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<title>Display递归打印 - Go语言圣经</title>
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<nav id="sidebar" class="sidebar" aria-label="Table of contents">
<div class="sidebar-scrollbox">
<ol class="chapter">
<li class="chapter-item expanded ">
<a href="../index.html" >Go语言圣经</a>
</li>
<li class="chapter-item expanded ">
<a href="../preface-zh.html" >译者序</a>
</li>
<li class="chapter-item expanded ">
<a href="../preface.html" >前言</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch1/ch1.html" ><strong aria-hidden="true">1.</strong> 入门</a>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch1/ch1-01.html" ><strong aria-hidden="true">1.1.</strong> Hello, World</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch1/ch1-02.html" ><strong aria-hidden="true">1.2.</strong> 命令行参数</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch1/ch1-03.html" ><strong aria-hidden="true">1.3.</strong> 查找重复的行</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch1/ch1-04.html" ><strong aria-hidden="true">1.4.</strong> GIF动画</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch1/ch1-05.html" ><strong aria-hidden="true">1.5.</strong> 获取URL</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch1/ch1-06.html" ><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch1/ch1-07.html" ><strong aria-hidden="true">1.7.</strong> Web服务</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch1/ch1-08.html" ><strong aria-hidden="true">1.8.</strong> 本章要点</a>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../ch2/ch2.html" ><strong aria-hidden="true">2.</strong> 程序结构</a>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch2/ch2-01.html" ><strong aria-hidden="true">2.1.</strong> 命名</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch2/ch2-02.html" ><strong aria-hidden="true">2.2.</strong> 声明</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch2/ch2-03.html" ><strong aria-hidden="true">2.3.</strong> 变量</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch2/ch2-04.html" ><strong aria-hidden="true">2.4.</strong> 赋值</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch2/ch2-05.html" ><strong aria-hidden="true">2.5.</strong> 类型</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch2/ch2-06.html" ><strong aria-hidden="true">2.6.</strong> 包和文件</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch2/ch2-07.html" ><strong aria-hidden="true">2.7.</strong> 作用域</a>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../ch3/ch3.html" ><strong aria-hidden="true">3.</strong> 基础数据类型</a>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch3/ch3-01.html" ><strong aria-hidden="true">3.1.</strong> 整型</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch3/ch3-02.html" ><strong aria-hidden="true">3.2.</strong> 浮点数</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch3/ch3-03.html" ><strong aria-hidden="true">3.3.</strong> 复数</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch3/ch3-04.html" ><strong aria-hidden="true">3.4.</strong> 布尔型</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch3/ch3-05.html" ><strong aria-hidden="true">3.5.</strong> 字符串</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch3/ch3-06.html" ><strong aria-hidden="true">3.6.</strong> 常量</a>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../ch4/ch4.html" ><strong aria-hidden="true">4.</strong> 复合数据类型</a>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch4/ch4-01.html" ><strong aria-hidden="true">4.1.</strong> 数组</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch4/ch4-02.html" ><strong aria-hidden="true">4.2.</strong> Slice</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch4/ch4-03.html" ><strong aria-hidden="true">4.3.</strong> Map</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch4/ch4-04.html" ><strong aria-hidden="true">4.4.</strong> 结构体</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch4/ch4-05.html" ><strong aria-hidden="true">4.5.</strong> JSON</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch4/ch4-06.html" ><strong aria-hidden="true">4.6.