# init(truncatingIfNeeded:) Creates a new instance from the bit pattern of the given instance by sign-extending or truncating to fit this type. ``` init(truncatingIfNeeded source: T) where T : BinaryInteger ``` ## Parameters `source` An integer to convert to this type. ## Discussion When the bit width of `T` (the type of `source`) is equal to or greater than this type’s bit width, the result is the truncated least-significant bits of `source`. For example, when converting a 16-bit value to an 8-bit type, only the lower 8 bits of `source` are used. ``` let p: Int16 = -500 // 'p' has a binary representation of 11111110_00001100 let q = Int8(truncatingIfNeeded: p) // q == 12 // 'q' has a binary representation of 00001100 ``` When the bit width of `T` is less than this type’s bit width, the result is *sign-extended* to fill the remaining bits. That is, if `source` is negative, the result is padded with ones; otherwise, the result is padded with zeros. ``` let u: Int8 = 21 // 'u' has a binary representation of 00010101 let v = Int16(truncatingIfNeeded: u) // v == 21 // 'v' has a binary representation of 00000000_00010101 let w: Int8 = -21 // 'w' has a binary representation of 11101011 let x = Int16(truncatingIfNeeded: w) // x == -21 // 'x' has a binary representation of 11111111_11101011 let y = UInt16(truncatingIfNeeded: w) // y == 65515 // 'y' has a binary representation of 11111111_11101011 ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)