Swift class/struct implicitly conforms to NSObjectProtocol?

Question

A while ago I faced strange Swift issue. When I try to cast Swift object to NSObjectProtocol in debug - it executes successfully. But when this code executes in AppStore build, it casts to nil.

import Foundation

final class MyClass {
    let testP: String = "123"
}

struct MyStruct {
    let testP: String = "123"
}

let myClass = MyClass()
let myStruct = MyStruct()

print(myClass) // >> __lldb_expr_1.MyClass
print(myClass as! NSObjectProtocol) // >> __lldb_expr_1.MyClass
print(myStruct as! NSObjectProtocol) // >> __lldb_expr_3.MyStruct(testP: "123")

When I cast struct to NSObjectProtocol I get such warning:

Cast from 'MyStruct' to unrelated type 'NSObjectProtocol' always fails

However, how you can see it successfully prints my struct.

So, the question is: is it bug or feature of Swift?)

PS: Pardon my French

https://forums.swift.org https://bugs.swift.org/secure/Dashboard.jspa there is some unresolved issues — Vyacheslav, Apr 17 '19 at 12:53

Agisight · Answer 1 · 2019-04-17T13:43:01.167

0

NSObjectProtocol has some methods for AnyClass and you need to conform with protocol's methods.

What you can read in offical Apple documentation

An object that conforms to this protocol can be considered a first-class object. Such an object can be asked about its:

Class, and the place of its class in the inheritance hierarchy.

Conformance to protocols.

Ability to respond to a particular message.

See in an image a methods to conform any object with NSObjectProtocol.

Some other helpful information for you here: https://medium.com/a-swift-misadventure/why-swift-protocol-conforming-values-are-treated-as-value-types-by-default-9482c6809583

edited Apr 17 '19 at 13:43

answered Apr 17 '19 at 13:04

Agisight

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I understand why I've got warning, the question is: why is this struct successfully casts? despite warning... – Dmitriy Stupivtsev Apr 17 '19 at 13:07
By the way, NSObjectProtocol is not marked as class or AnyObject, so any struct may conform it. – Dmitriy Stupivtsev Apr 17 '19 at 13:10
Ok, it's just contract, so I can create struct, which will conform this protocol and handle these properties and methods as I wish – Dmitriy Stupivtsev Apr 17 '19 at 13:23
Show me you code with struct that conforms to that protocol – Agisight Apr 17 '19 at 13:24
Sorry, but this is not an answer for my question) – Dmitriy Stupivtsev Apr 17 '19 at 13:24
Oh, my bad, but it strange, because in documentation NSObjectProtocol is not marked as class or AnyObject – Dmitriy Stupivtsev Apr 17 '19 at 13:26
For both struct and class i catch an error like this: `Thread 1: EXC_BREAKPOINT (code=1, subcode=0x102caeb90)`. – Agisight Apr 17 '19 at 13:30
To call `print(myClass as! NSObjectProtocol)` without errors you have to set `final class MyClass : NSObject {let testP: String = "123"}` – Agisight Apr 17 '19 at 13:31
I know how it should be, but I have no error with this code Swift 5.0 XCode 10.2 – Dmitriy Stupivtsev Apr 17 '19 at 13:32
My Xcode – Version 10.2 (10E125). And runtime crashes with your code – Agisight Apr 17 '19 at 13:34
Maybe you have earlier version of Swift? I run playground and separate iOS project with Swift 5 and execute this code with only one warning and without runtime crashes – Dmitriy Stupivtsev Apr 17 '19 at 13:38

score 0 · Accepted Answer · answered May 02 '19 at 08:27

0

Apparently, it’s both bug and feature. https://bugs.swift.org/browse/SR-10495

answered May 02 '19 at 08:27

Dmitriy Stupivtsev

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Swift class/struct implicitly conforms to NSObjectProtocol?

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