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< section class = "normal" id = "section-gitbook_50" >
< h1 id = "-" > 语法结构< / h1 >
< p > 本页包含内容：< / p >
< ul >
< li > 空白与注释（< em > Whitespace and Comments< / em > ） < / li >
< li > 标识符（< em > Identifiers< / em > ） < / li >
< li > 关键字（< em > Keywords< / em > ） < / li >
< li > 字面量（< em > Literals< / em > ） < / li >
< li > 运算符（< em > Operators< / em > ） < / li >
< / ul >
< p > Swift 的“语法结构（< em > lexical structure< / em > ）”描述了如何在该语言中用字符序列构建合法标记，组成该语言中最底层的代码块，并在之后的章节中用于描述语言的其他部分。< / p >
< p > 通常，标记在随后介绍的语法约束下，由 Swift 源文件的输入文本中提取可能的最长子串生成。这种方法称为“最长匹配项（< em > longest match< / em > ）”，或者“最大适合”（< em > maximal munch< / em > ）。< / p >
< h2 id = "-" > 空白与注释< / h2 >
< p > 空白（< em > whitespace< / em > ）有两个用途：分隔源文件中的标记和区分运算符属于前缀还是后缀，（参见 < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/doc/uid/TP40014097-CH30-XID_871" target = "_blank" > 运算符< / a > ）在其他情况下则会被忽略。以下的字符会被当作空白：空格（< em > space< / em > ） （ ） （ < em > line feed< / em > ） （ ） （ < em > carriage return< / em > ） （ ） （ < em > horizontal tab< / em > ） （ ） （ < em > vertical tab< / em > ） （ ） （ < em > form feed< / em > ） （ ） （ < em > null< / em > ） （ ） < / p >
< p > 注释（< em > comments< / em > ）被编译器当作空白处理。单行注释由 < code > //< / code > 开始直到该行结束。多行注释由 < code > /*< / code > 开始，以 < code > */< / code > 结束。可以嵌套注释，但注意注释标记必须匹配。< / p >
< h2 id = "-" > 标识符< / h2 >
< p > 标识符（< em > identifiers< / em > ）可以由以下的字符开始：大写或小写的字母 < code > A< / code > 到 < code > Z< / code > 、下划线 < code > _< / code > 、基本多语言面（< em > Basic Multilingual Plane< / em > ）中的 Unicode 非组合字符以及基本多语言面以外的非专用区（< em > Private Use Area< / em > ）字符。首字符之后，标识符允许使用数字和 Unicode 字符组合。< / p >
< p > 使用保留字（< em > reserved word< / em > ）作为标识符，需要在其前后增加反引号 < code > `< / code > 。例如，< code > class< / code > 不是合法的标识符，但可以使用 < code > `class`< / code > 。反引号不属于标识符的一部分，< code > `x`< / code > 和 < code > x< / code > 表示同一标识符。< / p >
< p > 闭包（< em > closure< / em > ）中如果没有明确指定参数名称，参数将被隐式命名为 < code > $0< / code > 、< code > $1< / code > 、< code > $2< / code > ... 这些命名在闭包作用域内是合法的标识符。< / p >
< blockquote >
< p > 标识符语法< / p >
< p > < em > identifier< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier-head" target = "_blank" > identifier-head < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier-characters" target = "_blank" > identifier-characters< / a > < em > opt< / em > < / p >
< p > < em > identifier< / em > → ` < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier-head" target = "_blank" > identifier-head < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier-characters" target = "_blank" > identifier-characters < / a > < em > opt < / em > ` < / p >
< p > < em > identifier< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/implicit-parameter-name" target = "_blank" > implicit-parameter-name < / a > < / p >
< p > < em > identifier-list< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier" target = "_blank" > identifier < / a > | < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier" target = "_blank" > identifier < / a > , < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier-list" target = "_blank" > < / a > < / p >
< p > < em > identifier-head< / em > → A 到 Z 大写或小写字母< / p >
< p > < em > identifier-head< / em > → U+00A8, U+00AA, U+00AD, U+00AF, U+00B2– – < / p >
< p > < em > identifier-head< / em > → U+00BC– – – – < / p >
< p > < em > identifier-head< / em > → U+0100– – – – < / p >
< p > < em > identifier-head< / em > → U+1E00– < / p >
< p > < em > identifier-head< / em > → U+200B– – – – < / p >
< p > < em > identifier-head< / em > → U+2070– – – – < / p >
< p > < em > identifier-head< / em > → U+2C00– – < / p >
< p > < em > identifier-head< / em > → U+3004– – – – < / p >
< p > < em > identifier-head< / em > → U+F900– – – – < / p >
< p > < em > identifier-head< / em > → U+FE47– < / p >
< p > < em > identifier-head< / em > → U+10000– – – – < / p >
< p > < em > identifier-head< / em > → U+50000– – – – < / p >
< p > < em > identifier-head< / em > → U+90000– – – – < / p >
