video in question?

Darcy Phillips
Darcy Phillips
4,087 Points
Darcy Phillips
Darcy Phillips
4,087 Points

The video is "Method Dispatch in a Protocol Extension", in the "Extensions and Protocols" section of the "Intermediate Swift 2" course. The quote is at 5:06 in video.

Thanks!