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<i class="fa fa-check"></i> <b>1.</b> 欢迎使用 Swift
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<i class="fa fa-check"></i> <b>1.1.</b> 关于 Swift
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<i class="fa fa-check"></i> <b>1.2.</b> Swift 初见
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<i class="fa fa-check"></i> <b>2.</b> Swift 教程
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<i class="fa fa-check"></i> <b>2.1.</b> 基础部分
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<a href="../chapter2/02_Basic_Operators.html">
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<i class="fa fa-check"></i> <b>2.2.</b> 基本运算符
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<a href="../chapter2/03_Strings_and_Characters.html">
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<i class="fa fa-check"></i> <b>2.3.</b> 字符串和字符
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</a>
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<li class="chapter " data-level="2.4" data-path="chapter2/04_Collection_Types.html">
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<a href="../chapter2/04_Collection_Types.html">
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<i class="fa fa-check"></i> <b>2.4.</b> 集合类型
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</a>
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<li class="chapter " data-level="2.5" data-path="chapter2/05_Control_Flow.html">
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<a href="../chapter2/05_Control_Flow.html">
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<i class="fa fa-check"></i> <b>2.5.</b> 控制流
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</a>
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<li class="chapter " data-level="2.6" data-path="chapter2/06_Functions.html">
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<a href="../chapter2/06_Functions.html">
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<i class="fa fa-check"></i> <b>2.6.</b> 函数
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</a>
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<li class="chapter " data-level="2.7" data-path="chapter2/07_Closures.html">
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<a href="../chapter2/07_Closures.html">
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<i class="fa fa-check"></i> <b>2.7.</b> 闭包
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</a>
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<li class="chapter " data-level="2.8" data-path="chapter2/08_Enumerations.html">
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<a href="../chapter2/08_Enumerations.html">
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<i class="fa fa-check"></i> <b>2.8.</b> 枚举
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</a>
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<li class="chapter " data-level="2.9" data-path="chapter2/09_Classes_and_Structures.html">
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<a href="../chapter2/09_Classes_and_Structures.html">
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<i class="fa fa-check"></i> <b>2.9.</b> 类和结构体
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</a>
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</li>
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<li class="chapter " data-level="2.10" data-path="chapter2/10_Properties.html">
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<a href="../chapter2/10_Properties.html">
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<i class="fa fa-check"></i> <b>2.10.</b> 属性
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</a>
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</li>
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<li class="chapter " data-level="2.11" data-path="chapter2/11_Methods.html">
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<a href="../chapter2/11_Methods.html">
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<i class="fa fa-check"></i> <b>2.11.</b> 方法
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</a>
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</li>
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<li class="chapter " data-level="2.12" data-path="chapter2/12_Subscripts.html">
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<a href="../chapter2/12_Subscripts.html">
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<i class="fa fa-check"></i> <b>2.12.</b> 附属脚本
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</a>
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</li>
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<li class="chapter " data-level="2.13" data-path="chapter2/13_Inheritance.html">
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<a href="../chapter2/13_Inheritance.html">
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<i class="fa fa-check"></i> <b>2.13.</b> 继承
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</a>
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</li>
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<li class="chapter " data-level="2.14" data-path="chapter2/14_Initialization.html">
