Answer: Copy-on-write means a value type shares its underlying storage with copies until one of them is mutated — only then is a real copy made. This makes copying large collections cheap.

Code Example:

var array1 = [1, 2, 3, 4, 5]
var array2 = array1  // no copy yet — shared storage

array2.append(6)     // copy happens NOW — array1 is unaffected

print(array1) // [1, 2, 3, 4, 5]
print(array2) // [1, 2, 3, 4, 5, 6]

Types that use COW in Swift:

• Array
• Dictionary
• Set
• String
• Data

Note: Custom structs do NOT get COW automatically. You have to implement it manually if needed.