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2231 lines
84 KiB
HTML
2231 lines
84 KiB
HTML
<!DOCTYPE HTML>
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<html lang="zh-tw" >
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<head>
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<meta charset="UTF-8">
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<meta http-equiv="X-UA-Compatible" content="IE=edge" />
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<title>常量 | Go语言圣经</title>
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<link rel="next" href="../ch4/ch4.html" />
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<link rel="prev" href="../ch3/ch3-05.html" />
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</head>
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<body>
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<div class="book" data-level="3.6" data-chapter-title="常量" data-filepath="ch3/ch3-06.md" data-basepath=".." data-revision="Thu Dec 31 2015 16:18:40 GMT+0800 (中国标准时间)">
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<div class="book-summary">
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<nav role="navigation">
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<ul class="summary">
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<li class="chapter " data-level="0" data-path="index.html">
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<a href="../index.html">
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<i class="fa fa-check"></i>
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前言
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</a>
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<ul class="articles">
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<li class="chapter " data-level="0.1" data-path="ch0/ch0-01.html">
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<a href="../ch0/ch0-01.html">
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<i class="fa fa-check"></i>
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<b>0.1.</b>
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Go語言起源
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</a>
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</li>
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<li class="chapter " data-level="0.2" data-path="ch0/ch0-02.html">
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<a href="../ch0/ch0-02.html">
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<i class="fa fa-check"></i>
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<b>0.2.</b>
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Go語言項目
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</a>
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</li>
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<li class="chapter " data-level="0.3" data-path="ch0/ch0-03.html">
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<a href="../ch0/ch0-03.html">
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<i class="fa fa-check"></i>
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<b>0.3.</b>
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本書的組織
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</a>
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</li>
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<li class="chapter " data-level="0.4" data-path="ch0/ch0-04.html">
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<a href="../ch0/ch0-04.html">
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<i class="fa fa-check"></i>
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<b>0.4.</b>
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更多的信息
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</a>
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</li>
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<li class="chapter " data-level="0.5" data-path="ch0/ch0-05.html">
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<a href="../ch0/ch0-05.html">
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<i class="fa fa-check"></i>
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<b>0.5.</b>
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致謝
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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="1" data-path="ch1/ch1.html">
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<a href="../ch1/ch1.html">
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<i class="fa fa-check"></i>
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<b>1.</b>
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入門
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</a>
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<ul class="articles">
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<li class="chapter " data-level="1.1" data-path="ch1/ch1-01.html">
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<a href="../ch1/ch1-01.html">
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<i class="fa fa-check"></i>
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<b>1.1.</b>
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Hello, World
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</a>
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</li>
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<li class="chapter " data-level="1.2" data-path="ch1/ch1-02.html">
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<a href="../ch1/ch1-02.html">
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<i class="fa fa-check"></i>
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<b>1.2.</b>
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命令行參數
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</a>
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</li>
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<li class="chapter " data-level="1.3" data-path="ch1/ch1-03.html">
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<a href="../ch1/ch1-03.html">
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<i class="fa fa-check"></i>
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<b>1.3.</b>
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査找重複的行
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</a>
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</li>
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<li class="chapter " data-level="1.4" data-path="ch1/ch1-04.html">
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<a href="../ch1/ch1-04.html">
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<i class="fa fa-check"></i>
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<b>1.4.</b>
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GIF動畵
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</a>
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</li>
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<li class="chapter " data-level="1.5" data-path="ch1/ch1-05.html">
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<a href="../ch1/ch1-05.html">
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<i class="fa fa-check"></i>
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<b>1.5.</b>
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獲取URL
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</a>
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</li>
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<li class="chapter " data-level="1.6" data-path="ch1/ch1-06.html">
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<a href="../ch1/ch1-06.html">
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<i class="fa fa-check"></i>
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<b>1.6.</b>
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併發獲取多個URL
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</a>
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</li>
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<li class="chapter " data-level="1.7" data-path="ch1/ch1-07.html">
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<a href="../ch1/ch1-07.html">
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<i class="fa fa-check"></i>
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<b>1.7.</b>
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Web服務
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</a>
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</li>
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<li class="chapter " data-level="1.8" data-path="ch1/ch1-08.html">
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<a href="../ch1/ch1-08.html">
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<i class="fa fa-check"></i>
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<b>1.8.</b>
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本章要點
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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="2" data-path="ch2/ch2.html">
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<a href="../ch2/ch2.html">
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<i class="fa fa-check"></i>
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<b>2.</b>
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程序結構
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</a>
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<ul class="articles">
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<li class="chapter " data-level="2.1" data-path="ch2/ch2-01.html">
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<a href="../ch2/ch2-01.html">
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<i class="fa fa-check"></i>
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<b>2.1.</b>
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命名
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</a>
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</li>
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<li class="chapter " data-level="2.2" data-path="ch2/ch2-02.html">
