How does Swift determine `AnyClass` equality?

nmgswift · July 17, 2024, 9:51pm

I've been playing around, trying to understand the internals of the Swift runtime, and I am curious to understand how it determines the equality of AnyClass variables. I see that AnyClass is a type alias for AnyObject.Type, so perhaps this would apply to all metatypes.

Really, I'm just looking for the section of code in the Swift runtime that would handle the equality check on line 3 in the following code:

let foo: AnyClass = someFuncReturningAnyClass()
let bar: AnyClass = someOtherFuncReturningAnyClass()
let equal = foo == equal

John_McCall · July 17, 2024, 11:03pm

It's this operator, which works on all types:

github.com

swiftlang/swift/blob/346bd23c926d6a91ac99285314d3c11e1bee92c7/stdlib/public/core/Builtin.swift#L167


      
            return !(lhs == rhs)
          }
          
          /// Returns a Boolean value indicating whether two types are identical.
          ///
          /// - Parameters:
          ///   - t0: A type to compare.
          ///   - t1: Another type to compare.
          /// - Returns: `true` if both `t0` and `t1` are `nil` or if they represent the
          ///   same type; otherwise, `false`.
          @inlinable @_transparent
          public func == (t0: Any.Type?, t1: Any.Type?) -> Bool {
            switch (t0, t1) {
            case (.none, .none): return true
            case let (.some(ty0), .some(ty1)):
              return Bool(Builtin.is_same_metatype(ty0, ty1))
            default: return false
            }
          }
          
          /// Returns a Boolean value indicating whether two types are not identical.

nmgswift · July 17, 2024, 11:08pm

Nice! Thank you very much!