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<title>flag.Value接口 - Go语言圣经</title>
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<a href="../index.html" >Go语言圣经</a>
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<a href="../preface-zh.html" >译者序</a>
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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 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 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" ><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" class="active"><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 ">
|
||
<a href="../ch9/ch9-08.html" ><strong aria-hidden="true">9.8.</strong> Goroutines和线程</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10.html" ><strong aria-hidden="true">10.</strong> 包和工具</a>
|
||
</li>
|
||
<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>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-02.html" ><strong aria-hidden="true">10.2.</strong> 导入路径</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-03.html" ><strong aria-hidden="true">10.3.</strong> 包声明</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-04.html" ><strong aria-hidden="true">10.4.</strong> 导入声明</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-05.html" ><strong aria-hidden="true">10.5.</strong> 包的匿名导入</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-06.html" ><strong aria-hidden="true">10.6.</strong> 包和命名</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-07.html" ><strong aria-hidden="true">10.7.</strong> 工具</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11.html" ><strong aria-hidden="true">11.</strong> 测试</a>
|
||
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|
||
<ol class="section">
|
||
<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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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-02.html" ><strong aria-hidden="true">11.2.</strong> 测试函数</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<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 ">
|
||
<a href="../ch11/ch11-04.html" ><strong aria-hidden="true">11.4.</strong> 基准测试</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-05.html" ><strong aria-hidden="true">11.5.</strong> 剖析</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-06.html" ><strong aria-hidden="true">11.6.</strong> 示例函数</a>
|
||
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|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12.html" ><strong aria-hidden="true">12.</strong> 反射</a>
|
||
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|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-01.html" ><strong aria-hidden="true">12.1.</strong> 为何需要反射?</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-02.html" ><strong aria-hidden="true">12.2.</strong> reflect.Type和reflect.Value</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-03.html" ><strong aria-hidden="true">12.3.</strong> Display递归打印</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-04.html" ><strong aria-hidden="true">12.4.</strong> 示例: 编码S表达式</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-05.html" ><strong aria-hidden="true">12.5.</strong> 通过reflect.Value修改值</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-06.html" ><strong aria-hidden="true">12.6.</strong> 示例: 解码S表达式</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-07.html" ><strong aria-hidden="true">12.7.</strong> 获取结构体字段标签</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-08.html" ><strong aria-hidden="true">12.8.</strong> 显示一个类型的方法集</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-09.html" ><strong aria-hidden="true">12.9.</strong> 几点忠告</a>
|
||
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|
||
</ol>
|
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<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13.html" ><strong aria-hidden="true">13.</strong> 底层编程</a>
|
||
</li>
|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-01.html" ><strong aria-hidden="true">13.1.</strong> unsafe.Sizeof, Alignof 和 Offsetof</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-02.html" ><strong aria-hidden="true">13.2.</strong> unsafe.Pointer</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<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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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-04.html" ><strong aria-hidden="true">13.4.</strong> 通过cgo调用C代码</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-05.html" ><strong aria-hidden="true">13.5.</strong> 几点忠告</a>
|
||
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|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../appendix/appendix.html" ><strong aria-hidden="true">14.</strong> 附录</a>
|
||
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|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<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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|
||
<li class="chapter-item expanded ">
|
||
<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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|
||
<li class="chapter-item expanded ">
|
||
<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>WaBook(Go语言实现的MD电子书构建工具): <a href="https://github.com/wa-lang/wabook">https://github.com/wa-lang/wabook</a></em></li></ul><hr>
|
||
|
||
<h2>7.4. flag.Value接口</h2>
|
||
<p>在本章，我们会学到另一个标准的接口类型flag.Value是怎么帮助命令行标记定义新的符号的。思考下面这个会休眠特定时间的程序：</p>
|
||
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch7/sleep<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
|
||
<pre><code class="language-go">var period = flag.Duration("period", 1*time.Second, "sleep period")
|
||
|
||
func main() {
|
||
flag.Parse()
|
||
fmt.Printf("Sleeping for %v...", *period)
|
||
time.Sleep(*period)
|
||
fmt.Println()
|
||
}
|
||
</code></pre>
|
||
<p>在它休眠前它会打印出休眠的时间周期。fmt包调用time.Duration的String方法打印这个时间周期是以用户友好的注解方式，而不是一个纳秒数字：</p>
|
||
<pre><code>$ go build gopl.io/ch7/sleep
|
||
$ ./sleep
|
||
Sleeping for 1s...
|
||
</code></pre>
|
||
<p>默认情况下，休眠周期是一秒，但是可以通过 -period 这个命令行标记来控制。flag.Duration函数创建一个time.Duration类型的标记变量并且允许用户通过多种用户友好的方式来设置这个变量的大小，这种方式还包括和String方法相同的符号排版形式。这种对称设计使得用户交互良好。</p>
|
||
<pre><code>$ ./sleep -period 50ms
|
||
Sleeping for 50ms...
