xcrun swift
Welcome to Swift!  Type :help for assistance.
  1> import Foundation
  2> var t : NSNumber = true
t: __NSCFBoolean = {}
  3> var f : NSObject = false
f: __NSCFBoolean = {}
  4>

读这个：https://developer.apple.com/library/prerelease/ios/documentation/swift/conceptual/buildingcocoaapps/WorkingWithCocoaDataTypes.html#//apple_ref/doc/uid/TP40014216-CH6-XID_43

Swift automatically bridges certain native number types,such as Int and Float,to NSNumber. This bridging lets you create an NSNumber from one of these types

All of the following types are automatically bridged to NSNumber:

• Int
• UInt
• Float
• Double
• Bool