</strong> 文本和HTML模板</a>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../ch5/ch5.html" ><strong aria-hidden="true">5.</strong> 函数</a>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch5/ch5-01.html" ><strong aria-hidden="true">5.1.</strong> 函数声明</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch5/ch5-02.html" ><strong aria-hidden="true">5.2.</strong> 递归</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch5/ch5-03.html" ><strong aria-hidden="true">5.3.</strong> 多返回值</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch5/ch5-04.html" ><strong aria-hidden="true">5.4.</strong> 错误</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch5/ch5-05.html" ><strong aria-hidden="true">5.5.</strong> 函数值</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch5/ch5-06.html" ><strong aria-hidden="true">5.6.</strong> 匿名函数</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch5/ch5-07.html" ><strong aria-hidden="true">5.7.</strong> 可变参数</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch5/ch5-08.html" ><strong aria-hidden="true">5.8.</strong> Deferred函数</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch5/ch5-09.html" ><strong aria-hidden="true">5.9.</strong> Panic异常</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch5/ch5-10.html" ><strong aria-hidden="true">5.10.</strong> Recover捕获异常</a>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../ch6/ch6.html" ><strong aria-hidden="true">6.</strong> 方法</a>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch6/ch6-01.html" ><strong aria-hidden="true">6.1.</strong> 方法声明</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch6/ch6-02.html" ><strong aria-hidden="true">6.2.</strong> 基于指针对象的方法</a>
</li>
<li class="chapter-item expanded ">
<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>
</li>
<li class="chapter-item expanded ">
<a href="../ch6/ch6-04.html" ><strong aria-hidden="true">6.4.</strong> 方法值和方法表达式</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch6/ch6-05.html" ><strong aria-hidden="true">6.5.</strong> 示例: Bit数组</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch6/ch6-06.html" ><strong aria-hidden="true">6.6.</strong> 封装</a>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../ch7/ch7.html" ><strong aria-hidden="true">7.</strong> 接口</a>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch7/ch7-01.html" ><strong aria-hidden="true">7.1.</strong> 接口是合约</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-02.html" ><strong aria-hidden="true">7.2.</strong> 接口类型</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-03.html" ><strong aria-hidden="true">7.3.</strong> 实现接口的条件</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-04.html" ><strong aria-hidden="true">7.4.</strong> flag.Value接口</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-05.html" ><strong aria-hidden="true">7.5.</strong> 接口值</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-06.html" ><strong aria-hidden="true">7.6.</strong> sort.Interface接口</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-07.html" ><strong aria-hidden="true">7.7.</strong> http.Handler接口</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-08.html" ><strong aria-hidden="true">7.8.</strong> error接口</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-09.html" ><strong aria-hidden="true">7.9.</strong> 示例: 表达式求值</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-10.html" ><strong aria-hidden="true">7.10.</strong> 类型断言</a>
</li>
<li class="chapter-item expanded ">
<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>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-12.html" ><strong aria-hidden="true">7.12.</strong> 通过类型断言查询接口</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-13.html" ><strong aria-hidden="true">7.13.</strong> 类型分支</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-14.html" ><strong aria-hidden="true">7.14.</strong> 示例: 基于标记的XML解码</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch7/ch7-15.html" ><strong aria-hidden="true">7.15.</strong> 补充几点</a>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../ch8/ch8.html" ><strong aria-hidden="true">8.