swift - 如何检查变量是否未在 Swift 中初始化？

Question

Swift 允许声明但不初始化变量。如何检查变量是否未在 Swift 中初始化？

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

你是对的——你不能将非可选变量与 nil 进行比较。当您声明非可选变量但未为其提供值时，它不会像可选变量一样设置为nil。没有办法在运行时测试未初始化的非可选变量的使用，因为这种使用的任何可能性都是一个可怕的、编译器检查的程序员错误。唯一可以编译的代码是保证每个变量在使用前都被初始化的代码。如果您希望能够将 nil 分配给一个变量并在运行时检查它的值，那么您必须使用一个可选的。

示例 1:正确用法

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.
}

示例2:编译错误

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.
}

示例 3:允许 nil

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.
}