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<a href="../ch2/ch2-02.html">
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<i class="fa fa-check"></i>
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<b>2.2.</b>
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聲明
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</a>
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</li>
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<li class="chapter " data-level="2.3" data-path="ch2/ch2-03.html">
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<a href="../ch2/ch2-03.html">
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<i class="fa fa-check"></i>
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<b>2.3.</b>
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變量
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</a>
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</li>
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<li class="chapter " data-level="2.4" data-path="ch2/ch2-04.html">
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<a href="../ch2/ch2-04.html">
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<i class="fa fa-check"></i>
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<b>2.4.</b>
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賦值
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</a>
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</li>
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<li class="chapter " data-level="2.5" data-path="ch2/ch2-05.html">
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<a href="../ch2/ch2-05.html">
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<i class="fa fa-check"></i>
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<b>2.5.</b>
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類型
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</a>
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</li>
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<li class="chapter " data-level="2.6" data-path="ch2/ch2-06.html">
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<a href="../ch2/ch2-06.html">
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<i class="fa fa-check"></i>
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<b>2.6.</b>
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包和文件
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</a>
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</li>
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<li class="chapter " data-level="2.7" data-path="ch2/ch2-07.html">
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<a href="../ch2/ch2-07.html">
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<i class="fa fa-check"></i>
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<b>2.7.</b>
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作用域
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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="ch3/ch3.html">
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<a href="../ch3/ch3.html">
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<i class="fa fa-check"></i>
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<b>3.</b>
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基礎數據類型
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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="ch3/ch3-01.html">
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<a href="../ch3/ch3-01.html">
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<i class="fa fa-check"></i>
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<b>3.1.</b>
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整型
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</a>
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</li>
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<li class="chapter " data-level="3.2" data-path="ch3/ch3-02.html">
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<a href="../ch3/ch3-02.html">
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<i class="fa fa-check"></i>
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<b>3.2.</b>
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浮點數
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</a>
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</li>
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<li class="chapter " data-level="3.3" data-path="ch3/ch3-03.html">
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<a href="../ch3/ch3-03.html">
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<i class="fa fa-check"></i>
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<b>3.3.</b>
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複數
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</a>
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</li>
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<li class="chapter " data-level="3.4" data-path="ch3/ch3-04.html">
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<a href="../ch3/ch3-04.html">
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<i class="fa fa-check"></i>
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<b>3.4.</b>
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布爾型
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</a>
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</li>
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<li class="chapter " data-level="3.5" data-path="ch3/ch3-05.html">
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<a href="../ch3/ch3-05.html">
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<i class="fa fa-check"></i>
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<b>3.5.</b>
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字符串
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</a>
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</li>
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<li class="chapter active" data-level="3.6" data-path="ch3/ch3-06.html">
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<a href="../ch3/ch3-06.html">
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<i class="fa fa-check"></i>
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<b>3.6.</b>
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常量
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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="4" data-path="ch4/ch4.html">
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<a href="../ch4/ch4.html">
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<i class="fa fa-check"></i>
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<b>4.</b>
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複合數據類型
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</a>
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<ul class="articles">
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<li class="chapter " data-level="4.1" data-path="ch4/ch4-01.html">
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<a href="../ch4/ch4-01.html">
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<i class="fa fa-check"></i>
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<b>4.1.</b>
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數組
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</a>
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</li>
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<li class="chapter " data-level="4.2" data-path="ch4/ch4-02.html">
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<a href="../ch4/ch4-02.html">
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<i class="fa fa-check"></i>
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<b>4.2.</b>
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Slice
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</a>
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</li>
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<li class="chapter " data-level="4.3" data-path="ch4/ch4-03.html">
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<a href="../ch4/ch4-03.html">
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<i class="fa fa-check"></i>
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<b>4.3.</b>
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Map
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</a>
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</li>
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<li class="chapter " data-level="4.4" data-path="ch4/ch4-04.html">
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<a href="../ch4/ch4-04.html">
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<i class="fa fa-check"></i>
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<b>4.4.</b>
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結構體
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</a>
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</li>
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<li class="chapter " data-level="4.5" data-path="ch4/ch4-05.html">
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<a href="../ch4/ch4-05.html">
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<i class="fa fa-check"></i>
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<b>4.5.</b>
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JSON
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</a>
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</li>
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<li class="chapter " data-level="4.6" data-path="ch4/ch4-06.html">
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<a href="../ch4/ch4-06.html">
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<i class="fa fa-check"></i>
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<b>4.6.</b>
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文本和HTML模闆
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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="5" data-path="ch5/ch5.html">
