Classes and Structures

Classes have additional capabilities that structures do not:

Inheritance enables one class to inherit the characteristics of another. Type casting enables you to check and interpret the type of a class instance at runtime. Deinitializers enable an instance of a class to free up any resources it has assigned. Reference counting allows more than one reference to a class instance.

Structures are always copied when they are passed around in your code, and do not use reference counting.

if tenEighty === alsoTenEighty {
    println("tenEighty and alsoTenEighty refer to the same Resolution instance.")
}
// prints "tenEighty and alsoTenEighty refer to the same Resolution instance.”

Whenever you assign a Dictionary instance to a constant or variable, or pass a Dictionary instance as an argument to a function or method call, the dictionary is copied at the point that the assignment or call takes place.

var ages = ["Peter": 23, "Wei": 35, "Anish": 65, "Katya": 19]
var copiedAges = ages  
copiedAges["Peter"] = 24
println(ages["Peter"])
// prints "23" 

If you assign an Array instance to a constant or variable, or pass an Array instance as an argument to a function or method call, the contents of the array are not copied at the point that the assignment or call takes place. Instead, both arrays share the same sequence of element values. When you modify an element value through one array, the result is observable through the other.

For arrays, copying only takes place when you perform an action that has the potential to modify the length of the array. This includes appending, inserting, or removing items, or using a ranged subscript to replace a range of items in the array.

var a = [1, 2, 3]
var b = a
var c = a

println(a[0])
// 1
println(b[0])
// 1
println(c[0])
// 1

a[0] = 42
println(a[0])
// 42
println(b[0])
// 42
println(c[0])
// 42

a.append(4)
a[0] = 777
println(a[0])
// 777
println(b[0])
// 42
println(c[0])
// 42

b.unshare()

b[0] = -105
println(a[0])
// 777
println(b[0])
// -105
println(c[0])
// 42

if b === c {
    println("b and c still share the same array elements.")
} else {
    println("b and c now refer to two independent sets of array elements.")
}
// prints "b and c now refer to two independent sets of array elements."       

var names = ["Mohsen", "Hilary", "Justyn", "Amy", "Rich", "Graham", "Vic"]
var copiedNames = names.copy()
copiedNames[0] = "Mo"
println(names[0])
// prints "Mohsen"  

If you simply need to be sure that your reference to an array’s contents is the only reference in existence, call the unshare method, not the copy method. The unshare method does not make a copy of the array unless it is necessary to do so. The copy method always copies the array, even if it is already unshared.