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Classes

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Swift Class

A class is used to programmatically represent a real-life object in code. Classes are defined by the keyword class followed by the class name and curly braces that store the class’s properties and methods.

// Using data types: class Student { var name: String var year: Int var gpa: Double var honors: Bool } // Using default property values: class Student { var name = "" var year = 0 var gpa = 0.0 var honors = false }

Instance of a Class

Creating a new instance of a class is done by calling a defined class name with parentheses () and any necessary arguments.

class Person { var name = "" var age = 0 } var sonny = Person() // sonny is now an instance of Person

Class Properties

Class properties are accessed using dot syntax, i.e. .property.

var ferris = Student() ferris.name = "Ferris Bueller" ferris.year = 12 ferris.gpa = 3.81 ferris.honors = false

init() Method

Classes can be initialized with an init() method and corresponding initialized properties. In the init() method, the self keyword is used to reference the actual instance of the class assign property values.

class Fruit { var hasSeeds = true var color: String init(color: String) { self.color = color } } let apple = Fruit(color: "red")

Inheritance

A class can inherit, or take on, another class’s properties and methods:

  • The new inheriting class is known as a subclass.
  • The class that the subclass inherits from is known as its superclass.
// Suppose we have a BankAccount class: class BankAccount { var balance = 0.0 func deposit(amount: Double) { balance += amount } func withdraw(amount: Double) { balance -= amount } } // And we want a new SavingsAccount class that inherits from BankAccount: class SavingsAccount: BankAccount { var interest = 0.0 func addInterest() { let interest = balance * 0.005 self.deposit(amount: interest) } } // Here, the new SavingsAccount class (subclass) automatically gains all of the characteristics of BankAccount class (superclass). In addition, the SavingsAccount class defines a .interest property and a .addInterest() method.

Overriding

A subclass can provide its own custom implementation of a property or method that is inherited from a superclass. This is known as overriding.

// Suppose we have a BankAccount class: class BankAccount { var balance = 0.0 func deposit(amount: Double) { balance += amount } func withdraw(amount: Double) { balance -= amount } } // Suppose we want a new SavingsAccount class and we want to override the .withdraw() method from its superclass BankAccount: class SavingsAccount: BankAccount { var interest = 0.0 var numWithdraw = 0 func addInterest() { let interest = balance * 0.01 self.deposit(amount: interest) } override func withdraw(amount: Double) { balance -= amount numWithdraw += 1 } }

Reference Types

Classes are reference types, while structures are value types, classes are reference types.

Unlike value types, reference types are not copied when they are assigned to a variable or constant, or when they are passed to a function. Rather than a copy, a reference to the same existing instance is used.