初始分配后可变变不变

发布于 2025-01-17 09:17:06 字数 447 浏览 2 评论 0原文

有没有一种方法可以使变量在初始化/分配后变得不可变，以便它可以在某个时刻发生变化，但稍后又变得不可变？我知道我可以创建一个新的 let 变量，但是有没有办法在不创建新变量的情况下做到这一点？

如果不是，那么安全地确保变量在需要时不被更改的最佳实践是什么？我想要完成的示例：

var x = 0         //VARIABLE DECLARATION


x += 1           //VARIABLE IS CHANGED AT SOME POINT


x = let x        //MAKE VARIABLE IMMUTABLE AFTER SOME FUNCTION IS PERFORMED


x += 5          //what I'm going for: ERROR - CANNOT ASSIGN TO IMMUTABLE VARIABLE

原文

Is there a way to make a variable immutable after initializing/assigning it, so that it can change at one point, but later become immutable? I know that I could create a new let variable, but is there a way to do so without creating a new variable?

If not, what is best practice to safely ensure a variable isn't changed after it needs to be?
Example of what I'm trying to accomplish:

var x = 0         //VARIABLE DECLARATION


x += 1           //VARIABLE IS CHANGED AT SOME POINT


x = let x        //MAKE VARIABLE IMMUTABLE AFTER SOME FUNCTION IS PERFORMED


x += 5          //what I'm going for: ERROR - CANNOT ASSIGN TO IMMUTABLE VARIABLE

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南烟 2025-01-24 09:17:06

您可以使用内联闭包初始化变量：

let x: Int = {
    var x = 0

    while x < 100 {
        x += 1
    }

    return x
}()

You can initialize a variable with an inline closure:

let x: Int = {
    var x = 0

    while x < 100 {
        x += 1
    }

    return x
}()

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烈酒灼喉 2025-01-24 09:17:06

我不知道这使您可以将var变量更改为LET以后常数。但是您可以将您的常数声明为让开始，而不是立即给它一个值。

let x: Int /// No initial value

x = 100 /// This works.

x += 5 /// Mutating operator '+=' may not be used on immutable value 'x'

只要您在使用它之前的某个时候为常数分配一个值，就可以，因为编译器可以确定最终将被填充。例如如果其他有效，则由于有条件的分支之一可以被调用。

let x: Int

if 5 < 10 {
    x = 0 /// This also works.
} else {
    x = 1 /// Either the first block or the `else` will be called.
}

x += 5 /// Mutating operator '+=' may not be used on immutable value 'x'

There's no way I know of that lets you change a var variable into a let constant later on. But you could declare your constant as let to begin with and not immediately give it a value.

let x: Int /// No initial value

x = 100 /// This works.

x += 5 /// Mutating operator '+=' may not be used on immutable value 'x'

As long as you assign a value to your constant sometime before you use it, you're fine, since the compiler can figure out that it will eventually be populated. For example if else works, since one of the conditional branches is guaranteed to get called.

let x: Int

if 5 < 10 {
    x = 0 /// This also works.
} else {
    x = 1 /// Either the first block or the `else` will be called.
}

x += 5 /// Mutating operator '+=' may not be used on immutable value 'x'

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