Copy on write:

COW.swift

class Person {
    var name: String
    
    init(name: String) {
        self.name = name
    }
}

let person1 = Person(name: "Mike")
let person2 = person1

let address1 = Unmanaged.passUnretained(person1).toOpaque()
let address2 = Unmanaged.passUnretained(person2).toOpaque()
// Both are same
print("Person1: \(address1)")
print("Person2: \(address2)")

struct Animal {
    var name: String
}

var animal1 = Animal(name: "Tiger")
var animal2 = animal1

// Both are not same
withUnsafeBytes(of: &animal1) { pointer in
    print("Animal1: \(pointer)")
}

withUnsafeBytes(of: &animal2) { pointer in
    print("Animal2: \(pointer)")
}

But, In value types like Arrays, Dictionaries, and Strings use a special trick. Even though they are structs, they don't actually copy their internal data until you try to modify one of them.

Before modification: Both structs point to the same internal "storage" on the heap.

After modification: The copy is physically made.

var array1 = Array(repeating: 1, count: 1000)
var array2 = array1 // This is a "Value Type" copy

print("--- Before Modification ---")
// Both will print same
array1.withUnsafeBufferPointer { pointer in
    print("array1: \(pointer)")
}
array2.withUnsafeBufferPointer { pointer in
    print("array2: \(pointer)")
}

// 2. Modify array2
print("\n--- After Modification ---")
array2.append(2)
// Both will print different
array1.withUnsafeBufferPointer { pointer in
    print("array1: \(pointer)")
}
array2.withUnsafeBufferPointer { pointer in
    print("array2: \(pointer)")
}