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<a href="../ch5/ch5.html">
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<i class="fa fa-check"></i>
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<b>5.</b>
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函數
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</a>
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<ul class="articles">
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<li class="chapter " data-level="5.1" data-path="ch5/ch5-01.html">
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<a href="../ch5/ch5-01.html">
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<i class="fa fa-check"></i>
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<b>5.1.</b>
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函數聲明
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</a>
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</li>
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<li class="chapter " data-level="5.2" data-path="ch5/ch5-02.html">
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<a href="../ch5/ch5-02.html">
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<i class="fa fa-check"></i>
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<b>5.2.</b>
|
|
|
|
遞歸
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="5.3" data-path="ch5/ch5-03.html">
|
|
|
|
|
|
<a href="../ch5/ch5-03.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>5.3.</b>
|
|
|
|
多返迴值
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="5.4" data-path="ch5/ch5-04.html">
|
|
|
|
|
|
<a href="../ch5/ch5-04.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>5.4.</b>
|
|
|
|
錯誤
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="5.5" data-path="ch5/ch5-05.html">
|
|
|
|
|
|
<a href="../ch5/ch5-05.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>5.5.</b>
|
|
|
|
函數值
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="5.6" data-path="ch5/ch5-06.html">
|
|
|
|
|
|
<a href="../ch5/ch5-06.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>5.6.</b>
|
|
|
|
匿名函數
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="5.7" data-path="ch5/ch5-07.html">
|
|
|
|
|
|
<a href="../ch5/ch5-07.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>5.7.</b>
|
|
|
|
可變參數
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="5.8" data-path="ch5/ch5-08.html">
|
|
|
|
|
|
<a href="../ch5/ch5-08.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>5.8.</b>
|
|
|
|
Deferred函數
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="5.9" data-path="ch5/ch5-09.html">
|
|
|
|
|
|
<a href="../ch5/ch5-09.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>5.9.</b>
|
|
|
|
Panic異常
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="5.10" data-path="ch5/ch5-10.html">
|
|
|
|
|
|
<a href="../ch5/ch5-10.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>5.10.</b>
|
|
|
|
Recover捕獲異常
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
</ul>
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="6" data-path="ch6/ch6.html">
|
|
|
|
|
|
<a href="../ch6/ch6.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>6.</b>
|
|
|
|
方法
|
|
</a>
|
|
|
|
|
|
<ul class="articles">
|
|
|
|
|
|
<li class="chapter " data-level="6.1" data-path="ch6/ch6-01.html">
|
|
|
|
|
|
<a href="../ch6/ch6-01.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>6.1.</b>
|
|
|
|
方法聲明
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="6.2" data-path="ch6/ch6-02.html">
|
|
|
|
|
|
<a href="../ch6/ch6-02.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>6.2.</b>
|
|
|
|
基於指針對象的方法
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="6.3" data-path="ch6/ch6-03.html">
|
|
|
|
|
|
<a href="../ch6/ch6-03.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>6.3.</b>
|
|
|
|
通過嵌入結構體來擴展類型
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="6.4" data-path="ch6/ch6-04.html">
|
|
|
|
|
|
<a href="../ch6/ch6-04.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>6.4.</b>
|
|
|
|
方法值和方法表達式
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="6.5" data-path="ch6/ch6-05.html">
|
|
|
|
|
|
<a href="../ch6/ch6-05.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>6.5.</b>
|
|
|
|
示例: Bit數組
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="6.6" data-path="ch6/ch6-06.html">
|
|
|
|
|
|
<a href="../ch6/ch6-06.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>6.6.</b>
|
|
|
|
封裝
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
</ul>
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7" data-path="ch7/ch7.html">
|
|
|
|
|
|
<a href="../ch7/ch7.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.</b>
|
|
|
|
接口
|
|
</a>
|
|
|
|
|
|
<ul class="articles">
|
|
|
|
|
|
<li class="chapter " data-level="7.1" data-path="ch7/ch7-01.html">
|
|
|
|
|
|
<a href="../ch7/ch7-01.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.1.</b>
|
|
|
|
接口是合約
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.2" data-path="ch7/ch7-02.html">
|
|
|
|
|
|
<a href="../ch7/ch7-02.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.2.</b>
|
|
|
|
接口類型
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.3" data-path="ch7/ch7-03.html">
|
|
|
|
|
|
<a href="../ch7/ch7-03.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.3.</b>
|
|
|
|
實現接口的條件
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.4" data-path="ch7/ch7-04.html">
|
|
|
|
|
|
<a href="../ch7/ch7-04.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.4.</b>
|
|
|
|
flag.Value接口
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.5" data-path="ch7/ch7-05.html">
|
|
|
|
|
|
<a href="../ch7/ch7-05.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.5.</b>
|
|
|
|
接口值
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.6" data-path="ch7/ch7-06.html">
|
|
|
|
|
|
<a href="../ch7/ch7-06.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.6.</b>
|
|
|
|
sort.Interface接口
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.7" data-path="ch7/ch7-07.html">
|
|
|
|
|
|
<a href="../ch7/ch7-07.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.7.</b>
|
|
|
|
http.Handler接口
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.8" data-path="ch7/ch7-08.html">
|
|
|
|
|
|
<a href="../ch7/ch7-08.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.8.</b>
|
|
|
|
error接口
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.9" data-path="ch7/ch7-09.html">
|
|
|
|
|
|
<a href="../ch7/ch7-09.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.9.</b>
|
|
|
|
示例: 表達式求值
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.10" data-path="ch7/ch7-10.html">
|
|
|
|
|
|
<a href="../ch7/ch7-10.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.10.</b>
|
|
|
|
類型斷言
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.11" data-path="ch7/ch7-11.html">
|
|
|
|
|
|
<a href="../ch7/ch7-11.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.11.</b>
|
|
|
|
基於類型斷言識别錯誤類型
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.12" data-path="ch7/ch7-12.html">
|
|
|
|
|
|
<a href="../ch7/ch7-12.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.12.</b>
|
|
|
|
通過類型斷言査詢接口
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.13" data-path="ch7/ch7-13.html">
|
|
|
|
|
|
<a href="../ch7/ch7-13.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.13.</b>
|
|
|
|
類型分支
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.14" data-path="ch7/ch7-14.html">
|
|
|
|
|
|
<a href="../ch7/ch7-14.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.14.</b>
|
|
|
|
示例: 基於標記的XML解碼
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="7.15" data-path="ch7/ch7-15.html">
|
|
|
|
|
|
<a href="../ch7/ch7-15.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>7.15.</b>
|
|
|
|
補充幾點
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
</ul>
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8" data-path="ch8/ch8.html">
|
|
|
|
|
|
<a href="../ch8/ch8.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.</b>
|
|
|
|
Goroutines和Channels
|
|
</a>
|
|
|
|
|
|
<ul class="articles">
|
|
|
|
|
|
<li class="chapter " data-level="8.1" data-path="ch8/ch8-01.html">
|
|
|
|
|
|
<a href="../ch8/ch8-01.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.1.</b>
|
|
|
|
Goroutines
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8.2" data-path="ch8/ch8-02.html">
|
|
|
|
|
|
<a href="../ch8/ch8-02.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.2.</b>
|
|
|
|
示例: 併發的Clock服務
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8.3" data-path="ch8/ch8-03.html">
|
|
|
|
|
|
<a href="../ch8/ch8-03.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.3.</b>
|
|
|
|
示例: 併發的Echo服務
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8.4" data-path="ch8/ch8-04.html">
|
|
|
|
|
|
<a href="../ch8/ch8-04.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.4.</b>
|
|
|
|
Channels
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8.5" data-path="ch8/ch8-05.html">
|
|
|
|
|
|
<a href="../ch8/ch8-05.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.5.</b>
|
|
|
|
併行的循環
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8.6" data-path="ch8/ch8-06.html">
|
|
|
|
|
|
<a href="../ch8/ch8-06.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.6.</b>
|
|
|
|
示例: 併發的Web爬蟲
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8.7" data-path="ch8/ch8-07.html">
|
|
|
|
|
|
<a href="../ch8/ch8-07.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.7.</b>
|
|
|
|
基於select的多路複用
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8.8" data-path="ch8/ch8-08.html">
|
|
|
|
|
|
<a href="../ch8/ch8-08.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.8.</b>
|
|
|
|
示例: 併發的字典遍歷
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8.9" data-path="ch8/ch8-09.html">
|
|
|
|
|
|
<a href="../ch8/ch8-09.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.9.</b>
|
|
|
|
併發的退出
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="8.10" data-path="ch8/ch8-10.html">
|
|
|
|
|
|
<a href="../ch8/ch8-10.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>8.10.</b>
|
|
|
|
示例: 聊天服務
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
