根据SE-0054,
ImplicitlyUnwrappedOptional<T>不再是唯一类型;只有
Optional<T>现在。
声明仍然可以被注释为隐含的未包装的可选内容
T!,但这只是添加一个隐藏的属性来通知编译器,在需要他们的未包装类型的上下文中,它们的值可能被强制包装
T;
他们的实际类型是现在
T?。
因此,您可以想到以下声明:
var str: String!
实际看起来像这样:
@_implicitlyUnwrapped // this attribute name is fictitious var str: String?
只有编译器可以看到此
@_implicitlyUnwrapped属性,但是它允许的是在
str需要a
String(其未包装类型)的上下文中隐式展开的值:
// `str` cannot be type-checked as a strong optional, so the compiler will// implicitly force unwrap it (causing a crash in this case)let x: String = str// We're accessing a member on the unwrapped type of `str`, so it'll also be// implicitly force unwrapped hereprint(str.count)
但是在所有其他
str可以作为强选项进行类型检查的情况下,它将是:
// `x` is inferred to be a `String?` (because we really are assigning a `String?`)let x = strlet y: Any = str // `str` is implicitly coerced from `String?` to `Any`print(str) // Same as the previous example, as `print` takes an `Any` parameter.
而且,与强制展开相比,编译器将始终更喜欢将其视为此类。
如提案所述(强调我的意思):
如果 可以使用强壮的可选类型 对表达式 进行显式类型检查,则它将为 。但是,如果需要,类型检查器将转为强制使用可选。此行为
的结果是,引用声明为的值的任何表达式的结果T!将具有typeT或typeT?。
关于字符串插值,在后台,编译器使用
_ExpressibleByStringInterpolation协议中的此初始化程序来评估字符串插值段:
/// Creates an instance containing the appropriate representation for the/// given value.////// Do not call this initializer directly. It is used by the compiler for/// each string interpolation segment when you use string interpolation. For/// example:////// let s = "(5) x (2) = (5 * 2)"/// print(s)/// // Prints "5 x 2 = 10"////// This initializer is called five times when processing the string literal/// in the example above; once each for the following: the integer `5`, the/// string `" x "`, the integer `2`, the string `" = "`, and the result of/// the expression `5 * 2`.////// - Parameter expr: The expression to represent.init<T>(stringInterpolationSegment expr: T)
因此,当您的代码隐式调用时:
var str: String!str = "Hello"print("The following should not be printed as an optional: (str)")
由于
str的实际类型是
String?,因此默认情况下,编译器将推断通用占位符
T为。因此,
str不会强行解开的值,您最终将看到可选内容的描述。
如果希望在字符串插值中使用时将IUO强制展开,则可以简单地使用force unwrap运算符
!:
var str: String!str = "Hello"print("The following should not be printed as an optional: (str!)")
或者,您可以强制使用其非可选类型(在这种情况下为
String),以强制编译器为您隐式强制对其进行解包:
print("The following should not be printed as an optional: (str as String)")
如果
str为,这两个当然都会崩溃
nil。
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