What is the difference between copying an array and copying a struct in Swift?

Copied structure is a new independent instance. Changes in one structure do not affect the other.

Copying an array (Array) in Swift by default is "copy-on-write". This means that when a new reference to an array is created, the data is not duplicated until one of the copies is modified. Until then, both variables refer to the same elements in memory. When one copy is modified, a new independent data instance is created.

struct Point {
    var x: Int
    var y: Int
}

var p1 = Point(x: 1, y: 2)
var p2 = p1 // Copying the structure. p2 is a new instance

p2.x = 10 // Changing p2 does not affect p1

print("p1: \(p1.x), \(p1.y)") // Output: p1: 1, 2
print("p2: \(p2.x), \(p2.y)") // Output: p2: 10, 2

var array1 = [1, 2, 3]
var array2 = array1 // Copying on assignment

array2[0] = 10 // Actual data copying occurs

print("array1: \(array1)") // Output: array1: [1, 2, 3]
print("array2: \(array2)") // Output: array2: [10, 2, 3]

The difference lies in the fundamental behavior: structures are always copied by value, while arrays, being structures, implement "copy-on-write" optimization for more efficient memory management when there are multiple references.