</ul>
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="9" data-path="ch9/ch9.html">
|
|
|
|
|
|
<a href="../ch9/ch9.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>9.</b>
|
|
|
|
基於共享變量的併發
|
|
</a>
|
|
|
|
|
|
<ul class="articles">
|
|
|
|
|
|
<li class="chapter " data-level="9.1" data-path="ch9/ch9-01.html">
|
|
|
|
|
|
<a href="../ch9/ch9-01.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>9.1.</b>
|
|
|
|
競爭條件
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="9.2" data-path="ch9/ch9-02.html">
|
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|
|
|
|
<a href="../ch9/ch9-02.html">
|
|
|
|
<i class="fa fa-check"></i>
|
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<b>9.2.</b>
|
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sync.Mutex互斥鎖
|
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</a>
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|
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</li>
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<li class="chapter " data-level="9.3" data-path="ch9/ch9-03.html">
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|
|
|
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<a href="../ch9/ch9-03.html">
|
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<i class="fa fa-check"></i>
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<b>9.3.</b>
|
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sync.RWMutex讀寫鎖
|
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</a>
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</li>
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<li class="chapter " data-level="9.4" data-path="ch9/ch9-04.html">
|
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|
<a href="../ch9/ch9-04.html">
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<i class="fa fa-check"></i>
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<b>9.4.</b>
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內存同步
|
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</a>
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</li>
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<li class="chapter " data-level="9.5" data-path="ch9/ch9-05.html">
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<a href="../ch9/ch9-05.html">
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<i class="fa fa-check"></i>
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<b>9.5.</b>
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sync.Once初始化
|
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</a>
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</li>
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<li class="chapter " data-level="9.6" data-path="ch9/ch9-06.html">
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|
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|
<a href="../ch9/ch9-06.html">
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<i class="fa fa-check"></i>
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<b>9.6.</b>
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競爭條件檢測
|
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</a>
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</li>
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<li class="chapter " data-level="9.7" data-path="ch9/ch9-07.html">
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<a href="../ch9/ch9-07.html">
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<i class="fa fa-check"></i>
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<b>9.7.</b>
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示例: 併發的非阻塞緩存
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</a>
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</li>
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<li class="chapter " data-level="9.8" data-path="ch9/ch9-08.html">
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<a href="../ch9/ch9-08.html">
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<i class="fa fa-check"></i>
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<b>9.8.</b>
|
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Goroutines和線程
|
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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="10" data-path="ch10/ch10.html">
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<a href="../ch10/ch10.html">
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<i class="fa fa-check"></i>
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<b>10.</b>
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包和工具
|
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</a>
|
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|
|
<ul class="articles">
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<li class="chapter " data-level="10.1" data-path="ch10/ch10-01.html">
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<a href="../ch10/ch10-01.html">
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<i class="fa fa-check"></i>
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<b>10.1.</b>
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簡介
|
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</a>
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</li>
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<li class="chapter " data-level="10.2" data-path="ch10/ch10-02.html">
|
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<a href="../ch10/ch10-02.html">
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<i class="fa fa-check"></i>
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<b>10.2.</b>
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|
導入路徑
|
|
</a>
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</li>
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<li class="chapter " data-level="10.3" data-path="ch10/ch10-03.html">
|
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<a href="../ch10/ch10-03.html">
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<i class="fa fa-check"></i>
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<b>10.3.</b>
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|
|
包聲明
|
|
</a>
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</li>
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<li class="chapter " data-level="10.4" data-path="ch10/ch10-04.html">
|
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<a href="../ch10/ch10-04.html">
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<i class="fa fa-check"></i>
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<b>10.4.</b>
|
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|
|
導入聲明
|
|
</a>
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</li>
|
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<li class="chapter " data-level="10.5" data-path="ch10/ch10-05.html">
|
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<a href="../ch10/ch10-05.html">
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<i class="fa fa-check"></i>
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<b>10.5.</b>
|
|
|
|
匿名導入
|
|
</a>
|
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</li>
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<li class="chapter " data-level="10.6" data-path="ch10/ch10-06.html">
|
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|
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|
<a href="../ch10/ch10-06.html">
|
|
|
|
<i class="fa fa-check"></i>
|
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<b>10.6.</b>
|
|
|
|
包和命名
|
|
</a>
|
|
|
|
|
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</li>
|
|
|
|
<li class="chapter " data-level="10.7" data-path="ch10/ch10-07.html">
|
|
|
|
|
|
<a href="../ch10/ch10-07.html">
|
|
|
|
<i class="fa fa-check"></i>
|
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<b>10.7.</b>
|
|
|
|
工具
|
|
</a>
|
|
|
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|
</li>
|
|
|
|
|
|
</ul>
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="11" data-path="ch11/ch11.html">
|
|
|
|
|
|
<a href="../ch11/ch11.html">
|
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|
|
<i class="fa fa-check"></i>
|
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|
|
<b>11.</b>
|
|
|
|
測試
|
|
</a>
|
|
|
|
|
|
<ul class="articles">
|
|
|
|
|
|
<li class="chapter " data-level="11.1" data-path="ch11/ch11-01.html">
|
|
|
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|
|
<a href="../ch11/ch11-01.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>11.1.</b>
|
|
|
|
go test
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="11.2" data-path="ch11/ch11-02.html">
|
|
|
|
|
|
<a href="../ch11/ch11-02.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>11.2.</b>
|
|
|
|
測試函數
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="11.3" data-path="ch11/ch11-03.html">
|
|
|
|
|
|
<a href="../ch11/ch11-03.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>11.3.</b>
|
|
|
|
測試覆蓋率
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="11.4" data-path="ch11/ch11-04.html">
|
|
|
|
|
|
<a href="../ch11/ch11-04.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>11.4.</b>
|
|
|
|
基準測試
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="11.5" data-path="ch11/ch11-05.html">
|
|
|
|
|
|
<a href="../ch11/ch11-05.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>11.5.</b>
|
|
|
|
剖析
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="11.6" data-path="ch11/ch11-06.html">
|
|
|
|
|
|
<a href="../ch11/ch11-06.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>11.6.</b>
|
|
|
|
示例函數
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
</ul>
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="12" data-path="ch12/ch12.html">
|
|
|
|
|
|
<a href="../ch12/ch12.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.</b>
|
|
|
|
反射
|
|
</a>
|
|
|
|
|
|
<ul class="articles">
|
|
|
|
|
|
<li class="chapter " data-level="12.1" data-path="ch12/ch12-01.html">
|
|
|
|
|
|
<a href="../ch12/ch12-01.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.1.</b>
|
|
|
|
爲何需要反射?
