To convert integer types in Swift while preserving bit patterns, use the following initializer.
init<T>(truncatingIfNeeded source: T) where T : BinaryInteger
This can be used to convert between unsigned and signed integers. For example, 0x00, 0x7F, 0x80, 0xFF in UInt8 would look like this in decimal.
| Hex | Decimal in UInt8 |
|---|---|
| 0x00 | 0 |
| 0x7F | 127 |
| 0x80 | 128 |
| 0xFF | 255 |
The same value for Int8 would look like this.
| Hex | Decimal in Int8 |
|---|---|
| 0x00 | 0 |
| 0x7F | 127 |
| 0x80 | -128 |
| 0xFF | -1 |
You can confirm that you get the expected conversion by typing the following in Playground and running it.
let u1 = UInt8(0x00)
let u2 = UInt8(0x7F)
let u3 = UInt8(0x80)
let u4 = UInt8(0xFF)
let i1 = Int8(truncatingIfNeeded: u1)
let i2 = Int8(truncatingIfNeeded: u2)
let i3 = Int8(truncatingIfNeeded: u3)
let i4 = Int8(truncatingIfNeeded: u4)
If no argument label is specified, the next initializer is used.
init<T>(_ source: T) where T : BinaryInteger
This initializer can also do the conversion, but an error occurs when the value is out of range. Specifically, the conversion from UInt8 to Int8 results in an error for values equal or greater than 128.
Run the following code in Playground.
let u1 = UInt8(0x00)
let u2 = UInt8(0x7F)
let u3 = UInt8(0x80)
let u4 = UInt8(0xFF)
let i1 = Int8(u1)
let i2 = Int8(u2)
let i3 = Int8(u3)
let i4 = Int8(u4)
Then the following is output to the console and an error occurs when making Int8 from u3.
Swift/Integers.swift:3564: Fatal error: Not enough bits to represent the passed value
init(truncatingIfNeeded:) succeeds because it converts the type while preserving the bit pattern of the number. 128 in UInt8 and -128 in Int8 are both 10000000 in binary.
init(_ source:) occurs out of range error because it interprets numeric values.The possible values for Int8 are between -128 and 127. u3 is 128, so it is out of range.