</strong> Goroutines和Channels</a>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch8/ch8-01.html" ><strong aria-hidden="true">8.1.</strong> Goroutines</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch8/ch8-02.html" ><strong aria-hidden="true">8.2.</strong> 示例: 并发的Clock服务</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch8/ch8-03.html" ><strong aria-hidden="true">8.3.</strong> 示例: 并发的Echo服务</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch8/ch8-04.html" ><strong aria-hidden="true">8.4.</strong> Channels</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch8/ch8-05.html" ><strong aria-hidden="true">8.5.</strong> 并发的循环</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch8/ch8-06.html" ><strong aria-hidden="true">8.6.</strong> 示例: 并发的Web爬虫</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch8/ch8-07.html" ><strong aria-hidden="true">8.7.</strong> 基于select的多路复用</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch8/ch8-08.html" ><strong aria-hidden="true">8.8.</strong> 示例: 并发的目录遍历</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch8/ch8-09.html" ><strong aria-hidden="true">8.9.</strong> 并发的退出</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch8/ch8-10.html" ><strong aria-hidden="true">8.10.</strong> 示例: 聊天服务</a>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../ch9/ch9.html" ><strong aria-hidden="true">9.</strong> 基于共享变量的并发</a>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch9/ch9-01.html" ><strong aria-hidden="true">9.1.</strong> 竞争条件</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch9/ch9-02.html" ><strong aria-hidden="true">9.2.</strong> sync.Mutex互斥锁</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch9/ch9-03.html" ><strong aria-hidden="true">9.3.</strong> sync.RWMutex读写锁</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch9/ch9-04.html" ><strong aria-hidden="true">9.4.</strong> 内存同步</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch9/ch9-05.html" ><strong aria-hidden="true">9.5.</strong> sync.Once惰性初始化</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch9/ch9-06.html" ><strong aria-hidden="true">9.6.</strong> 竞争条件检测</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch9/ch9-07.html" ><strong aria-hidden="true">9.7.</strong> 示例: 并发的非阻塞缓存</a>
</li>
<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>
</li>
<ol class="section">
<li class="chapter-item expanded ">
<a href="../ch11/ch11-01.html" ><strong aria-hidden="true">11.1.</strong> go test</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch11/ch11-02.html" ><strong aria-hidden="true">11.2.</strong> 测试函数</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch11/ch11-03.html" ><strong aria-hidden="true">11.3.</strong> 测试覆盖率</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch11/ch11-04.html" ><strong aria-hidden="true">11.4.</strong> 基准测试</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch11/ch11-05.html" ><strong aria-hidden="true">11.5.</strong> 剖析</a>
</li>
<li class="chapter-item expanded ">
<a href="../ch11/ch11-06.html" ><strong aria-hidden="true">11.6.</strong> 示例函数</a>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../ch12/ch12.html" ><strong aria-hidden="true">12.</strong> 反射</a>
</li>
<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>
</li>
<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" class="active"><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>
</li>
</ol>
<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>
</li>
<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>
</li>
</ol>
<li class="chapter-item expanded ">
<a href="../appendix/appendix.html" ><strong aria-hidden="true">14.</strong> 附录</a>
</li>
<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>
</li>
<li class="chapter-item expanded ">
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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>12.3. Display，一个递归的值打印器</h2>
<p>接下来，让我们看看如何改善聚合数据类型的显示。我们并不想完全克隆一个fmt.Sprint函数，我们只是构建一个用于调试用的Display函数：给定任意一个复杂类型 x，打印这个值对应的完整结构，同时标记每个元素的发现路径。让我们从一个例子开始。</p>
<pre><code class="language-Go">e, _ := eval.Parse(&quot;sqrt(A / pi)&quot;)
Display(&quot;e&quot;, e)
</code></pre>