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="12.2" data-path="ch12/ch12-02.html">
|
|
|
|
|
|
<a href="../ch12/ch12-02.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.2.</b>
|
|
|
|
reflect.Type和reflect.Value
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="12.3" data-path="ch12/ch12-03.html">
|
|
|
|
|
|
<a href="../ch12/ch12-03.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.3.</b>
|
|
|
|
Display遞歸打印
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="12.4" data-path="ch12/ch12-04.html">
|
|
|
|
|
|
<a href="../ch12/ch12-04.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.4.</b>
|
|
|
|
示例: 編碼S表達式
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="12.5" data-path="ch12/ch12-05.html">
|
|
|
|
|
|
<a href="../ch12/ch12-05.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.5.</b>
|
|
|
|
通過reflect.Value脩改值
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="12.6" data-path="ch12/ch12-06.html">
|
|
|
|
|
|
<a href="../ch12/ch12-06.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.6.</b>
|
|
|
|
示例: 解碼S表達式
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="12.7" data-path="ch12/ch12-07.html">
|
|
|
|
|
|
<a href="../ch12/ch12-07.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.7.</b>
|
|
|
|
獲取結構體字段標識
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="12.8" data-path="ch12/ch12-08.html">
|
|
|
|
|
|
<a href="../ch12/ch12-08.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.8.</b>
|
|
|
|
顯示一個類型的方法集
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="12.9" data-path="ch12/ch12-09.html">
|
|
|
|
|
|
<a href="../ch12/ch12-09.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>12.9.</b>
|
|
|
|
幾點忠告
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
</ul>
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="13" data-path="ch13/ch13.html">
|
|
|
|
|
|
<a href="../ch13/ch13.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>13.</b>
|
|
|
|
底層編程
|
|
</a>
|
|
|
|
|
|
<ul class="articles">
|
|
|
|
|
|
<li class="chapter " data-level="13.1" data-path="ch13/ch13-01.html">
|
|
|
|
|
|
<a href="../ch13/ch13-01.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>13.1.</b>
|
|
|
|
unsafe.Sizeof, Alignof 和 Offsetof
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="13.2" data-path="ch13/ch13-02.html">
|
|
|
|
|
|
<a href="../ch13/ch13-02.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>13.2.</b>
|
|
|
|
unsafe.Pointer
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="13.3" data-path="ch13/ch13-03.html">
|
|
|
|
|
|
<a href="../ch13/ch13-03.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>13.3.</b>
|
|
|
|
示例: 深度相等判斷
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="13.4" data-path="ch13/ch13-04.html">
|
|
|
|
|
|
<a href="../ch13/ch13-04.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>13.4.</b>
|
|
|
|
通過cgo調用C代碼
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="13.5" data-path="ch13/ch13-05.html">
|
|
|
|
|
|
<a href="../ch13/ch13-05.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>13.5.</b>
|
|
|
|
幾點忠告
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
</ul>
|
|
|
|
</li>
|
|
|
|
<li class="chapter " data-level="14" data-path="CONTRIBUTORS.html">
|
|
|
|
|
|
<a href="../CONTRIBUTORS.html">
|
|
|
|
<i class="fa fa-check"></i>
|
|
|
|
<b>14.</b>
|
|
|
|
附録
|
|
</a>
|
|
|
|
|
|
</li>
|
|
|
|
|
|
|
|
|
|
<li class="divider"></li>
|
|
<li>
|
|
<a href="https://www.gitbook.com" target="blank" class="gitbook-link">
|
|
本書使用 GitBook 釋出
|
|
</a>
|
|
</li>
|
|
|
|
</ul>
|
|
</nav>
|
|
</div>
|
|
|
|
<div class="book-body">
|
|
<div class="body-inner">
|
|
<div class="book-header" role="navigation">
|
|
<!-- Actions Left -->
|
|
|
|
|
|
<!-- Title -->
|
|
<h1>
|
|
<i class="fa fa-circle-o-notch fa-spin"></i>
|
|
<a href="../" >Go语言圣经</a>
|
|
</h1>
|
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</div>
|
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|
<div class="page-wrapper" tabindex="-1" role="main">
|
|
<div class="page-inner">
|
|
|
|
|
|
<section class="normal" id="section-">
|
|
|
|
<h2 id="36-常量">3.6. 常量</h2>
|
|
<p>常量表達式的值在編譯期計算，而不是在運行期。每種常量的潛在類型都是基礎類型：boolean、string或數字。</p>
|
|
<p>一個常量的聲明語句定義了常量的名字，和變量的聲明語法類似，常量的值不可脩改，這樣可以防止在運行期被意外或惡意的脩改。例如，常量比變量更適合用於表達像π之類的數學常數，因爲它們的值不會發生變化：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">const</span> pi = <span class="hljs-number">3.14159</span> <span class="hljs-comment">// approximately; math.Pi is a better approximation</span>
