iOS编程语言-Swift中结构体和枚举是值类型

柔情只为你懂 2022-03-16 15:40 132阅读 0赞

### 文章目录 ###

*   *   *  Structures and Enumerations Are Value Types
         *   *  An Example to describe the Essence
             *  The Same Behavior applies to Enumerations:
             *  Choosing Between Structures and Classes

### Structures and Enumerations Are Value Types ###

*  A *value type* is a type whose value is *copied* when it’s assigned to a variable or constant, or when it’s passed to a function.
 *  In fact, all of the basic types in Swift-integers, floating-point numbers, Booleans, strings, arrays and dictionaries-are value types, and are implemented as structures behind the scenes.
 *  All structures and enumerations are value types in Swift. This meas that any structure and enumeration instances you create-and any value types they have as properties-are always copied when they are passed around in your code.

#### An Example to describe the Essence ####

//声明结构体
    struct Resolution {
        
          var width = 0
          var hetight = 0
    }
    //创建一个结构体实例
    let hd = Resolution(width: 1920, height: 1080)
    //将实例hd赋值给变量cinema
    var cinema = hd
    //修改变量cinema的宽度
    cinema.width = 2048
    //打印输出hd和cinema中的width
    print("\(hd.widht)") //输出1920
    print("\(cinema.widht)")//输出2048

*  When **cinema** was given the current value of **hd**, the *values* stored in **hd** were copied into the new **cinema** instance. The end result was two completely separate instances that contained the same numeric values. However, because they are separate instances, setting the width of **cinema** to **2048** doesn’t affect the width stored in **hd**, as shown in the figure below:  
    ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0ZURF9TTA_size_16_color_FFFFFF_t_70][]

#### The Same Behavior applies to Enumerations: ####

//声明枚举
    enum CompassPoint {
        
        case north, south, east, west
        mutating func turnNorth() {
        
    		self = .north
    	}
    }
    //创建当前方向实例
    var currentDirection = CompassPoint.west
    //记录当前方向
    let rememberedDirection = currentDirection
    //修改当前方向
    currentDirection.turnNorth()
    //打印输出
    print("\(currentDirection)")//输出north
    print("\(rememberedDirection)")//输出west

*  When **rememberedDirection** is assigned the value of **currentDirection**, it’s acutally set to a copy of that value. Changing the value of **currentDirection** thereafter doesn’t affect the copy of the original value that was stored in **rememberedDirection**.

#### Choosing Between Structures and Classes ####

*  Structures and classes are good choices for storing data and modeling behavior in your apps, but their similarities can make it difficult to choose one over the other.
 *  Consider the following recommendations to help choose which option makes sence when adding a new data type to your app.  
    ![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0ZURF9TTA_size_16_color_FFFFFF_t_70 1][]
 *  Using structures makes it easier to reason about a portion of your code without needing to consider the whole state of your app. Because structures are value types-unlike classes-local changes to a structure aren’t visible to the rest of your app unless you intentionally communicate those changes as part of the flow of your app. As a result, you can look at a section of code and be more confident that changes to instances in that secion will be made explicitly, rather than being made invisibly from a tangentially related function call.

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