感叹号在Swift语言中是什么意思?

Swift编程语言指南中有如下示例:

class Person {
    let name: String
    init(name: String) { self.name = name }
    var apartment: Apartment?
    deinit { println("\(name) is being deinitialized") }
}

class Apartment {
    let number: Int
    init(number: Int) { self.number = number }
    var tenant: Person?
    deinit { println("Apartment #\(number) is being deinitialized") }
}

var john: Person?
var number73: Apartment?

john = Person(name: "John Appleseed")
number73 = Apartment(number: 73)

//From Apple's “The Swift Programming Language” guide (https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/AutomaticReferenceCounting.html)

然后，在给这个人分配公寓时，他们使用感叹号来“打开实例”:

john!.apartment = number73

“展开实例”是什么意思?为什么有必要?它与仅仅做以下事情有什么不同:

john.apartment = number73

我对Swift语言很不熟悉。我只是想把基本的东西记下来。

更新: 我遗漏了一个很大的谜团(答案中没有直接说明——至少在写这篇文章的时候没有)，那就是当你做以下事情的时候:

var john: Person?

这并不意味着“约翰是人的类型，它可能是nil”，因为我最初的想法。我只是误解了《人与人》是完全不同的类型。一旦我明白了，所有其他的?，!疯狂，以及下面这些精彩的回答，让我觉得更有道理。

当前回答

简单地说

USING感叹号表示变量必须包含非nil值(它永远不会是nil)

2016-05-23 13:03:00

其他回答

!意味着您正在强制打开对象!遵循。更多信息可以在苹果文档中找到，可以在这里找到:https://developer.apple.com/library/ios/documentation/swift/conceptual/Swift_Programming_Language/TheBasics.html

2014-11-16 02:30:05

总之(!): 在你声明了一个变量并且你确定这个变量有一个值之后。

let assumedString: String! = "Some message..."
let implicitString: String = assumedString

否则你必须在每次传递值之后都这样做…

let possibleString: String? = "An optional string."
let forcedString: String = possibleString! // requires an exclamation mark

2016-01-07 07:53:49

简单地说

USING感叹号表示变量必须包含非nil值(它永远不会是nil)

2016-05-23 13:03:00

The entire story begins with a feature of swift called optional vars. These are the vars which may have a value or may not have a value. In general swift doesn't allow us to use a variable which isn't initialised, as this may lead to crashes or unexpected reasons and also server a placeholder for backdoors. Thus in order to declare a variable whose value isn't initially determined we use a '?'. When such a variable is declared, to use it as a part of some expression one has to unwrap them before use, unwrapping is an operation through which value of a variable is discovered this applies to objects. Without unwrapping if you try to use them you will have compile time error. To unwrap a variable which is an optional var, exclamation mark "!" is used.

有时候你知道这样的可选变量会被系统或者你自己的程序赋值，但是稍后，比如UI outlet，在这种情况下，我们不会用问号"?"来声明可选变量，而是用"!"

因此，系统知道这个用“!”声明的变量现在是可选的，没有值，但将在其生命周期的后期接收一个值。