< p > < em > identifier-head< / em > → U+D0000– – < / p >
< p > < em > identifier-character< / em > → 数字 0 到 9< / p >
< p > < em > identifier-character< / em > → U+0300– – – – < / p >
< p > < em > identifier-character< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier-head" target = "_blank" > identifier-head < / a > < / p >
< p > < em > identifier-characters< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier-character" target = "_blank" > identifier-character< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/identifier-characters" target = "_blank" > < / a > < em > opt < / em > < / p >
< p > < em > implicit-parameter-name< / em > → < strong > $ < / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-digits" target = "_blank" > decimal-digits < / a > < / p >
< / blockquote >
< h2 id = "-" > 关键字< / h2 >
< p > 被保留的关键字（< em > keywords< / em > ）不允许用作标识符，除非被反引号转义，参见 < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/doc/uid/TP40014097-CH30-XID_796" target = "_blank" > 标识符< / a > 。< / p >
< ul >
< li > < p > < strong > 用作声明的关键字：< / strong > < em > class< / em > 、< em > deinit< / em > 、< em > enum< / em > 、< em > extension< / em > 、< em > func< / em > 、< em > import< / em > 、< em > init< / em > 、< em > let< / em > 、< em > protocol< / em > 、< em > static< / em > 、< em > struct< / em > 、< em > subscript< / em > 、< em > typealias< / em > 、< em > var< / em > < / p >
< / li >
< li > < p > < strong > 用作语句的关键字：< / strong > < em > break< / em > 、< em > case< / em > 、< em > continue< / em > 、< em > default< / em > 、< em > do< / em > 、< em > else< / em > 、< em > fallthrough< / em > 、< em > if< / em > 、< em > in< / em > 、< em > for< / em > 、< em > return< / em > 、< em > switch< / em > 、< em > where< / em > 、< em > while< / em > < / p >
< / li >
< li > < p > < strong > 用作表达和类型的关键字：< / strong > < em > as< / em > 、< em > dynamicType< / em > 、< em > is< / em > 、< em > new< / em > 、< em > super< / em > 、< em > self< / em > 、< em > Self< / em > 、< em > Type< / em > 、< em > __COLUMN__< / em > 、< em > __FILE__< / em > 、< em > __FUNCTION__< / em > 、< em > __LINE__< / em > < / p >
< / li >
< li > < p > < strong > 特定上下文中被保留的关键字：< / strong > < em > associativity< / em > 、< em > didSet< / em > 、< em > get< / em > 、< em > infix< / em > 、< em > inout< / em > 、< em > left< / em > 、< em > mutating< / em > 、< em > none< / em > 、< em > nonmutating< / em > 、< em > operator< / em > 、< em > override< / em > 、< em > postfix< / em > 、< em > precedence< / em > 、< em > prefix< / em > 、< em > right< / em > 、< em > set< / em > 、< em > unowned< / em > 、< em > unowned(safe)< / em > 、< em > unowned(unsafe)< / em > 、< em > weak< / em > 、< em > willSet< / em > ，这些关键字在特定上下文之外可以被用于标识符。< / p >
< / li >
< / ul >
< h2 id = "-" > 字面量< / h2 >
< p > 字面值表示整型、浮点型数字或文本类型的值，举例如下：< / p >
< pre > < code > 42 // 整型字面量
3.14159 // 浮点型字面量
" Hello, world!" // 文本型字面量
< / code > < / pre > < blockquote >
< p > 字面量语法< / p >
< p > < em > literal< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/integer-literal" target = "_blank" > integer-literal < / a > | < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/floating-point-literal" target = "_blank" > floating-point-literal< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/string-literal" target = "_blank" > string-literal < / a > < / p >
< / blockquote >
< h3 id = "-" > 整型字面量< / h3 >
< p > 整型字面量（< em > integer literals< / em > ）表示未指定精度整型数的值。整型字面量默认用十进制表示，可以加前缀来指定其他的进制，二进制字面量加 < code > 0b< / code > ，八进制字面量加 < code > 0o< / code > ，十六进制字面量加 < code > 0x< / code > 。< / p >
< p > 十进制字面量包含数字 < code > 0< / code > 至 < code > 9< / code > 。二进制字面量只包含 < code > 0< / code > 或 < code > 1< / code > ，八进制字面量包含数字 < code > 0< / code > 至 < code > 7< / code > ，十六进制字面量包含数字 < code > 0< / code > 至 < code > 9< / code > 以及字母 < code > A< / code > 至 < code > F< / code > （大小写均可）。< / p >
< p > 负整数的字面量在数字前加减号 < code > -< / code > ，比如 < code > -42< / code > 。< / p >
