Swift Integer Overflow: Why Addition Can Crash

Adding two valid integers can crash your Swift app at runtime.

The Problem

let a: Int8 = 120
let b: Int8 = 10
let c = a + b  // Crashes at runtime

Int8 holds values from -128 to 127. The result 130 exceeds this range, causing an overflow.

Why Swift Crashes

Swift’s arithmetic operators trap on overflow by default. Unlike C, which silently wraps to garbage values, Swift crashes immediately. This prevents subtle bugs where incorrect data propagates through your app.

Solution: Overflow Operators

Use &+, &-, and &* when you want wrapping behavior:

let a: Int8 = 120
let b: Int8 = 10
let c = a &+ b  // c = -126 (wraps around)

The value wraps from 127 to -128 and continues counting, resulting in -126.

When to Use Overflow Operators

• Cryptographic algorithms requiring modular arithmetic
• Hash function implementations
• Low-level bit manipulation
• Performance-critical code where you’ve validated inputs

Quick Reference

Operator	Behavior	Example (Int8: 127 + 1)
+	Traps on overflow	Crashes
&+	Wraps on overflow	-128

Standard operators are safer for typical app logic. Reserve overflow operators for intentional wrapping scenarios.