Swift Class Example: init, self

Define a class. Create a new instances of a class with the init keyword.

Class. With a class we group together related data and methods. We form a unit. And with many units together we create complex models.

With init methods, we construct class instances. Classes are a reference type—they are stored in a referenced region of memory. Structs are not.

A simple example. This program declares a class named Box. There are two stored properties on the class—width and height. These are stored in the memory for the class.

Init: This is a special method on the class. It is called when we create a new instance by calling Box().

Self: This keyword (present in init and area) refers to the current instance of the class.

Note: We create a new class instance with width 10 and height 5. We call the area() func, which computes the result.

Swift program that uses class, init method class Box { var width: Int var height: Int init(width: Int, height: Int) { self.width = width self.height = height } func area() -> Int { // Return width times height. return self.width * self.height } } // Create new instance of Box class. let test = Box(width: 10, height: 5) // Compute area for Box instance. let area = test.area() // Display area. print(area) Output 50

Overloaded init. A class may have more than one init method. We can provide overloaded init methods that have formal parameter lists.

Here: We can create a Unit with a String, Int argument list, or use just an Int.

Note: If a class has an optional stored property, like "model," we do not need to assign it in all the init methods.

Stored Property

Swift program that uses overloaded init methods class Unit { var model: String? var modelId: Int init(model: String, modelId: Int) { // This init requires two arguments. self.model = model self.modelId = modelId } init(modelId: Int) { // Only one argument. self.modelId = modelId } } // Create a Unit with the 2-argument init. let example = Unit(model: "ABC12", modelId: 12) print(example.modelId) // Use 1-argument init. let example2 = Unit(modelId: 13) print(example2.modelId) Output 12 13

Deinit. Swift provides the deinit keyword. This is used to run code before deinitialization. The deinit code is run automatically.

Tip: If we assign a class instance to nil, the deinit code block is executed. In this example, we create an Optional Item.

And: When the Item instance is assigned to nil, the deinit block is run. Cleanup of system resources can occur here.

Quote: Deinitializers are called automatically, just before instance deallocation takes place. You are not allowed to call a deinitializer yourself (Swift Programming Language).

Swift program that uses deinit class Item { init() { print("init called") } deinit { // Called whenever class stops existing. print("deinit called") } } // Create optional Item variable. var i: Item? = Item() // Set optional to nil to force deinit. i = nil Output init called deinit called

Failable init. An init can be failable. We use the "init?" syntax for this. Here we disallow a Square class to be created if the width or height is less than or equal to zero.

Note: A failable init method returns an Optional class instance. The return value may be nil.

So: We must test the return value in a let-conditional statement (or with another Optional test).

Tip: A failable init makes sense if the class cannot logically exist if an argument is invalid.

Swift program that uses failable in it class Square { var width: Int! // Has default value of nil. var height: Int! init?(width: Int, height: Int) { // Do not allow negative width or height. if width <= 0 { return nil } if height <= 0 { return nil } self.width = width self.height = height } } // The initializer succeeds. var square = Square(width: 10, height: 20) if let s = square { print(s.width) print(s.height) } // The failable initializer returns nil. var squareNegative = Square(width: -1, height: -1) if let s = squareNegative { // Not reached. print(s.width) print(s.height) } Output 10 20

CustomStringConvertible. A type can inherit from CustomStringConvertible (formerly Printable). This means the class has a description property.

And: We must define the "description" String property. This should return a string that contains important data about the class instance.

Item: This class provides a description that includes its size Int. Print() displays the description when passed a class instance.

Swift program that uses CustomStringConvertible, description class Item: CustomStringConvertible { var size: Int init(size: Int) { self.size = size } var description: String { // Custom string. return "Item: \(self.size)" } } // Create new instance. // ... Print using CustomStringConvertible description. let item = Item(size: 10) print(item) // Access the description directly. let description = item.description print("Description = \(description)") Output Item: 10 Description = Item: 10

Subscript. Some accesses to a class are more complex: they might use two or more arguments. A subscript can be used to make accesses to a class similar to those of an array or dictionary.Subscript

With classes and structs, we build programs with more reusable parts. In larger programs, this leads to simplifications. Classes can even reduce copying and improve performance.

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Swift Class Example: init, self

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