< p > 允许使用下划线 < code > _< / code > 来增加数字的可读性，下划线不会影响字面量的值。整型字面量也可以在数字前加 < code > 0< / code > ，同样不会影响字面量的值。< / p >
< pre > < code > 1000_000 // 等于 1000000
005 // 等于 5
< / code > < / pre > < p > 除非特殊指定，整型字面量的默认类型为 Swift 标准库类型中的 < code > Int< / code > 。Swift 标准库还定义了其他不同长度以及是否带符号的整数类型，请参考 < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html#//apple_ref/doc/uid/TP40014097-CH5-XID_411" target = "_blank" > 整数类型< / a > 。< / p >
< blockquote >
< p > 整型字面量语法< / p >
< p > < em > integer-literal< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/binary-literal" target = "_blank" > binary-literal < / a > < / p >
< p > < em > integer-literal< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/octal-literal" target = "_blank" > octal-literal < / a > < / p >
< p > < em > integer-literal< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-literal" target = "_blank" > decimal-literal < / a > < / p >
< p > < em > integer-literal< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-literal" target = "_blank" > hexadecimal-literal< / a > < / p >
< p > < em > binary-literal< / em > → < strong > 0b< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/binary-digit" target = "_blank" > binary-digit< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/binary-literal-characters" target = "_blank" > binary-literal-characters < / a > < em > opt < / em > < / p >
< p > < em > binary-digit< / em > → 数字 0 或 1< / p >
< p > < em > binary-literal-character< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/binary-digit" target = "_blank" > binary-digit< / a > < / p >
< p > < em > binary-literal-characters< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/binary-literal-character" target = "_blank" > binary-literal-character< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/binary-literal-characters" target = "_blank" > binary-literal-characters < / a > < em > opt < / em > < / p >
< p > < em > octal-literal< / em > → < strong > 0o< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/octal-digit" target = "_blank" > octal-digit< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/octal-literal-characters" target = "_blank" > octal-literal-characters < / a > < em > opt < / em > < / p >
< p > < em > octal-digit< / em > → 数字 0 至 7< / p >
< p > < em > octal-literal-character< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/octal-digit" target = "_blank" > octal-digit< / a > < / p >
< p > < em > octal-literal-characters< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/octal-literal-character" target = "_blank" > octal-literal-character< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/octal-literal-characters" target = "_blank" > < / a > < em > opt < / em > < / p >
< p > < em > decimal-literal< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-digit" target = "_blank" > decimal-digit< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-literal-characters" target = "_blank" > < / a > < em > opt < / em > < / p >
< p > < em > decimal-digit< / em > → 数字 0 至 9< / p >
< p > < em > decimal-digits< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-digit" target = "_blank" > decimal-digit < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-digits" target = "_blank" > decimal-digits < / a > < em > opt < / em > < / p >
< p > < em > decimal-literal-character< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-digit" target = "_blank" > decimal-digit < / a > | _ < / p >
< p > < em > decimal-literal-characters< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-literal-character" target = "_blank" > decimal-literal-character< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-literal-characters" target = "_blank" > decimal-literal-characters < / a > < em > opt < / em > < / p >
< p > < em > hexadecimal-literal< / em > → < strong > 0x< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-literal-characters" target = "_blank" > < / a > < em > opt < / em > < / p >
< p > < em > hexadecimal-digit< / em > → 数字 0 到 9, a 到 f, 或 A 到 F< / p >
