What causes 'Constant captured by a closure before being initialized' error

问题

In a following class

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

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

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

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

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
       }
    }

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

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?

回答1:

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

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

is transformed by the compiler to

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

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).

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
}

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
    }
}

Or replace ?? by an if-statement:

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
        }
    }
}

Addendum: Related resources:

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

Quote from the bug report:

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

来源：https://stackoverflow.com/questions/38291580/what-causes-constant-captured-by-a-closure-before-being-initialized-error

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