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<a href="../chapter2/14_Initialization.html">
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<i class="fa fa-check"></i> <b>2.14.</b> 构造过程
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</a>
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</li>
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<li class="chapter " data-level="2.15" data-path="chapter2/15_Deinitialization.html">
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<a href="../chapter2/15_Deinitialization.html">
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<i class="fa fa-check"></i> <b>2.15.</b> 析构过程
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</a>
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</li>
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<li class="chapter " data-level="2.16" data-path="chapter2/16_Automatic_Reference_Counting.html">
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<a href="../chapter2/16_Automatic_Reference_Counting.html">
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<i class="fa fa-check"></i> <b>2.16.</b> 自动引用计数
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</a>
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</li>
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<li class="chapter " data-level="2.17" data-path="chapter2/17_Optional_Chaining.html">
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<a href="../chapter2/17_Optional_Chaining.html">
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<i class="fa fa-check"></i> <b>2.17.</b> 可选链
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</a>
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</li>
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<li class="chapter " data-level="2.18" data-path="chapter2/18_Type_Casting.html">
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<a href="../chapter2/18_Type_Casting.html">
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<i class="fa fa-check"></i> <b>2.18.</b> 类型检查
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</a>
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</li>
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<li class="chapter " data-level="2.19" data-path="chapter2/19_Nested_Types.html">
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<a href="../chapter2/19_Nested_Types.html">
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<i class="fa fa-check"></i> <b>2.19.</b> 类型嵌套
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</a>
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</li>
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<li class="chapter " data-level="2.20" data-path="chapter2/20_Extensions.html">
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<a href="../chapter2/20_Extensions.html">
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<i class="fa fa-check"></i> <b>2.20.</b> 扩展
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</a>
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||
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</li>
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<li class="chapter " data-level="2.21" data-path="chapter2/21_Protocols.html">
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<a href="../chapter2/21_Protocols.html">
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<i class="fa fa-check"></i> <b>2.21.</b> 协议
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||
</a>
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||
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</li>
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<li class="chapter " data-level="2.22" data-path="chapter2/22_Generics.html">
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<a href="../chapter2/22_Generics.html">
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<i class="fa fa-check"></i> <b>2.22.</b> 泛型
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</a>
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</li>
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<li class="chapter " data-level="2.23" data-path="chapter2/23_Advanced_Operators.html">
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<a href="../chapter2/23_Advanced_Operators.html">
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<i class="fa fa-check"></i> <b>2.23.</b> 高级操作符
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</a>
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</li>
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</ul>
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</li>
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<li class="chapter " data-level="3" data-path="chapter3/chapter3.html">
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<a href="../chapter3/chapter3.html">
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<i class="fa fa-check"></i> <b>3.</b> 语言参考
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</a>
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<ul class="articles">
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<li class="chapter " data-level="3.1" data-path="chapter3/01_About_the_Language_Reference.html">
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<a href="../chapter3/01_About_the_Language_Reference.html">