< p > < em > hexadecimal-literal-character< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit< / a > < / p >
< p > < em > hexadecimal-literal-characters< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-literal-character" target = "_blank" > hexadecimal-literal-character < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-literal-characters" target = "_blank" > hexadecimal-literal-characters< / a > < em > opt < / em > < / p >
< / blockquote >
< h3 id = "-" > 浮点型字面量< / h3 >
< p > 浮点型字面量（< em > floating-point literals< / em > ）表示未指定精度浮点数的值。< / p >
< p > 浮点型字面量默认用十进制表示（无前缀），也可以用十六进制表示（加前缀 < code > 0x< / code > ）。< / p >
< p > 十进制浮点型字面量（< em > decimal floating-point literals< / em > ）由十进制数字串后跟小数部分或指数部分（或两者皆有）组成。十进制小数部分由小数点 < code > .< / code > 后跟十进制数字串组成。指数部分由大写或小写字母 < code > e< / code > 后跟十进制数字串组成，这串数字表示 < code > e< / code > 之前的数量乘以 10 的几次方。例如：< code > 1.25e2< / code > 表示 < code > 1.25 ⨉ 10^2< / code > ，也就是 < code > 125.0< / code > ；同样，< code > 1.25e－ < / code > 表示 < code > 1.25 ⨉ 10^－ < / code > ，也就是 < code > 0.0125< / code > 。< / p >
< p > 十六进制浮点型字面量（< em > hexadecimal floating-point literals< / em > ）由前缀 < code > 0x< / code > 后跟可选的十六进制小数部分以及十六进制指数部分组成。十六进制小数部分由小数点后跟十六进制数字串组成。指数部分由大写或小写字母 < code > p< / code > 后跟十进制数字串组成，这串数字表示 < code > p< / code > 之前的数量乘以 2 的几次方。例如：< code > 0xFp2< / code > 表示 < code > 15 ⨉ 2^2< / code > ，也就是 < code > 60< / code > ；同样，< code > 0xFp-2< / code > 表示 < code > 15 ⨉ 2^-2< / code > ，也就是 < code > 3.75< / code > 。< / p >
< p > 与整型字面量不同，负的浮点型字面量由一元运算符减号 < code > -< / code > 和浮点型字面量组成，例如 < code > -42.0< / code > 。这代表一个表达式，而不是一个浮点整型字面量。< / p >
< p > 允许使用下划线 < code > _< / code > 来增强可读性，下划线不会影响字面量的值。浮点型字面量也可以在数字前加 < code > 0< / code > ，同样不会影响字面量的值。< / p >
< pre > < code > 10_000.56 // 等于 10000.56
005000.76 // 等于 5000.76
< / code > < / pre > < p > 除非特殊指定，浮点型字面量的默认类型为 Swift 标准库类型中的 < code > Double< / code > ， < code > Float< / code > 类型， < / p >
< blockquote >
< p > 浮点型字面量语法< / p >
< p > < em > floating-point-literal< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-literal" target = "_blank" > decimal-literal< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-fraction" target = "_blank" > decimal-fraction < / a > < em > opt< / em > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-exponent" target = "_blank" > decimal-exponent < / a > < em > opt < / em > < / p >
< p > < em > floating-point-literal< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-literal" target = "_blank" > hexadecimal-literal< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-fraction" target = "_blank" > hexadecimal-fraction< / a > < em > opt < / em > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-exponent" target = "_blank" > hexadecimal-exponent< / a > < / p >
< p > < em > decimal-fraction< / em > → . < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-literal" target = "_blank" > < / a > < / p >
< p > < em > decimal-exponent< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/floating-point-e" target = "_blank" > floating-point-e< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/sign" target = "_blank" > < / a > < em > opt < / em > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/decimal-literal" target = "_blank" > decimal-literal< / a > < / p >
< p > < em > hexadecimal-fraction< / em > → . < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-literal" target = "_blank" > < / a > < em > opt < / em > < / p >
< p > < em > hexadecimal-exponent< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/floating-point-p" target = "_blank" > floating-point-p < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/sign" target = "_blank" > sign < / a > < em > opt < / em > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-literal" target = "_blank" > hexadecimal-literal < / a > < / p >
< p > < em > floating-point-e< / em > → < strong > e < / strong > | < strong > E< / strong > < / p >
< p > < em > floating-point-p< / em > → < strong > p< / strong > < strong > P< / strong > < / p >
