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638 lines
31 KiB
HTML
<!DOCTYPE HTML>
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<title>声明 - Go语言圣经</title>
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<nav id="sidebar" class="sidebar" aria-label="Table of contents">
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<div class="sidebar-scrollbox">
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<ol class="chapter">
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<li class="chapter-item expanded ">
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<a href="../index.html" >Go语言圣经</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../preface-zh.html" >译者序</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../preface.html" >前言</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1.html" ><strong aria-hidden="true">1.</strong> 入门</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-01.html" ><strong aria-hidden="true">1.1.</strong> Hello, World</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-02.html" ><strong aria-hidden="true">1.2.</strong> 命令行参数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-03.html" ><strong aria-hidden="true">1.3.</strong> 查找重复的行</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-04.html" ><strong aria-hidden="true">1.4.</strong> GIF动画</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-05.html" ><strong aria-hidden="true">1.5.</strong> 获取URL</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-06.html" ><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-07.html" ><strong aria-hidden="true">1.7.</strong> Web服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-08.html" ><strong aria-hidden="true">1.8.</strong> 本章要点</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2.html" ><strong aria-hidden="true">2.</strong> 程序结构</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-01.html" ><strong aria-hidden="true">2.1.</strong> 命名</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-02.html" class="active"><strong aria-hidden="true">2.2.</strong> 声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-03.html" ><strong aria-hidden="true">2.3.</strong> 变量</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-04.html" ><strong aria-hidden="true">2.4.</strong> 赋值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-05.html" ><strong aria-hidden="true">2.5.</strong> 类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-06.html" ><strong aria-hidden="true">2.6.</strong> 包和文件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-07.html" ><strong aria-hidden="true">2.7.</strong> 作用域</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3.html" ><strong aria-hidden="true">3.</strong> 基础数据类型</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-01.html" ><strong aria-hidden="true">3.1.</strong> 整型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-02.html" ><strong aria-hidden="true">3.2.</strong> 浮点数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-03.html" ><strong aria-hidden="true">3.3.</strong> 复数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-04.html" ><strong aria-hidden="true">3.4.</strong> 布尔型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-05.html" ><strong aria-hidden="true">3.5.</strong> 字符串</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-06.html" ><strong aria-hidden="true">3.6.</strong> 常量</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4.html" ><strong aria-hidden="true">4.</strong> 复合数据类型</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-01.html" ><strong aria-hidden="true">4.1.</strong> 数组</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-02.html" ><strong aria-hidden="true">4.2.</strong> Slice</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-03.html" ><strong aria-hidden="true">4.3.</strong> Map</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-04.html" ><strong aria-hidden="true">4.4.</strong> 结构体</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-05.html" ><strong aria-hidden="true">4.5.</strong> JSON</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-06.html" ><strong aria-hidden="true">4.6.</strong> 文本和HTML模板</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5.html" ><strong aria-hidden="true">5.</strong> 函数</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-01.html" ><strong aria-hidden="true">5.1.</strong> 函数声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-02.html" ><strong aria-hidden="true">5.2.</strong> 递归</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-03.html" ><strong aria-hidden="true">5.3.</strong> 多返回值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-04.html" ><strong aria-hidden="true">5.4.</strong> 错误</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-05.html" ><strong aria-hidden="true">5.5.