|
|
</code></pre>
|
|
<p>和變量聲明一樣，可以批量聲明多個常量；這比較適合聲明一組相關的常量：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">const</span> (
|
|
e = <span class="hljs-number">2.71828182845904523536028747135266249775724709369995957496696763</span>
|
|
pi = <span class="hljs-number">3.14159265358979323846264338327950288419716939937510582097494459</span>
|
|
)
|
|
</code></pre>
|
|
<p>所有常量的運算都可以在編譯期完成，這樣可以減少運行時的工作，也方便其他編譯優化。當操作數是常量時，一些運行時的錯誤也可以在編譯時被發現，例如整數除零、字符串索引越界、任何導致無效浮點數的操作等。</p>
|
|
<p>常量間的所有算術運算、邏輯運算和比較運算的結果也是常量，對常量的類型轉換操作或以下函數調用都是返迴常量結果：len、cap、real、imag、complex和unsafe.Sizeof（§13.1）。</p>
|
|
<p>因爲它們的值是在編譯期就確定的，因此常量可以是構成類型的一部分，例如用於指定數組類型的長度：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">const</span> IPv4Len = <span class="hljs-number">4</span>
|
|
|
|
<span class="hljs-comment">// parseIPv4 parses an IPv4 address (d.d.d.d).</span>
|
|
<span class="hljs-keyword">func</span> parseIPv4(s <span class="hljs-typename">string</span>) IP {
|
|
<span class="hljs-keyword">var</span> p [IPv4Len]<span class="hljs-typename">byte</span>
|
|
<span class="hljs-comment">// ...</span>
|
|
}
|
|
</code></pre>
|
|
<p>一個常量的聲明也可以包含一個類型和一個值，但是如果沒有顯式指明類型，那麽將從右邊的表達式推斷類型。在下面的代碼中，time.Duration是一個命名類型，底層類型是int64，time.Minute是對應類型的常量。下面聲明的兩個常量都是time.Duration類型，可以通過%T參數打印類型信息：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">const</span> noDelay time.Duration = <span class="hljs-number">0</span>
|
|
<span class="hljs-keyword">const</span> timeout = <span class="hljs-number">5</span> * time.Minute
|
|
fmt.Printf(<span class="hljs-string">"%T %[1]v\n"</span>, noDelay) <span class="hljs-comment">// "time.Duration 0"</span>
|
|
fmt.Printf(<span class="hljs-string">"%T %[1]v\n"</span>, timeout) <span class="hljs-comment">// "time.Duration 5m0s</span>
|
|
fmt.Printf(<span class="hljs-string">"%T %[1]v\n"</span>, time.Minute) <span class="hljs-comment">// "time.Duration 1m0s"</span>
|
|
</code></pre>
|
|
<p>如果是批量聲明的常量，除了第一個外其它的常量右邊的初始化表達式都可以省略，如果省略初始化表達式則表示使用前面常量的初始化表達式寫法，對應的常量類型也一樣的。例如：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">const</span> (
|
|
a = <span class="hljs-number">1</span>
|
|
b
|
|
c = <span class="hljs-number">2</span>
|
|
d
|
|
)
|
|
|
|
fmt.Println(a, b, c, d) <span class="hljs-comment">// "1 1 2 2"</span>
|
|
</code></pre>
|
|
<p>如果隻是簡單地複製右邊的常量表達式，其實併沒有太實用的價值。但是它可以帶來其它的特性，那就是iota常量生成器語法。</p>
|
|
<h3 id="361-iota-常量生成器">3.6.1. iota 常量生成器</h3>
|
|
<p>常量聲明可以使用iota常量生成器初始化，它用於生成一組以相似規則初始化的常量，但是不用每行都寫一遍初始化表達式。在一個const聲明語句中，在第一個聲明的常量所在的行，iota將會被置爲0，然後在每一個有常量聲明的行加一。</p>
|
|
<p>下面是來自time包的例子，它首先定義了一個Weekday命名類型，然後爲一週的每天定義了一個常量，從週日0開始。在其它編程語言中，這種類型一般被稱爲枚舉類型。</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">type</span> Weekday <span class="hljs-typename">int</span>
|
|
|
|
<span class="hljs-keyword">const</span> (
|
|
Sunday Weekday = <span class="hljs-constant">iota</span>
|
|
Monday
|
|
Tuesday
|
|
Wednesday
|
|
Thursday
|
|
Friday
|
|
Saturday
|
|
)
|
|
</code></pre>
|
|
<p>週一將對應0，週一爲1，如此等等。</p>
|
|
<p>我們也可以在複雜的常量表達式中使用iota，下面是來自net包的例子，用於給一個無符號整數的最低5bit的每個bit指定一個名字：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">type</span> Flags <span class="hljs-typename">uint</span>
|
|
|
|
<span class="hljs-keyword">const</span> (
|
|
FlagUp Flags = <span class="hljs-number">1</span> << <span class="hljs-constant">iota</span> <span class="hljs-comment">// is up</span>
|
|
FlagBroadcast <span class="hljs-comment">// supports broadcast access capability</span>
|
|
FlagLoopback <span class="hljs-comment">// is a loopback interface</span>
|
|
FlagPointToPoint <span class="hljs-comment">// belongs to a point-to-point link</span>
|
|
FlagMulticast <span class="hljs-comment">// supports multicast access capability</span>
|
|
)
|
|
</code></pre>
|
|
<p>隨着iota的遞增，每個常量對應表達式1 << iota，是連續的2的冪，分别對應一個bit位置。使用這些常量可以用於測試、設置或清除對應的bit位的值：</p>
|
|
<pre><code class="lang-Go">gopl.io/ch3/netflag
|
|
|
|
<span class="hljs-keyword">func</span> IsUp(v Flags) <span class="hljs-typename">bool</span> { <span class="hljs-keyword">return</span> v&FlagUp == FlagUp }
|
|
<span class="hljs-keyword">func</span> TurnDown(v *Flags) { *v &^= FlagUp }
|
|
<span class="hljs-keyword">func</span> SetBroadcast(v *Flags) { *v |= FlagBroadcast }
|
|
