如何檢查變量是否未在Swift中初始化？

Question

Swift允許聲明變量但不初始化。我如何檢查變量是否在Swift中未初始化？

class myClass {} 
var classVariable: myClass // a variable of class type - not initialized and no errors! 
//if classVariable == nil {} // doesn't work - so, how can I check it? 

Answer 1

你說的沒錯，你可能不是一個非可選變量比較nil。當您爲非可選變量聲明但未提供值時，它是而不是設置爲nil就像一個可選變量是。沒有辦法在運行時測試未初始化的非可選變量，因爲這種使用的任何可能性都是可怕的，編譯器檢查的程序員錯誤。唯一可編譯的代碼是確保每個變量在使用前都被初始化的代碼。如果您希望能夠將nil分配給變量並在運行時檢查其值，那麼您必須使用可選的變量。

實施例1：正確使用

func pickThing(choice: Bool) { 
    let variable: String //Yes, we can fail to provide a value here... 

    if choice { 
     variable = "Thing 1" 
    } else { 
     variable = "Thing 2" 
    } 

    print(variable) //...but this is okay because the variable is definitely set by now. 
} 

實施例2：編譯錯誤

func pickThing2(choice: Bool) { 
    let variable: String //Yes, we can fail to provide a value here, but... 

    if choice { 
     variable = "Thing 1" 
    } else { 
     //Uh oh, if choice is false, variable will be uninitialized... 
    } 

    print(variable) //...that's why there's a compilation error. Variables ALWAYS must have a value. You may assume that they always do! The compiler will catch problems like these. 
} 

實施例3：允許零

func pickThing3(choice: Bool) { 
    let variable: String? //Optional this time! 

    if choice { 
     variable = "Thing 1" 
    } else { 
     variable = nil //Yup, this is allowed. 
    } 

    print(variable) //This works fine, although if choice is false, it'll print nil. 
} 

Answer 2

這可能是編譯器的異常，你不會得到一個錯誤聲明一個變量這樣

class MyClass {} 
var myClass : MyClass 

，但在遊樂場你會得到一個運行時錯誤時，你剛纔讀的變量

myClass 

variable 'myClass' used before being initialized

Swift最重要的功能之一是非可選變量永遠不會是零。如果你嘗試訪問這個變量，你會得到一個運行時錯誤，也就是崩潰。