</strong> 函数值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-06.html" ><strong aria-hidden="true">5.6.</strong> 匿名函数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-07.html" ><strong aria-hidden="true">5.7.</strong> 可变参数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-08.html" ><strong aria-hidden="true">5.8.</strong> Deferred函数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-09.html" ><strong aria-hidden="true">5.9.</strong> Panic异常</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-10.html" ><strong aria-hidden="true">5.10.</strong> Recover捕获异常</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6.html" ><strong aria-hidden="true">6.</strong> 方法</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-01.html" ><strong aria-hidden="true">6.1.</strong> 方法声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-02.html" ><strong aria-hidden="true">6.2.</strong> 基于指针对象的方法</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-03.html" ><strong aria-hidden="true">6.3.</strong> 通过嵌入结构体来扩展类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-04.html" ><strong aria-hidden="true">6.4.</strong> 方法值和方法表达式</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-05.html" ><strong aria-hidden="true">6.5.</strong> 示例: Bit数组</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-06.html" ><strong aria-hidden="true">6.6.</strong> 封装</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7.html" ><strong aria-hidden="true">7.</strong> 接口</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-01.html" ><strong aria-hidden="true">7.1.</strong> 接口是合约</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-02.html" ><strong aria-hidden="true">7.2.</strong> 接口类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-03.html" ><strong aria-hidden="true">7.3.</strong> 实现接口的条件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-04.html" ><strong aria-hidden="true">7.4.</strong> flag.Value接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-05.html" ><strong aria-hidden="true">7.5.</strong> 接口值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-06.html" ><strong aria-hidden="true">7.6.</strong> sort.Interface接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-07.html" ><strong aria-hidden="true">7.7.</strong> http.Handler接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-08.html" ><strong aria-hidden="true">7.8.</strong> error接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-09.html" ><strong aria-hidden="true">7.9.</strong> 示例: 表达式求值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-10.html" ><strong aria-hidden="true">7.10.</strong> 类型断言</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-11.html" ><strong aria-hidden="true">7.11.</strong> 基于类型断言识别错误类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-12.html" ><strong aria-hidden="true">7.12.</strong> 通过类型断言查询接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-13.html" ><strong aria-hidden="true">7.13.</strong> 类型分支</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-14.html" ><strong aria-hidden="true">7.14.</strong> 示例: 基于标记的XML解码</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-15.html" ><strong aria-hidden="true">7.15.</strong> 补充几点</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8.html" ><strong aria-hidden="true">8.</strong> Goroutines和Channels</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-01.html" ><strong aria-hidden="true">8.1.</strong> Goroutines</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-02.html" ><strong aria-hidden="true">8.2.</strong> 示例: 并发的Clock服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-03.html" ><strong aria-hidden="true">8.3.</strong> 示例: 并发的Echo服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-04.html" ><strong aria-hidden="true">8.4.</strong> Channels</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-05.html" ><strong aria-hidden="true">8.5.</strong> 并发的循环</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-06.html" ><strong aria-hidden="true">8.6.</strong> 示例: 并发的Web爬虫</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-07.html" ><strong aria-hidden="true">8.7.</strong> 基于select的多路复用</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-08.html" ><strong aria-hidden="true">8.8.</strong> 示例: 并发的目录遍历</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-09.html" ><strong aria-hidden="true">8.9.</strong> 并发的退出</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-10.html" ><strong aria-hidden="true">8.10.</strong> 示例: 聊天服务</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9.html" ><strong aria-hidden="true">9.</strong> 基于共享变量的并发</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-01.html" ><strong aria-hidden="true">9.1.</strong> 竞争条件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-02.html" ><strong aria-hidden="true">9.2.