如何检查变量是否未在Swift中初始化？

Question

Swift允许声明变量但不初始化.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 - 你可能无法比较非可选变量nil.当您为非可选变量声明但不提供值时,它不会nil像可选变量那样设置.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.将编译的唯一代码是保证每个变量在使用之前被初始化的代码.如果您希望能够nil在运行时分配给变量并检查其值,则必须使用可选项.

示例1:正确使用

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.
}

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.
}

例3:允许为零

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.
}