7.1 - Comparing Classes and Structures.md

7.1 Comparing Classes and Structures

Classes and Structures have a great deal in common. Both can:

• Define properties to store values
• Define methods to provide functionality
• Define subscripts to provide access to their values using subscript syntax
• Define initializers to set up their initial state
• Be extended to expand their functionality beyond a default implementation
• Conform to protocols to provide standard functionality of a certain kind

For information on these features, please search the specific section.

Classes however, have functionality that Structures do not, namely:

• 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.

These are all advanced features. Please see the relevant sections for them.

Definition Syntax

Classes and Structures are defined in an analogous way to C-based languages, using the struct and class keywords:

struct Resolution {
    var width = 0
    var height = 0
}
class VideoMode {
    var resolution = Resolution()
    var interlaced = false
    var frameRate = 0.0
    var name: String?
}

Class and Structure Instances

Classes and Structures are instantiated in the same way:

let someResolution = Resolution()
let someVideoMode = VideoMode()

Accessing Properties

Both classes and structures use a simple dot-based (class.property) based syntax to access properties and subproperties:

print("The width of someResolution is \(someResolution.width)")
// Prints "The width of someResolution is 0"

print("The width of someVideoMode is \(someVideoMode.resolution.width)")
// Prints "The width of someVideoMode is 0"

Memberwise Initializers for Structure Types

Structures, and only structures, receive an automatically-generated memberwise initializer which you can use to initialise properties.

let vga = Resolution(width: 640, height: 480)

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