</strong> sync.Mutex互斥锁</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-03.html" ><strong aria-hidden="true">9.3.</strong> sync.RWMutex读写锁</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-04.html" ><strong aria-hidden="true">9.4.</strong> 内存同步</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-05.html" ><strong aria-hidden="true">9.5.</strong> sync.Once惰性初始化</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-06.html" ><strong aria-hidden="true">9.6.</strong> 竞争条件检测</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-07.html" ><strong aria-hidden="true">9.7.</strong> 示例: 并发的非阻塞缓存</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-08.html" ><strong aria-hidden="true">9.8.</strong> Goroutines和线程</a>
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</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10.html" ><strong aria-hidden="true">10.</strong> 包和工具</a>
|
||
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|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-01.html" ><strong aria-hidden="true">10.1.</strong> 包简介</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch10/ch10-02.html" ><strong aria-hidden="true">10.2.</strong> 导入路径</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-03.html" ><strong aria-hidden="true">10.3.</strong> 包声明</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch10/ch10-04.html" ><strong aria-hidden="true">10.4.</strong> 导入声明</a>
|
||
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|
||
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|
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<a href="../ch10/ch10-05.html" ><strong aria-hidden="true">10.5.</strong> 包的匿名导入</a>
|
||
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|
||
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|
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<a href="../ch10/ch10-06.html" ><strong aria-hidden="true">10.6.</strong> 包和命名</a>
|
||
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|
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|
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<a href="../ch10/ch10-07.html" ><strong aria-hidden="true">10.7.</strong> 工具</a>
|
||
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|
||
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|
||
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|
||
<a href="../ch11/ch11.html" ><strong aria-hidden="true">11.</strong> 测试</a>
|
||
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||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-01.html" ><strong aria-hidden="true">11.1.</strong> go test</a>
|
||
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|
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<a href="../ch11/ch11-02.html" ><strong aria-hidden="true">11.2.</strong> 测试函数</a>
|
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|
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<li class="chapter-item expanded ">
|
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<a href="../ch11/ch11-03.html" ><strong aria-hidden="true">11.3.</strong> 测试覆盖率</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch11/ch11-04.html" ><strong aria-hidden="true">11.4.</strong> 基准测试</a>
|
||
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|
||
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|
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<a href="../ch11/ch11-05.html" ><strong aria-hidden="true">11.5.</strong> 剖析</a>
|
||
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|
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|
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<a href="../ch11/ch11-06.html" ><strong aria-hidden="true">11.6.</strong> 示例函数</a>
|
||
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<a href="../ch12/ch12.html" ><strong aria-hidden="true">12.</strong> 反射</a>
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<a href="../ch12/ch12-01.html" ><strong aria-hidden="true">12.1.</strong> 为何需要反射?</a>
|
||
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<a href="../ch12/ch12-02.html" ><strong aria-hidden="true">12.2.</strong> reflect.Type和reflect.Value</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch12/ch12-03.html" ><strong aria-hidden="true">12.3.</strong> Display递归打印</a>
|
||
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|
||
<li class="chapter-item expanded ">
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<a href="../ch12/ch12-04.html" ><strong aria-hidden="true">12.4.</strong> 示例: 编码S表达式</a>
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||
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|
||
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<a href="../ch12/ch12-05.html" ><strong aria-hidden="true">12.5.</strong> 通过reflect.Value修改值</a>
