为什么 NSLog 不支持 Swift 对象（转）

import Foundation

print("Hello, World!")

class Foo: CustomDebugStringConvertible {
    var debugDescription: String {
        return "debug Foo."
    }
}

NSLog("\(Foo())")

class Bar: NSObject {
    override var description: String {
        return "debug Bar."
    }
}

NSLog("%@", Bar())

Hello, World!
2016-06-09 10:59:30.465 NSLog[9843:180519] debug Foo.
2016-06-09 10:59:30.468 NSLog[9843:180519] debug Bar.