(infinitespeak.wordpress.com) Classes and Structs. Dr. Sarah Abraham

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1 (infinitespeak.wordpress.com) Classes and Structs Dr. Sarah Abraham University of Texas at Austin CS329e Fall 2018

2 Classes and Structures General-purpose, flexible constructs to build blocks of code Properties and methods add functionality Defining classes and structs in a file makes external interface automatically available in Swift

3 Classes vs Structs Both classes and structs define: Properties, methods, initializers Classes allow for: Inheritance Type-casting to check type of class at runtime Deinitialization and reference counting Structs passed by value, classes passed by reference

4 Defining a Struct struct Point { var x = 0.0 var y = 0.0 //Create a struct instance and change its x value var p1 = Point() p1.x = 10.0

5 Defining a Class class Person { var firstname:string? var lastname:string? func description() -> String { return \(lastname!), \(firstname!)

6 Creating an Instance Each instance has its own memory and set of properties: let p1 = Person() Instances can call on instance methods p1.description() Method is called on the instance itself

7 Initializers Automatically called after memory is allocated Creates an object with a good starting state init() { self.firstname = Unknown self.lastname = Unknown If no init() is provided, will auto-generate a default init() with an empty method body

8 Initializing Property Values Can provide default values for properties var firstname: String = Unknown var lastname: String = Unknown init() { Can overload initializers to determine property values init(firstname: String, lastname: String) { self.firstname = firstname self.lastname = lastname

9 Designated Initializer Main initializer used for a class All other initializers funnel through this initializer Ensures initialization occurs through superclass chain Must call designated initializer from its superclass if it has one Set all properties of class while letting user send in customized values

10 Convenience Initializers Secondary, supporting initializers for a class Must call another initializer from the same class Must ultimately call the designated initializer init method is prefixed with convenience

11 Convenience Initializer Example //Designated initializer init(firstname: String, lastname: String) { self.firstname = firstname self.lastname = lastname //Convenience initializer convenience init() { self.init(firstname: Lev, lastname: Tolstoy )

12 Why All The Hassle? All this setup might be redundant in these toy examples but it s useful when there s inheritance: Person Super/Base class Player Sub/Derived class Prevents nil from creating unexpected behavior Swift passes nil values!

13 Class-Level Methods and Properties Type methods called on the type itself rather than an instance class keyword defines type-methods Allows subclasses to override superclass implementation static also works but methods cannot be overwritten by subclass Class-level properties are defined at the type, rather than instance, level static keyword defines class-level properties

14 Type-Method Example class Player { static var unlockedlevels = 1 class func unlocklevels(levels: Int) { unlockedlevels += levels var currentlevel = 1 func updatecurrentlevel(selectedlevel : Int) { if selectedlevel < Player.unlockedLevels { currentlevel = selectedlevel else { currentlevel = Player.unlockedLevels

15 Quiz Question! What is the difference between an instance method and a type method? A. Instance methods are called by the class instance, type methods are called at the class-level B. Instance methods can be used by all types, whereas type methods are specific to one type C. Instance methods cannot be accessed outside the class, type methods are available anywhere in the code

16 Private Properties and Methods Cannot be accessed outside of the class Preserve internal workings of classes Maintain modular, black box nature of classes Reduce unexpected class access patterns private keyword declared before type: private var currentsprite: Sprite private func setsprite(newsprite: Sprite) { currentsprite = newsprite