Classes and Structures

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comparing classes and structures

都可以：

  • 定义 property 来存储值
  • 定义函数
  • 定义下标，以供下表式调用
  • 定义初始化函数
  • Be extended to expand their functionality beyond a default implementation
  • Conform to protocols to provide standard functionality of a certain kind

class可以但structure不行：

  • Inheritance enables one class to inherit the characteristics of another.（继承另一个类）

  • Type casting enables you to check and interpret the type of a class instance at runtime.（类型转换）

  • Deinitializers enable an instance of a class to free up any resources it has assigned.（析构函数）

  • Reference counting allows more than one reference to a class instance.（引用计数）

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

syntax

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class SomeClass {
    // class definition goes here
}
struct SomeStructrue {
    // structure definition goes here
}

定义一个 class 或 struct 实际上是定义了一种类型，跟 String，Int 等类似。一般来说 class 和 struct 的名字用 UpperCamelCase，属性和方法名则用 lowerCamelCase。例如

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

上面的 struct 有两个属性，class 有四个属性，其中一个是 struct 类型，name 属性会有一个默认值是 nil，因为它是 optional。

class and structure instances

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

上面都使用初始化函数的方式声明对象。

accessing properties

使用 . 来访问属性：

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print("The width of resolution is \(someResolution.width)")

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

除了访问，也可以赋值：

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someVideoMode.resolution.width = 1280
print("The width of someVideoMode is \(someVideoMode.resolution.width)")

可以直接设定 struct 的属性值。

memberwise initializers for structure types

struct 按成员的初始化函数：

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

上面的方式，class 不支持。

structures and enumerations are value types

原话：A value type is a type whose value is copied when it is assigned to a variable or constant, or when it is passed to a function.

其实swift中所有的基本类型——integers, floating-point, Booleans, strings, arrays and dictionaries——都是 value type。

也就是说，当你创建 struct 或 enumeration 类型的实例时，都是拷贝。例如：

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et hd = Resolution(width: 1920, height: 1080)
var cinema = hd

此时，cinema 就是 hd 的一份拷贝，尽管现在它们两者有相同的长和宽，它们是完全独立的两个实例。例如：

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cinema.width = 2048
print("cinema is now \(cinema.width) pixels wide.")
// prints "cinema is now 2048 pixels wide."
print("hd is still \(hd.width) pixels wide.")
// prints "hd is still 1920 pixels wide."

类似的，enumeration 也有：

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enum CompassPoint {
    case North, South, East, West
}
var currentDirection = CompassPoint.West
let rememberDirection = currentDirection
currentDirection = .East
if rememberDirection == .West {
    print("The remembered direction is still .west")
}

classes are reference types

与 value type 不同，reference type 会取代现有的实例：

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let tenEighty = VideoMode()
tenEighty.resolution = hd
tenEighty.interlaced = true
tenEighty.name = "1080i"
tenEighty.frameRate = 25.0

let alsoTenEighty = tenEighty
alsoTenEighty.frameRate = 30.0

print("The frameRate property of tenEighty is now \(tenEighty.frameRate)")
// prints "The frameRate property of tenEighty is now 30.0"

identity operators

因为 class 是 reference type，所以有必要比较两个常量或变量是否 refer 到同一个类的实例上，有两个操作符可以完成这件事：

  • Identical to (===)
  • Not identitcal to (!==)
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if tenEighty === alsoTenEighty {
    print("tenEight and alsoTenEight refer to the same VideoMode instance.")
}
// prints "tenEight and alsoTenEight refer to the same VideoMode instance."
  • “Identical to”: means that two constants or variables of class type refer to exactly the same class instance.
  • “Equal to”: means that two instances are considered “equal” or “equivalent” in value, for some appropriate meaning of “equal”, as defined by the type’s designer.

Pointers

swift 中的 refer 到某个 reference type 的实例跟 C，C++ 中的指针是类似的，但不需要 * 来定义。

choose structure or class

structure instances are always passed by value, and class instances are always passed by reference.

如果满足下面一个或多个条件，考虑使用 structure：

  • The structure’s primary purpose is to encapsulate a few relatively simple data values.（为了概括一些数据）
  • It is reasonable to expect that the encapsulated values will be copied rather than referenced when you assign or pass around an instance of that structure.（如果数据一般是拷贝而不是refer）
  • Any properties stored by the structure are themselves value types, which would also be expected to be copied rather than referenced.（value types）
  • The structure does not need to inherit properties or behavior from another existing type.（不需要继承性）

一些可以使用 structure 的例子：

  • The size of a geometric shape, perhaps encapsulating a width property and a height property, both of type Double.（几何形状的大小）
  • A way to refer to ranges within a series, perhaps encapsulating a start property and a length property, both of type Int.（一系列的范围）
  • A point in a 3D coordinate system, perhaps encapsulating x, y and z properties, each of type Double.（坐标系统）

Assignment and Copy Behavior for Strings, Arrays, and Dictionaries

In Swift, many basic data types such as String, Array, and Dictionary are implemented as structures.意思是说，当这些类型赋给一个新的 constant 或 variable ，或传递给函数时，都是原始数据的拷贝。