< p > < em > sign< / em > → < strong > +< / strong > < strong > -< / strong > < / p >
< / blockquote >
< h3 id = "-" > 文本型字面量< / h3 >
< p > 文本型字面量（< em > string literal< / em > ）由双引号中的字符串组成，形式如下：< / p >
< pre > < code > " characters"
< / code > < / pre > < p > 文本型字面量中不能包含未转义的双引号 < code > " < / code > 、未转义的反斜线< code > \< / code > 、回车符（< em > carriage return< / em > ）或换行符（< em > line feed< / em > ）。< / p >
< p > 可以在文本型字面量中使用的转义特殊符号如下：< / p >
< ul >
< li > 空字符（ ） < code > \0< / code > < / li >
< li > 反斜线（ ） < code > \\< / code > < / li >
< li > 水平 Tab （ ） < code > \t< / code > < / li >
< li > 换行符（ ） < code > \n< / code > < / li >
< li > 回车符（ ） < code > \r< / code > < / li >
< li > 双引号（ ） < code > \" < / code > < / li >
< li > 单引号（ ） < code > \' < / code > < / li >
< / ul >
< p > 字符也可以用以下方式表示：< / p >
< ul >
< li > < code > \x< / code > 后跟两位十六进制数字< / li >
< li > < code > \u< / code > 后跟四位十六进制数字< / li >
< li > < code > \U< / code > 后跟八位十六进制数字< / li >
< / ul >
< p > 后跟的数字表示一个 Unicode 码点。< / p >
< p > 文本型字面量允许在反斜线小括号 < code > \()< / code > 中插入表达式的值。插入表达式（< em > interpolated expression< / em > ）不能包含未转义的双引号 < code > " < / code > 、反斜线 < code > \< / code > 、回车符或者换行符。表达式值的类型必须在 < em > String< / em > 类中有对应的初始化方法。< / p >
< p > 例如，以下所有文本型字面量的值相同：< / p >
< pre > < code > " 1 2 3"
" 1 2 \(3)"
" 1 2 \(1 + 2)"
var x = 3; " 1 2 \(x)"
< / code > < / pre > < p > 文本型字面量的默认类型为 < code > String< / code > 。组成字符串的字符类型为 < code > Character< / code > 。更多有关 < code > String< / code > 和 < code > Character< / code > 的信息请参照 < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/StringsAndCharacters.html#//apple_ref/doc/uid/TP40014097-CH7-XID_368" target = "_blank" > 字符串和字符< / a > 。< / p >
< blockquote >
< p > 文本型字面量语法< / p >
< p > < em > string-literal< / em > → < strong > " < / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/quoted-text" target = "_blank" > quoted-text< / a > < strong > " < / strong > < / p >
< p > < em > quoted-text< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/quoted-text-item" target = "_blank" > quoted-text-item < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/quoted-text" target = "_blank" > quoted-text< / a > < em > opt < / em > < / p >
< p > < em > quoted-text-item< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/escaped-character" target = "_blank" > escaped-character < / a > < / p >
< p > < em > quoted-text-item< / em > → < strong > (< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Expressions.html#//apple_ref/swift/grammar/expression" target = "_blank" > expression < / a > < strong > )< / strong > < / p >
< p > < em > quoted-text-item< / em > → 除 < code > " < / code > 、< code > \< / code > 、< code > U+000A< / code > 或 < code > U+000D< / code > 以外的任何 Unicode 扩展字符集< / p >
< p > < em > escaped-character< / em > → < strong > \0 < / strong > | < strong > \< / strong > < strong > \t < / strong > | < strong > \n< / strong > < strong > \r < / strong > | < strong > \" < / strong > | < strong > \' < / strong > < / p >
< p > < em > escaped-character< / em > → < strong > \x < / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < / p >
< p > < em > escaped-character< / em > → < strong > \u< / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit< / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < / p >
< p > < em > escaped-character< / em > → < strong > \U < / strong > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/hexadecimal-digit" target = "_blank" > hexadecimal-digit < / a > < / p >
< / blockquote >
< h2 id = "-" > 运算符< / h2 >
< p > Swift 标准库定义了许多可供使用的运算符，其中大部分在 < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/BasicOperators.html#//apple_ref/doc/uid/TP40014097-CH6-XID_70" target = "_blank" > 基础运算符< / a > 和 < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/AdvancedOperators.html#//apple_ref/doc/uid/TP40014097-CH27-XID_28" target = "_blank" > 高级运算符< / a > 中进行了阐述。这里将描述哪些字符能用作运算符。< / p >
< p > 运算符由一个或多个以下字符组成：