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<i class="fa fa-check"></i> <b>3.1.</b> 关于语言参考
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</a>
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</li>
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<li class="chapter " data-level="3.2" data-path="chapter3/02_Lexical_Structure.html">
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<a href="../chapter3/02_Lexical_Structure.html">
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<i class="fa fa-check"></i> <b>3.2.</b> 词法结构
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</a>
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</li>
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<li class="chapter " data-level="3.3" data-path="chapter3/03_Types.html">
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<a href="../chapter3/03_Types.html">
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<i class="fa fa-check"></i> <b>3.3.</b> 类型
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</a>
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||
|
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</li>
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<li class="chapter " data-level="3.4" data-path="chapter3/04_Expressions.html">
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<a href="../chapter3/04_Expressions.html">
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<i class="fa fa-check"></i> <b>3.4.</b> 表达式
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</a>
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||
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</li>
|
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|
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<li class="chapter " data-level="3.5" data-path="chapter3/10_Statements.html">
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<a href="../chapter3/10_Statements.html">
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<i class="fa fa-check"></i> <b>3.5.</b> 语句
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</a>
|
||
|
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|
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</li>
|
||
|
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<li class="chapter " data-level="3.6" data-path="chapter3/05_Declarations.html">
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<a href="../chapter3/05_Declarations.html">
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<i class="fa fa-check"></i> <b>3.6.</b> 声明
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</a>
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|
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</li>
|
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|
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<li class="chapter " data-level="3.7" data-path="chapter3/06_Attributes.html">
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<a href="../chapter3/06_Attributes.html">
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<i class="fa fa-check"></i> <b>3.7.</b> 特性
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</a>
|
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|
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|
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</li>
|
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|
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<li class="chapter " data-level="3.8" data-path="chapter3/07_Patterns.html">
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<a href="../chapter3/07_Patterns.html">
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<i class="fa fa-check"></i> <b>3.8.</b> 模式
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</a>
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|
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</li>
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|
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<li class="chapter " data-level="3.9" data-path="chapter3/08_Generic_Parameters_and_Arguments.html">
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<a href="../chapter3/08_Generic_Parameters_and_Arguments.html">
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<i class="fa fa-check"></i> <b>3.9.</b> 泛型参数
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</a>
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</li>
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|
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<li class="chapter " data-level="3.10" data-path="chapter3/09_Summary_of_the_Grammar.html">
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<a href="../chapter3/09_Summary_of_the_Grammar.html">
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<i class="fa fa-check"></i> <b>3.10.</b> 语法总结
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</a>
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|
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</li>
|
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|
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</ul>
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<div class="page-inner">
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<section class="normal" id="section-gitbook_179">
