[swift-users] odd `==` `!=` behavior with class that inherits `NSObject`

[Zhao Xin]

Everyone should notice that in the do-block (Sample 2). The result is just
opposite of the code out of it (Sample 1). Or say, out the do-block, Swift
calls `==` directly. In the do-block, Swift just call `isEqual(_)` instead
of `==`.

That is the inconsistency I am confused.

Zhaoxin

On Wed, Apr 19, 2017 at 10:33 AM, Zhao Xin <owenzx at gmail.com> wrote:

> Sample 1: both `==` and `!=` is true.
>
> import Foundation
>
>
> class Foo:NSObject {
>
>     let name:String
>
>
>     init(name:String) {
>
>         self.name = name
>
>     }
>
>
>     public static func ==(lhs: Foo, rhs: Foo) -> Bool {
>
>         guard type(of:lhs) == type(of:rhs) else { return false }
>
>         return lhs.name == rhs.name
>
>     }
>
> }
>
>
> let a = Foo(name: "bar")
>
> let b = Foo(name: "bar")
>
>
> print(a == b) // true
>
> print(a != b) // true
>
> Sample 2: Add above code to a do-block, behavior changes to expect
>
> do {
>
>     class Foo:NSObject {
>
>         let name:String
>
>
>
>         init(name:String) {
>
>             self.name = name
>
>         }
>
>
>
>         public static func ==(lhs: Foo, rhs: Foo) -> Bool {
>
>             guard type(of:lhs) == type(of:rhs) else { return false }
>
>             return lhs.name == rhs.name
>
>         }
>
>     }
>
>
>
>     let a = Foo(name: "bar")
>
>     let b = Foo(name: "bar")
>
>
>
>     print(a == b) // false
>
>     print(a != b) // true
>
> }
>
> Sample 3: A little investigation shows that `==` didn't call NSObject's `
> func isEqual(_ object: Any?) -> Bool` but `!=` did.
>
> class Foo:NSObject {
>
>     let name:String
>
>
>
>     init(name:String) {
>
>         self.name = name
>
>     }
>
>
>
>     public static func ==(lhs: Foo, rhs: Foo) -> Bool {
>
>         guard type(of:lhs) == type(of:rhs) else { return false }
>
>         return lhs.name == rhs.name
>
>     }
>
>
>
>     override func isEqual(to object: Any?) -> Bool {
>
>         print("111")
>
>         return super.isEqual(to: object)
>
>     }
>
>
>
>     override func isEqual(_ object: Any?) -> Bool {
>
>         print("2222")
>
>         return super.isEqual(object)
>
>     }
>
> }
>
>
> let a = Foo(name: "bar")
>
> let b = Foo(name: "bar")
>
>
> print(a == b) // true
>
> print(a != b) // 2222, true
>
> print(!(a == b)) // false
>
>
> So I am wondering what is the future? Will we keep on using `isEqual(_
>  object: Any?)` with class that inherits `NSObject`, or we are trying to
> drop it?
>
> Xcode  8.3.1 (8E1000a), 3.1 (swiftlang-802.0.51 clang-802.0.41), macOS 10.12.4
> (16E195)
>
> Zhaoxin
>
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