---
title: "init(truncatingIfNeeded:)"
framework: swift
role: symbol
role_heading: Initializer
path: "swift/int64/init(truncatingifneeded:)"
---

# init(truncatingIfNeeded:)

Creates a new instance from the bit pattern of the given instance by sign-extending or truncating to fit this type.

## Declaration

```swift
init<T>(truncatingIfNeeded source: T) where T : BinaryInteger
```

## Parameters

- `source`: An integer to convert to this type.

## Discussion

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