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<blockquote>
|
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<p>翻译：fd5788</p>
|
||
<p>校对：yankuangshi</p>
|
||
</blockquote>
|
||
<h1 id="-">泛型参数</h1>
|
||
<hr>
|
||
<p>本页包含内容：</p>
|
||
<ul>
|
||
<li><a href="#generic_parameter">泛型形参语句</a></li>
|
||
<li><a href="#generic_argument">泛型实参语句</a></li>
|
||
</ul>
|
||
<p>本节涉及泛型类型、泛型函数以及泛型构造器的参数，包括形参和实参。声明泛型类型、函数或构造器时，须指定相应的类型参数。类型参数相当于一个占位符，当实例化泛型类型、调用泛型函数或泛型构造器时，就用具体的类型实参替代之。</p>
|
||
<p>关于 Swift 语言的泛型概述，见<a href="../charpter2/22_Generics.html">泛型</a>(第二部分第22章)。</p>
|
||
<h2 id="-a-name-generic_parameter-a-"><a name="generic_parameter">泛型形参语句</a></h2>
|
||
<p>泛型形参语句指定泛型类型或函数的类型形参，以及这些参数的关联约束和要求。泛型形参语句用尖括号（<>）包住，并且有以下两种形式：</p>
|
||
<pre><code><generic parameter list>
|
||
<generic parameter list where requirements >
|
||
</code></pre><p>泛型形参列表中泛型形参用逗号分开，每一个采用以下形式：</p>
|
||
<pre><code>type parameter : constrain
|
||
</code></pre><p>泛型形参由两部分组成：类型形参及其后的可选约束。类型形参只是占位符类型（如T，U，V，KeyType，ValueType等）的名字而已。你可以在泛型类型、函数的其余部分或者构造器声明，以及函数或构造器的签名中使用它。</p>
|
||
<p>约束用于指明该类型形参继承自某个类或者遵守某个协议或协议的一部分。例如，在下面的泛型中，泛型形参<code>T: Comparable</code>表示任何用于替代类型形参<code>T</code>的类型实参必须满足<code>Comparable</code>协议。</p>
|
||
<pre><code>func simpleMin<T: COmparable>(x: T, y: T) -> T {
|
||
if x < y {
|
||
return y
|
||
}
|
||
return x
|
||
}
|
||
</code></pre><p>如，<code>Int</code>和<code>Double</code>均满足<code>Comparable</code>协议，该函数接受任何一种类型。与泛型类型相反，调用泛型函数或构造器时不需要指定泛型实参语句。类型实参由传递给函数或构造器的实参推断而出。</p>
|
||
<pre><code>simpleMin(17, 42) // T is inferred to be Int
|
||
simpleMin(3.14159, 2.71828) // T is inferred to be Double
|
||
</code></pre><h2 id="where-">Where语句</h2>
|
||
<p>要想对类型形参及其关联类型指定额外要求，可以在泛型形参列表之后添加<code>where</code>语句。<code>where</code>语句由关键字<code>where</code>及其后的用逗号分割的多个要求组成。</p>
|
||
<p><code>where</code>语句中的要求用于指明该类型形参继承自某个类或遵守某个协议或协议的一部分。尽管<code>where</code>语句有助于表达类型形参上的简单约束（如<code>T: Comparable</code>等同于<code>T where T: Comparable</code>，等等），但是依然可以用来对类型形参及其关联约束提供更复杂的约束。如，<code><T where T: C, T: P></code>表示泛型类型<code>T</code>继承自类<code>C</code>且遵守协议<code>P</code>。</p>
|
||
<p>如上所述，可以强制约束类型形参的关联类型遵守某个协议。<code><T: Generator where T.Element: Equatable></code>表示<code>T</code>遵守<code>Generator</code>协议，而且<code>T</code>的关联类型<code>T.Element</code>遵守<code>Eauatable</code>协议（<code>T</code>有关联类型是因为<code>Generator</code>声明了<code>Element</code>，而<code>T</code>遵守<code>Generator</code>协议）。</p>
|
||
<p>也可以用操作符<code>==</code>来指定两个类型等效的要求。例如，有这样一个约束：<code>T</code>和<code>U</code>遵守<code>Generator</code>协议，同时要求它们的关联类型等同，可以这样来表达：<code><T: Generator, U: Generator where T.Element == U.Element></code>。</p>
|
||
<p>当然，替代类型形参的类型实参必须满足所有类型形参所要求的约束和要求。</p>
|
||
<p>泛型函数或构造器可以重载，但在泛型形参语句中的类型形参必须有不同的约束或要求，抑或二者皆不同。当调用重载的泛型函数或构造器时，编译器会用这些约束来决定调用哪个重载函数或构造器。</p>
|
||
<p>泛型类可以生成一个子类，但是这个子类也必须是泛型类。</p>
|
||
<blockquote>
|
||
<p>Grammar of a generic parameter clause</p>
|
||
<p>parameter-clause → <generic-parameter-listrequirement-clause ></p>
|
||
<p>generic-parameter-list → generic-parameter generic-parameter,generic-parameter-list </p>
|
||
<p>generic-parameter → type-name</p>
|
||
<p>generic-parameter → type-name:type-identifier</p>
|
||
<p>generic-parameter → type-name:protocol-composition-type</p>
|
||
<p>requirement-clause → <code>where</code>requirement-list</p>
|
||
<p>requirement-list → requirement requirement,requirement-list</p>
|
||
<p>requirement → conformance-requirement same-type-requirement</p>
|
||
<p>conformance-requirement → type-identifier:type-identifier</p>
|
||
<p>conformance-requirement → type-identifier:protocol-composition-type</p>
|
||
<p>same-type-requirement → type-identifier==type-identifier</p>
|
||
</blockquote>
|
||
<h2 id="-a-name-generic_argument-a-"><a name="generic_argument">泛型实参语句</a></h2>
|
||
<p>泛型实参语句指定<em>泛型类型</em>的类型实参。泛型实参语句用尖括号（<>）包住，形式如下：</p>
|
||
<pre><code>< generic argument list >
|
||
</code></pre><p>泛型实参列表中类型实参有逗号分开。类型实参是实际具体类型的名字，用来替代泛型类型的泛型形参语句中的相应的类型形参。从而得到泛型类型的一个特化版本。如，Swift标准库的泛型字典类型定义如下：</p>
|
||
<pre><code> struct Dictionary<KeyTypel: Hashable, ValueType>: Collection,
|
||
DictionaryLiteralConvertible {
|
||
/* .. */
|
||
}
|
||
</code></pre><p>泛型<code>Dictionary</code>类型的特化版本，<code>Dictionary<String, Int></code>就是用具体的<code>String</code>和<code>Int</code>类型替代泛型类型<code>KeyType: Hashable</code>和<code>ValueType</code>产生的。每一个类型实参必须满足它所替代的泛型形参的所有约束，包括任何<code>where</code>语句所指定的额外的要求。上面的例子中，类型形参<code>KeyType</code>要求满足<code>Hashable</code>协议，因此<code>String</code>也必须满足<code>Hashable</code>协议。</p>
|
||
<p>可以用本身就是泛型类型的特化版本的类型实参替代类型形参（假设已满足合适的约束和要求）。例如，为了生成一个元素类型是整型数组的数组，可以用数组的特化版本<code>Array<Int></code>替代泛型类型<code>Array<T></code>的类型形参<code>T</code>来实现。</p>
|
||
<pre><code>let arrayOfArrays: Array<Array<Int>> = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
|
||
</code></pre><p>如<a href="#generic_parameter">泛型形参语句</a>所述，不能用泛型实参语句来指定泛型函数或构造器的类型实参。</p>
|
||
<blockquote>
|
||
<p>Grammar of a generic argument clause</p>
|
||
<p>generic-argument-clause → <generic-argument-list></p>
|
||
<p>generic-argument-list → generic-argument generic-argument,generic-argument-list</p>
|
||
<p>generic-argument → type</p>
|
||
</blockquote>
|
||
<p>==============================================
|
||
上篇：<a href="07_Patterns.html">模式</a></p>
|
||
<p>下篇：<a href="09_Summary_of_the_Grammar.html">语法总结</a></p>
|
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