<span class="hljs-keyword">func</span> IsCast(v Flags) <span class="hljs-typename">bool</span> { <span class="hljs-keyword">return</span> v&(FlagBroadcast|FlagMulticast) != <span class="hljs-number">0</span> }
|
|
|
|
unc main() {
|
|
<span class="hljs-keyword">var</span> v Flags = FlagMulticast | FlagUp
|
|
fmt.Printf(<span class="hljs-string">"%b %t\n"</span>, v, IsUp(v)) <span class="hljs-comment">// "10001 true"</span>
|
|
TurnDown(&v)
|
|
fmt.Printf(<span class="hljs-string">"%b %t\n"</span>, v, IsUp(v)) <span class="hljs-comment">// "10000 false"</span>
|
|
SetBroadcast(&v)
|
|
fmt.Printf(<span class="hljs-string">"%b %t\n"</span>, v, IsUp(v)) <span class="hljs-comment">// "10010 false"</span>
|
|
fmt.Printf(<span class="hljs-string">"%b %t\n"</span>, v, IsCast(v)) <span class="hljs-comment">// "10010 true"</span>
|
|
}
|
|
</code></pre>
|
|
<p>下面是一個更複雜的例子，每個常量都是1024的冪：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">const</span> (
|
|
_ = <span class="hljs-number">1</span> << (<span class="hljs-number">10</span> * <span class="hljs-constant">iota</span>)
|
|
KiB <span class="hljs-comment">// 1024</span>
|
|
MiB <span class="hljs-comment">// 1048576</span>
|
|
GiB <span class="hljs-comment">// 1073741824</span>
|
|
TiB <span class="hljs-comment">// 1099511627776 (exceeds 1 << 32)</span>
|
|
PiB <span class="hljs-comment">// 1125899906842624</span>
|
|
EiB <span class="hljs-comment">// 1152921504606846976</span>
|
|
ZiB <span class="hljs-comment">// 1180591620717411303424 (exceeds 1 << 64)</span>
|
|
YiB <span class="hljs-comment">// 1208925819614629174706176</span>
|
|
)
|
|
</code></pre>
|
|
<p>不過iota常量生成規則也有其局限性。例如，它併不能用於産生1000的冪（KB、MB等），因爲Go語言併沒有計算冪的運算符。</p>
|
|
<p><strong>練習 3.13：</strong> 編寫KB、MB的常量聲明，然後擴展到YB。</p>
|
|
<h3 id="362-無類型常量">3.6.2. 無類型常量</h3>
|
|
<p>Go語言的常量有個不同尋常之處。雖然一個常量可以有任意有一個確定的基礎類型，例如int或float64，或者是類似time.Duration這樣命名的基礎類型，但是許多常量併沒有一個明確的基礎類型。編譯器爲這些沒有明確的基礎類型的數字常量提供比基礎類型更高精度的算術運算；你可以認爲至少有256bit的運算精度。這里有六種未明確類型的常量類型，分别是無類型的布爾型、無類型的整數、無類型的字符、無類型的浮點數、無類型的複數、無類型的字符串。</p>
|
|
<p>通過延遲明確常量的具體類型，無類型的常量不僅可以提供更高的運算精度，而且可以直接用於更多的表達式而不需要顯式的類型轉換。例如，例子中的ZiB和YiB的值已經超出任何Go語言中整數類型能表達的范圍，但是它們依然是合法的常量，而且可以像下面常量表達式依然有效（譯註：YiB/ZiB是在編譯期計算出來的，併且結果常量是1024，是Go語言int變量能有效表示的）：</p>
|
|
<pre><code class="lang-Go">fmt.Println(YiB/ZiB) <span class="hljs-comment">// "1024"</span>
|
|
</code></pre>
|
|
<p>另一個例子，math.Pi無類型的浮點數常量，可以直接用於任意需要浮點數或複數的地方：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">var</span> x <span class="hljs-typename">float32</span> = math.Pi
|
|
<span class="hljs-keyword">var</span> y <span class="hljs-typename">float64</span> = math.Pi
|
|
<span class="hljs-keyword">var</span> z <span class="hljs-typename">complex128</span> = math.Pi
|
|
</code></pre>
|
|
<p>如果math.Pi被確定爲特定類型，比如float64，那麽結果精度可能會不一樣，同時對於需要float32或complex128類型值的地方則會強製需要一個明確的類型轉換：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">const</span> Pi64 <span class="hljs-typename">float64</span> = math.Pi
|
|
|
|
<span class="hljs-keyword">var</span> x <span class="hljs-typename">float32</span> = <span class="hljs-typename">float32</span>(Pi64)
|
|
<span class="hljs-keyword">var</span> y <span class="hljs-typename">float64</span> = Pi64
|
|
<span class="hljs-keyword">var</span> z <span class="hljs-typename">complex128</span> = <span class="hljs-typename">complex128</span>(Pi64)
|
|
</code></pre>
|
|
<p>對於常量面值，不同的寫法可能會對應不同的類型。例如0、0.0、0i和'\u0000'雖然有着相同的常量值，但是它們分别對應無類型的整數、無類型的浮點數、無類型的複數和無類型的字符等不同的常量類型。同樣，true和false也是無類型的布爾類型，字符串面值常量是無類型的字符串類型。</p>
|
|
<p>前面説過除法運算符/會根據操作數的類型生成對應類型的結果。因此，不同寫法的常量除法表達式可能對應不同的結果：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">var</span> f <span class="hljs-typename">float64</span> = <span class="hljs-number">212</span>
|
|
fmt.Println((f - <span class="hljs-number">32</span>) * <span class="hljs-number">5</span> / <span class="hljs-number">9</span>) <span class="hljs-comment">// "100"; (f - 32) * 5 is a float64</span>
|
|
fmt.Println(<span class="hljs-number">5</span> / <span class="hljs-number">9</span> * (f - <span class="hljs-number">32</span>)) <span class="hljs-comment">// "0"; 5/9 is an untyped integer, 0</span>
|
|
fmt.Println(<span class="hljs-number">5.0</span> / <span class="hljs-number">9.0</span> * (f - <span class="hljs-number">32</span>)) <span class="hljs-comment">// "100"; 5.0/9.0 is an untyped float</span>
|
|
</code></pre>
|
|
