swift – 字符串值为UnsafePointer函数参数行为

func foo(p:UnsafePointer<UInt8>) {
    var p = p
    for p; p.memory != 0; p++ {
        print(String(format:"%2X",p.memory))
    }
}

let str:String = "今日"
foo(str)

这打印E4BB8AE697A5,这是今日的有效UTF8表示

据我所知,这是无证的行为.从the document:

When a function is declared as taking a UnsafePointer argument,it can accept any of the following:

• nil,which is passed as a null pointer
• An UnsafePointer,UnsafeMutablePointer,or AutoreleasingUnsafeMutablePointer value,which is converted to UnsafePointer if necessary
• An in-out expression whose operand is an lvalue of type Type,which is passed as the address of the lvalue
• A [Type] value,which is passed as a pointer to the start of the array,and lifetime-extended for the duration of the call

在这种情况下,str不是它们.

我错过了什么吗？

添加：

如果参数类型为UnsafePointer< UInt16>而不起作用

func foo(p:UnsafePointer<UInt16>) {
    var p = p
    for p; p.memory != 0; p++ {
        print(String(format:"%4X",p.memory))
    }
}
let str:String = "今日"
foo(str)
//  ^ 'String' is not convertible to 'UnsafePointer<UInt16>'

即使内部String表示形式是UTF16

let str = "今日"
var p = UnsafePointer<UInt16>(str._core._baseAddress)
for p; p.memory != 0; p++ {
    print(String(format:"%4X",p.memory)) // prints 4ECA65E5 which is UTF16 今日
}