|
||
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|
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<a href="../ch12/ch12-06.html" ><strong aria-hidden="true">12.6.</strong> 示例: 解码S表达式</a>
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<a href="../ch12/ch12-07.html" ><strong aria-hidden="true">12.7.</strong> 获取结构体字段标签</a>
|
||
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|
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<a href="../ch12/ch12-08.html" ><strong aria-hidden="true">12.8.</strong> 显示一个类型的方法集</a>
|
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<a href="../ch12/ch12-09.html" ><strong aria-hidden="true">12.9.</strong> 几点忠告</a>
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<a href="../ch13/ch13.html" ><strong aria-hidden="true">13.</strong> 底层编程</a>
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<a href="../ch13/ch13-01.html" ><strong aria-hidden="true">13.1.</strong> unsafe.Sizeof, Alignof 和 Offsetof</a>
|
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<a href="../ch13/ch13-02.html" ><strong aria-hidden="true">13.2.</strong> unsafe.Pointer</a>
|
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<a href="../ch13/ch13-03.html" ><strong aria-hidden="true">13.3.</strong> 示例: 深度相等判断</a>
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||
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|
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<a href="../ch13/ch13-04.html" ><strong aria-hidden="true">13.4.</strong> 通过cgo调用C代码</a>
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<a href="../ch13/ch13-05.html" ><strong aria-hidden="true">13.5.</strong> 几点忠告</a>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../appendix/appendix-a-errata.html" ><strong aria-hidden="true">14.1.</strong> 附录A：原文勘误</a>
|
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|
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<a href="../appendix/appendix-b-author.html" ><strong aria-hidden="true">14.2.</strong> 附录B：作者译者</a>
|
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|
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<a href="../appendix/appendix-c-cpoyright.html" ><strong aria-hidden="true">14.3.</strong> 附录C：译文授权</a>
|
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|
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<a href="../appendix/appendix-d-translations.html" ><strong aria-hidden="true">14.4.</strong> 附录D：其它语言</a>
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<ul dir="auto"><li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li><li><em>MnBook(Go语言实现的MD电子书构建工具): <a href="https://github.com/wa-lang/mnbook">https://github.com/wa-lang/mnbook</a></em></li></ul><hr>
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<h2>2.2. 声明</h2>
|
||
<p>声明语句定义了程序的各种实体对象以及部分或全部的属性。Go语言主要有四种类型的声明语句：var、const、type和func，分别对应变量、常量、类型和函数实体对象的声明。这一章我们重点讨论变量和类型的声明，第三章将讨论常量的声明，第五章将讨论函数的声明。</p>
|
||
<p>一个Go语言编写的程序对应一个或多个以.go为文件后缀名的源文件。每个源文件中以包的声明语句开始，说明该源文件是属于哪个包。包声明语句之后是import语句导入依赖的其它包，然后是包一级的类型、变量、常量、函数的声明语句，包一级的各种类型的声明语句的顺序无关紧要（译注：函数内部的名字则必须先声明之后才能使用）。例如，下面的例子中声明了一个常量、一个函数和两个变量：</p>
|
||
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch2/boiling<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
|
||
<pre><code class="language-Go">// Boiling prints the boiling point of water.
|
||
package main
|
||
|
||
import "fmt"
|
||
|
||
const boilingF = 212.0
|
||
|
||
func main() {
|
||
var f = boilingF
|
||
var c = (f - 32) * 5 / 9
|
||
fmt.Printf("boiling point = %g°F or %g°C\n", f, c)
|
||
// Output:
|
||
// boiling point = 212°F or 100°C
|
||
}
|
||
</code></pre>
|
||
<p>其中常量boilingF是在包一级范围声明语句声明的，然后f和c两个变量是在main函数内部声明的声明语句声明的。在包一级声明语句声明的名字可在整个包对应的每个源文件中访问，而不是仅仅在其声明语句所在的源文件中访问。相比之下，局部声明的名字就只能在函数内部很小的范围被访问。</p>
|
||
<p>一个函数的声明由一个函数名字、参数列表（由函数的调用者提供参数变量的具体值）、一个可选的返回值列表和包含函数定义的函数体组成。如果函数没有返回值，那么返回值列表是省略的。执行函数从函数的第一个语句开始，依次顺序执行直到遇到return返回语句，如果没有返回语句则是执行到函数末尾，然后返回到函数调用者。</p>
|
||
<p>我们已经看到过很多函数声明和函数调用的例子了，在第五章将深入讨论函数的相关细节，这里只简单解释下。下面的fToC函数封装了温度转换的处理逻辑，这样它只需要被定义一次，就可以在多个地方多次被使用。在这个例子中，main函数就调用了两次fToC函数，分别使用在局部定义的两个常量作为调用函数的参数。</p>
|
||
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch2/ftoc<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
|
||
<pre><code class="language-Go">// Ftoc prints two Fahrenheit-to-Celsius conversions.
|
||
package main
|
||
|
||
import "fmt"
|
||
|
||
func main() {
|
||
const freezingF, boilingF = 32.0, 212.0
|
||
fmt.Printf("%g°F = %g°C\n", freezingF, fToC(freezingF)) // "32°F = 0°C"
|
||
fmt.Printf("%g°F = %g°C\n", boilingF, fToC(boilingF)) // "212°F = 100°C"
|
||
}
|
||
|
||
func fToC(f float64) float64 {
|
||
return (f - 32) * 5 / 9
|
||
}
|
||
</code></pre>
|
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