<p>隻有常量可以是無類型的。當一個無類型的常量被賦值給一個變量的時候，就像上面的第一行語句，或者是像其餘三個語句中右邊表達式中含有明確類型的值，無類型的常量將會被隱式轉換爲對應的類型，如果轉換合法的話。</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">var</span> f <span class="hljs-typename">float64</span> = <span class="hljs-number">3</span> + <span class="hljs-number">0i</span> <span class="hljs-comment">// untyped complex -> float64</span>
|
|
f = <span class="hljs-number">2</span> <span class="hljs-comment">// untyped integer -> float64</span>
|
|
f = <span class="hljs-number">1e123</span> <span class="hljs-comment">// untyped floating-point -> float64</span>
|
|
f = <span class="hljs-string">'a'</span> <span class="hljs-comment">// untyped rune -> float64</span>
|
|
</code></pre>
|
|
<p>上面的語句相當於:</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">var</span> f <span class="hljs-typename">float64</span> = <span class="hljs-typename">float64</span>(<span class="hljs-number">3</span> + <span class="hljs-number">0i</span>)
|
|
f = <span class="hljs-typename">float64</span>(<span class="hljs-number">2</span>)
|
|
f = <span class="hljs-typename">float64</span>(<span class="hljs-number">1e123</span>)
|
|
f = <span class="hljs-typename">float64</span>(<span class="hljs-string">'a'</span>)
|
|
</code></pre>
|
|
<p>無論是隱式或顯式轉換，將一種類型轉換爲另一種類型都要求目標可以表示原始值。對於浮點數和複數，可能會有舍入處理：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">const</span> (
|
|
deadbeef = <span class="hljs-number">0xdeadbeef</span> <span class="hljs-comment">// untyped int with value 3735928559</span>
|
|
a = <span class="hljs-typename">uint32</span>(deadbeef) <span class="hljs-comment">// uint32 with value 3735928559</span>
|
|
b = <span class="hljs-typename">float32</span>(deadbeef) <span class="hljs-comment">// float32 with value 3735928576 (rounded up)</span>
|
|
c = <span class="hljs-typename">float64</span>(deadbeef) <span class="hljs-comment">// float64 with value 3735928559 (exact)</span>
|
|
d = <span class="hljs-typename">int32</span>(deadbeef) <span class="hljs-comment">// compile error: constant overflows int32</span>
|
|
e = <span class="hljs-typename">float64</span>(<span class="hljs-number">1e309</span>) <span class="hljs-comment">// compile error: constant overflows float64</span>
|
|
f = <span class="hljs-typename">uint</span>(-<span class="hljs-number">1</span>) <span class="hljs-comment">// compile error: constant underflows uint</span>
|
|
)
|
|
</code></pre>
|
|
<p>對於一個沒有顯式類型的變量聲明語法（包括短變量聲明語法），無類型的常量會被隱式轉爲默認的變量類型，就像下面的例子：</p>
|
|
<pre><code class="lang-Go">i := <span class="hljs-number">0</span> <span class="hljs-comment">// untyped integer; implicit int(0)</span>
|
|
r := <span class="hljs-string">'\000'</span> <span class="hljs-comment">// untyped rune; implicit rune('\000')</span>
|
|
f := <span class="hljs-number">0.0</span> <span class="hljs-comment">// untyped floating-point; implicit float64(0.0)</span>
|
|
c := <span class="hljs-number">0i</span> <span class="hljs-comment">// untyped complex; implicit complex128(0i)</span>
|
|
</code></pre>
|
|
<p>註意默認類型是規則的：無類型的整數常量默認轉換爲int，對應不確定的內存大小，但是浮點數和複數常量則默認轉換爲float64和complex128。Go語言本身併沒有不確定內存大小的浮點數和複數類型，而且如果不知道浮點數類型的話將很難寫出正確的數值算法。</p>
|
|
<p>如果要給變量一個不同的類型，我們必鬚顯式地將無類型的常量轉化爲所需的類型，或給聲明的變量指定明確的類型，像下面例子這樣：</p>
|
|
<pre><code class="lang-Go"><span class="hljs-keyword">var</span> i = <span class="hljs-typename">int8</span>(<span class="hljs-number">0</span>)
|
|
<span class="hljs-keyword">var</span> i <span class="hljs-typename">int8</span> = <span class="hljs-number">0</span>
|
|
</code></pre>
|
|
<p>當嚐試將這些無類型的常量轉爲一個接口值時（見第7章），這些默認類型將顯得尤爲重要，因爲要靠它們明確接口對應的動態類型。</p>
|
|
<pre><code class="lang-Go">fmt.Printf(<span class="hljs-string">"%T\n"</span>, <span class="hljs-number">0</span>) <span class="hljs-comment">// "int"</span>
|
|
fmt.Printf(<span class="hljs-string">"%T\n"</span>, <span class="hljs-number">0.0</span>) <span class="hljs-comment">// "float64"</span>
|
|
fmt.Printf(<span class="hljs-string">"%T\n"</span>, <span class="hljs-number">0i</span>) <span class="hljs-comment">// "complex128"</span>
|
|
fmt.Printf(<span class="hljs-string">"%T\n"</span>, <span class="hljs-string">'\000'</span>) <span class="hljs-comment">// "int32" (rune)</span>
|
|
</code></pre>
|
|
<p>現在我們已經講述了Go語言中全部的基礎數據類型。下一步將演示如何用基礎數據類型組合成數組或結構體等複雜數據類型，然後構建用於解決實際編程問題的數據結構，這將是第四章的討論主題。</p>
|
|
|
|
|
|
</section>
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