|
||
$ ./sleep -period 2m30s
|
||
Sleeping for 2m30s...
|
||
$ ./sleep -period 1.5h
|
||
Sleeping for 1h30m0s...
|
||
$ ./sleep -period "1 day"
|
||
invalid value "1 day" for flag -period: time: invalid duration 1 day
|
||
</code></pre>
|
||
<p>因为时间周期标记值非常的有用，所以这个特性被构建到了flag包中；但是我们为我们自己的数据类型定义新的标记符号是简单容易的。我们只需要定义一个实现flag.Value接口的类型，如下：</p>
|
||
<pre><code class="language-go">package flag
|
||
|
||
// Value is the interface to the value stored in a flag.
|
||
type Value interface {
|
||
String() string
|
||
Set(string) error
|
||
}
|
||
</code></pre>
|
||
<p>String方法格式化标记的值用在命令行帮助消息中；这样每一个flag.Value也是一个fmt.Stringer。Set方法解析它的字符串参数并且更新标记变量的值。实际上，Set方法和String是两个相反的操作，所以最好的办法就是对他们使用相同的注解方式。</p>
|
||
<p>让我们定义一个允许通过摄氏度或者华氏温度变换的形式指定温度的celsiusFlag类型。注意celsiusFlag内嵌了一个Celsius类型（§2.5），因此不用实现本身就已经有String方法了。为了实现flag.Value，我们只需要定义Set方法：</p>
|
||
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch7/tempconv<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
|
||
<pre><code class="language-go">// *celsiusFlag satisfies the flag.Value interface.
|
||
type celsiusFlag struct{ Celsius }
|
||
|
||
func (f *celsiusFlag) Set(s string) error {
|
||
var unit string
|
||
var value float64
|
||
fmt.Sscanf(s, "%f%s", &value, &unit) // no error check needed
|
||
switch unit {
|
||
case "C", "°C":
|
||
f.Celsius = Celsius(value)
|
||
return nil
|
||
case "F", "°F":
|
||
f.Celsius = FToC(Fahrenheit(value))
|
||
return nil
|
||
}
|
||
return fmt.Errorf("invalid temperature %q", s)
|
||
}
|
||
</code></pre>
|
||
<p>调用fmt.Sscanf函数从输入s中解析一个浮点数（value）和一个字符串（unit）。虽然通常必须检查Sscanf的错误返回，但是在这个例子中我们不需要。因为如果有错误发生，就没有switch case会匹配到。</p>
|
||
<p>下面的CelsiusFlag函数将所有逻辑都封装在一起。它返回一个内嵌在celsiusFlag变量f中的Celsius指针给调用者。Celsius字段是一个会通过Set方法在标记处理的过程中更新的变量。调用Var方法将标记加入应用的命令行标记集合中，有异常复杂命令行接口的全局变量flag.CommandLine.Programs可能有几个这个类型的变量。调用Var方法将一个<code>*celsiusFlag</code>参数赋值给一个flag.Value参数，导致编译器去检查<code>*celsiusFlag</code>是否有必须的方法。</p>
|
||
<pre><code class="language-go">// CelsiusFlag defines a Celsius flag with the specified name,
|
||
// default value, and usage, and returns the address of the flag variable.
|
||
// The flag argument must have a quantity and a unit, e.g., "100C".
|
||
func CelsiusFlag(name string, value Celsius, usage string) *Celsius {
|
||
f := celsiusFlag{value}
|
||
flag.CommandLine.Var(&f, name, usage)
|
||
return &f.Celsius
|
||
}
|
||
</code></pre>
|
||
<p>现在我们可以开始在我们的程序中使用新的标记：</p>
|
||
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch7/tempflag<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
|
||
<pre><code class="language-go">var temp = tempconv.CelsiusFlag("temp", 20.0, "the temperature")
|
||
|
||
func main() {
|
||
flag.Parse()
|
||
fmt.Println(*temp)
|
||
}
|
||
</code></pre>
|
||
<p>下面是典型的场景：</p>
|
||
<pre><code>$ go build gopl.io/ch7/tempflag
|
||
$ ./tempflag
|
||
20°C
|
||
$ ./tempflag -temp -18C
|
||
-18°C
|
||
$ ./tempflag -temp 212°F
|
||
100°C
|
||
$ ./tempflag -temp 273.15K
|
||
invalid value "273.15K" for flag -temp: invalid temperature "273.15K"
|
||
Usage of ./tempflag:
|
||
-temp value
|
||
the temperature (default 20°C)
|
||
$ ./tempflag -help
|
||
Usage of ./tempflag:
|
||
-temp value
|
||
the temperature (default 20°C)
|
||
</code></pre>
|
||
<p><strong>练习 7.6：</strong> 对tempFlag加入支持开尔文温度。</p>
|
||
<p><strong>练习 7.7：</strong> 解释为什么帮助信息在它的默认值是20.0没有包含°C的情况下输出了°C。</p>
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