Question about SE-0057 "Importing Objective-C Lightweight Generics"

Michael_Peternell · 2016-04-14T21:06:15.000Z

apple/swift-evolution/blob/master/proposals/0057-importing-objc-generics.md

# Importing Objective-C Lightweight Generics

* Proposal: [SE-0057](0057-importing-objc-generics.md)
* Author: [Doug Gregor](https://github.com/DougGregor)
* Review Manager: [Chris Lattner](https://github.com/lattner)
* Status: **Implemented (Swift 3)**
* Decision Notes: [Rationale](https://lists.swift.org/pipermail/swift-evolution-announce/2016-April/000097.html)
* Previous Revision: [Originally Accepted Proposal](https://github.com/apple/swift-evolution/blob/3abbed3edd12dd21061181993df7952665d660dd/proposals/0057-importing-objc-generics.md)


## Introduction

Objective-C's *lightweight generics* feature allows Objective-C
classes to be parameterized on the types they work with, similarly to
Swift's generics syntax. Their adoption in Foundation's collection
classes allow Objective-C APIs to be bridged more effectively into
Swift. For example, an `NSArray<NSString *> *` bridges to `[String]`
rather than the far-weaker `[AnyObject]`. However, parameterized
Objective-C classes lose their type parameters when they are imported
into Swift, so uses of type parameters outside of bridged, typed

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How can Swift know if an Objective-C class implements +/- classForGenericArgumentAtIndex: at compile time? Does it look at the object code? Or does it look at the header file? Or maybe there is a third option?

I'm asking, because this may lead to misunderstandings if the method needs to be there in the header file, but Objective-C programmers are not aware that they have to declare that method.

-Michael

Joe_Groff · 2016-04-14T21:56:58.000Z

It needs to be present in the header at compile time.

-Joe

···

On Apr 14, 2016, at 2:06 PM, Michael Peternell via swift-evolution <swift-evolution@swift.org> wrote:

https://github.com/apple/swift-evolution/blob/master/proposals/0057-importing-objc-generics.md

How can Swift know if an Objective-C class implements +/- classForGenericArgumentAtIndex: at compile time? Does it look at the object code? Or does it look at the header file? Or maybe there is a third option?

I'm asking, because this may lead to misunderstandings if the method needs to be there in the header file, but Objective-C programmers are not aware that they have to declare that method.