Michele Dall'Agata Michele Dall'Agata - 6 months ago 32
iOS Question

Int to UInt (and vice versa) bit casting in Swift

I am looking for a direct way to bit cast the bit values of an Int to UInt and vice versa. For example (using the 8 bits integers for simplicity) I want to achieve the following:

let unsigned: UInt8 = toUInt8(-1) // unsigned is 255 or 0xff
let signed: Int8 = toInt8(0xff) // signed is -1


At first I came out with the following solution:

let unsigned = unsafeBitCast(Int8(-1), UInt8.self)
let signed = unsafeBitCast(UInt8(0xff), Int8.self)


But Apple in the "unsafeBitCast()" documentation states the following:


.. Caution:: Breaks the guarantees of Swift's type system; use with
extreme care. There's almost always a better way to do anything.


Does anyone have the better way?

Answer

You can do:

let unsigned = UInt8(bitPattern: Int8(-1)) // -> 255
let signed   = Int8(bitPattern: UInt8(0xff)) // -> -1

Many similar initializers exist:

extension Int8 {
    init(_ v: UInt8)
    init(_ v: UInt16)
    init(truncatingBitPattern: UInt16)
    init(_ v: Int16)
    init(truncatingBitPattern: Int16)
    init(_ v: UInt32)
    init(truncatingBitPattern: UInt32)
    init(_ v: Int32)
    init(truncatingBitPattern: Int32)
    init(_ v: UInt64)
    init(truncatingBitPattern: UInt64)
    init(_ v: Int64)
    init(truncatingBitPattern: Int64)
    init(_ v: UInt)
    init(truncatingBitPattern: UInt)
    init(_ v: Int)
    init(truncatingBitPattern: Int)
    init(bitPattern: UInt8)
}