是什么导致“常量在初始化之前被闭包捕获"错误 [英] What causes 'Constant captured by a closure before being initialized' error

问题描述

在下一堂课

class Foo {
   let _defaultValue = "N/A"
   let value: String 

   init (dict: NSDictionary) {
       self.value = dict["bar"] as? String! ?? _defaultValue
   }
}

编译器失败，并显示以下消息 constant 'self.value' captured by a closure before being initialized

compiler fails with the message constant 'self.value' captured by a closure before being initialized

据我所知，没有运算符读取`self.value.该消息似乎有些混乱.

As far as I can see, no operators read `self.value. The message seems somewhat confusing.

我不小心想到了一种解决方法.我应该说这让我更加困惑:

I accidentally came up with a workaround. I should say it confuses me even more:

class Foo {
       let value: String 

       init (dict: NSDictionary) {
           let _defaultValue = "N/A"
           self.value = dict["bar"] as? String! ?? _defaultValue
       }
    }

声明_defaultValue并在构造函数中对其进行初始化，使代码得以编译.

Declaring _defaultValue and initializing it within the constructor makes the code compile.

如果有人可以解释为什么在第一种情况下会发生错误，而在第二种情况下编译器会更快乐，那将是一个巨大的帮助?

It'd be an immense help if someone could explain why an error occurs in the first case and what is the compiler happier in the second case?

推荐答案

错误消息的原因是nil-coalescing运算符 定义为

The reason for the error message is that the nil-coalescing operator is defined as

public func ??<T>(optional: T?, defaultValue: @autoclosure () throws -> T) rethrows -> T

并在第二个参数上执行自动关闭"(以获取 短路行为).所以

and does a "auto closure" on the second argument (in order to get a short-circuiting behaviour). So

self.value = dict["bar"] as? String ?? _defaultValue

由编译器转换为

self.value = dict["bar"] as? String ?? { self._defaultValue }()

，在这里编译器会抱怨，因为self是在之前捕获的 正在完全初始化. (错误消息略有不同 在Swift 2和Swift 3之间).

and here the compiler complains because self is captured before being fully initialised. (The error messages are slightly different between Swift 2 and Swift 3).

可能的解决方法.您可以先将属性分配给局部变量:

Possible workarounds. You can assign the property to a local variable first:

init(dict: NSDictionary){
    let defValue = _defaultValue
    self.value = dict["bar"] as? String! ?? defValue
}

或者您可以将其设为该类的静态属性:

Or you can make it a static property of the class:

class Foo {
    static let _defaultValue = "N/A"
    let value: String

    init(dict: NSDictionary) {
        self.value = dict["bar"] as? String ?? Foo._defaultValue
    }
}

或将??替换为if语句:

class Foo {
    let _defaultValue = "N/A"
    let value: String

    init (dict: NSDictionary) {
        if let value = dict["bar"] as? String {
            self.value = value
        } else {
            self.value = _defaultValue
        }
    }
}

附录:相关资源:

• 在Swift中将自身自动封闭在初始化器中论坛.
• SR-944无法在&&的RHS中使用自我"在初始化所有属性之前错误报告.

• Autoclosure of self in initializers in the Swift forum.
• SR-944 Cannot use 'self' in RHS of && before all properties are initialized bug report.

错误报告中的语录:

乔丹·罗斯(Jordan Rose):自&&是使用@autoclosure实现的，但肯定不是最佳选择.

Jordan Rose: This is true since && is implemented using @autoclosure, but it's certainly suboptimal.

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