< code > /< / code > 、< code > =< / code > 、< code > -< / code > 、< code > +< / code > 、< code > !< / code > 、< code > *< / code > 、< code > %< / code > 、< code > < < / code > 、< code > > < / code > 、< code > & < / code > 、< code > |< / code > 、< code > ^< / code > 、< code > ~< / code > 、< code > .< / code > 。也就是说，标记 < code > =< / code > , < code > -> < / code > 、< code > //< / code > 、< code > /*< / code > 、< code > */< / code > 、< code > .< / code > 以及一元前缀运算符 < code > & < / code > 属于保留字，这些标记不能被重写或用于自定义运算符。< / p >
< p > 运算符两侧的空白被用来区分该运算符是否为前缀运算符（< em > prefix operator< / em > ）、后缀运算符（< em > postfix operator< / em > ）或二元运算符（< em > binary operator< / em > ）。规则总结如下：< / p >
< ul >
< li > < p > 如果运算符两侧都有空白或两侧都无空白，将被看作二元运算符。例如：< code > a+b< / code > 和 < code > a + b< / code > 中的运算符 < code > +< / code > 被看作二元运算符。< / p >
< / li >
< li > < p > 如果运算符只有左侧空白，将被看作前缀一元运算符。例如 < code > a ++b< / code > 中的 < code > ++< / code > 被看作前缀一元运算符。< / p >
< / li >
< li > < p > 如果运算符只有右侧空白，将被看作后缀一元运算符。例如 < code > a++ b< / code > 中的 < code > ++< / code > 被看作后缀一元运算符。< / p >
< / li >
< li > < p > 如果运算符左侧没有空白并紧跟 < code > .< / code > ，将被看作后缀一元运算符。例如 < code > a++.b< / code > 中的 < code > ++< / code > 被看作后缀一元运算符（同理， < code > a++ . b< / code > 中的 < code > ++< / code > 是后缀一元运算符而 < code > a ++ .b< / code > 中的 < code > ++< / code > 不是）.< / p >
< / li >
< / ul >
< p > 鉴于这些规则，运算符前的字符 < code > (< / code > 、< code > [< / code > 和 < code > {< / code > ；运算符后的字符 < code > )< / code > 、< code > ]< / code > 和 < code > }< / code > 以及字符 < code > ,< / code > 、< code > ;< / code > 和 < code > :< / code > 都将用于空白检测。< / p >
< p > 以上规则需注意一点，如果运算符 < code > !< / code > 或 < code > ?< / code > 左侧没有空白，则不管右侧是否有空白都将被看作后缀运算符。如果将 < code > ?< / code > 用作可选类型（< em > optional type< / em > ）修饰，左侧必须无空白。如果用于条件运算符 < code > ? :< / code > ，必须两侧都有空白。 < / p >
< p > 在特定构成中 ，以 < code > < < / code > 或 < code > > < / code > 开头的运算符会被分离成两个或多个标记，剩余部分以同样的方式会被再次分离。因此，在 < code > Dictionary< String, Array< Int> > < / code > 中没有必要添加空白来消除闭合字符 < code > > < / code > 的歧义。在这个例子中， 闭合字符 < code > > < / code > 被看作单字符标记，而不会被误解为移位运算符 < code > > > < / code > 。< / p >
< p > 要学习如何自定义新的运算符，请参考 < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/AdvancedOperators.html#//apple_ref/doc/uid/TP40014097-CH27-XID_48" target = "_blank" > 自定义操作符< / a > 和 < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Declarations.html#//apple_ref/doc/uid/TP40014097-CH34-XID_644" target = "_blank" > 运算符声明< / a > 。学习如何重写现有运算符，请参考 < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/AdvancedOperators.html#//apple_ref/doc/uid/TP40014097-CH27-XID_43" target = "_blank" > 运算符方法< / a > 。< / p >
< blockquote >
< p > 运算符语法< / p >
< p > < em > operator< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/operator-character" target = "_blank" > operator-character < / a > < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/operator" target = "_blank" > operator < / a > < em > opt < / em > < / p >
< p > < em > operator-character< / em > → < strong > / < / strong > | < strong > = < / strong > | < strong > - < / strong > | < strong > + < / strong > | < strong > ! < / strong > | < strong > * < / strong > | < strong > % < / strong > | < strong > < < / strong > | < strong > > < / strong > | < strong > & < / strong > | < strong > | < / strong > | < strong > ^ < / strong > | < strong > ~ < / strong > | < strong > . < / strong > < / p >
< p > < em > binary-operator< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/operator" target = "_blank" > operator < / a > < / p >
< p > < em > prefix-operator< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/operator" target = "_blank" > operator < / a > < / p >
< p > < em > postfix-operator< / em > → < a href = "https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/LexicalStructure.html#//apple_ref/swift/grammar/operator" target = "_blank" > operator < / a > < / p >
< / blockquote >
< / section >
< / div >
< / div >
< / div >
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