<p>在上面的调用中，传入Display函数的参数是在7.9节一个表达式求值函数返回的语法树。Display函数的输出如下：</p>
<pre><code class="language-Go">Display e (eval.call):
e.fn = &quot;sqrt&quot;
e.args[0].type = eval.binary
e.args[0].value.op = 47
e.args[0].value.x.type = eval.Var
e.args[0].value.x.value = &quot;A&quot;
e.args[0].value.y.type = eval.Var
e.args[0].value.y.value = &quot;pi&quot;
</code></pre>
<p>你应该尽量避免在一个包的API中暴露涉及反射的接口。我们将定义一个未导出的display函数用于递归处理工作，导出的是Display函数，它只是display函数简单的包装以接受interface{}类型的参数：</p>
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch12/display<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
<pre><code class="language-Go">func Display(name string, x interface{}) {
fmt.Printf(&quot;Display %s (%T):\n&quot;, name, x)
display(name, reflect.ValueOf(x))
}
</code></pre>
<p>在display函数中，我们使用了前面定义的打印基础类型——基本类型、函数和chan等——元素值的formatAtom函数，但是我们会使用reflect.Value的方法来递归显示复杂类型的每一个成员。在递归下降过程中，path字符串，从最开始传入的起始值（这里是“e”），将逐步增长来表示是如何达到当前值（例如“e.args[0].value”）的。</p>
<p>因为我们不再模拟fmt.Sprint函数，我们将直接使用fmt包来简化我们的例子实现。</p>
<pre><code class="language-Go">func display(path string, v reflect.Value) {
switch v.Kind() {
case reflect.Invalid:
fmt.Printf(&quot;%s = invalid\n&quot;, path)
case reflect.Slice, reflect.Array:
for i := 0; i &lt; v.Len(); i++ {
display(fmt.Sprintf(&quot;%s[%d]&quot;, path, i), v.Index(i))
}
case reflect.Struct:
for i := 0; i &lt; v.NumField(); i++ {
fieldPath := fmt.Sprintf(&quot;%s.%s&quot;, path, v.Type().Field(i).Name)
display(fieldPath, v.Field(i))
}
case reflect.Map:
for _, key := range v.MapKeys() {
display(fmt.Sprintf(&quot;%s[%s]&quot;, path,
formatAtom(key)), v.MapIndex(key))
}
case reflect.Ptr:
if v.IsNil() {
fmt.Printf(&quot;%s = nil\n&quot;, path)
} else {
display(fmt.Sprintf(&quot;(*%s)&quot;, path), v.Elem())
}
case reflect.Interface:
if v.IsNil() {
fmt.Printf(&quot;%s = nil\n&quot;, path)
} else {
fmt.Printf(&quot;%s.type = %s\n&quot;, path, v.Elem().Type())
display(path+&quot;.value&quot;, v.Elem())
}
default: // basic types, channels, funcs
fmt.Printf(&quot;%s = %s\n&quot;, path, formatAtom(v))
}
}
</code></pre>
<p>让我们针对不同类型分别讨论。</p>
<p><strong>Slice和数组：</strong> 两种的处理逻辑是一样的。Len方法返回slice或数组值中的元素个数，Index(i)获得索引i对应的元素，返回的也是一个reflect.Value；如果索引i超出范围的话将导致panic异常，这与数组或slice类型内建的len(a)和a[i]操作类似。display针对序列中的每个元素递归调用自身处理，我们通过在递归处理时向path附加“[i]”来表示访问路径。</p>
<p>虽然reflect.Value类型带有很多方法，但是只有少数的方法能对任意值都安全调用。例如，Index方法只能对Slice、数组或字符串类型的值调用，如果对其它类型调用则会导致panic异常。</p>
<p><strong>结构体：</strong> NumField方法报告结构体中成员的数量，Field(i)以reflect.Value类型返回第i个成员的值。成员列表也包括通过匿名字段提升上来的成员。为了在path添加“.f”来表示成员路径，我们必须获得结构体对应的reflect.Type类型信息，然后访问结构体第i个成员的名字。</p>
<p><strong>Maps:</strong> MapKeys方法返回一个reflect.Value类型的slice，每一个元素对应map的一个key。和往常一样，遍历map时顺序是随机的。MapIndex(key)返回map中key对应的value。我们向path添加“[key]”来表示访问路径。（我们这里有一个未完成的工作。其实map的key的类型并不局限于formatAtom能完美处理的类型；数组、结构体和接口都可以作为map的key。针对这种类型，完善key的显示信息是练习12.1的任务。）</p>
<p><strong>指针：</strong> Elem方法返回指针指向的变量，依然是reflect.Value类型。即使指针是nil，这个操作也是安全的，在这种情况下指针是Invalid类型，但是我们可以用IsNil方法来显式地测试一个空指针，这样我们可以打印更合适的信息。我们在path前面添加“*”，并用括弧包含以避免歧义。</p>
<p><strong>接口：</strong> 再一次，我们使用IsNil方法来测试接口是否是nil，如果不是，我们可以调用v.Elem()来获取接口对应的动态值，并且打印对应的类型和值。</p>
<p>现在我们的Display函数总算完工了，让我们看看它的表现吧。下面的Movie类型是在4.5节的电影类型上演变来的：</p>
<pre><code class="language-Go">type Movie struct {
Title, Subtitle string
Year int
Color bool
Actor map[string]string
Oscars []string
Sequel *string
}
</code></pre>
<p>让我们声明一个该类型的变量，然后看看Display函数如何显示它：</p>
<pre><code class="language-Go">strangelove := Movie{
Title: &quot;Dr. Strangelove&quot;,
Subtitle: &quot;How I Learned to Stop Worrying and Love the Bomb&quot;,
Year: 1964,
Color: false,
Actor: map[string]string{
&quot;Dr. Strangelove&quot;: &quot;Peter Sellers&quot;,
&quot;Grp. Capt. Lionel Mandrake&quot;: &quot;Peter Sellers&quot;,
&quot;Pres. Merkin Muffley&quot;: &quot;Peter Sellers&quot;,
&quot;Gen. Buck Turgidson&quot;: &quot;George C. Scott&quot;,
&quot;Brig. Gen. Jack D. Ripper&quot;: &quot;Sterling Hayden&quot;,
`Maj. T.J. &quot;King&quot; Kong`: &quot;Slim Pickens&quot;,
},
Oscars: []string{
&quot;Best Actor (Nomin.)&quot;,
&quot;Best Adapted Screenplay (Nomin.)&quot;,
&quot;Best Director (Nomin.)&quot;,
&quot;Best Picture (Nomin.)&quot;,
},
}
</code></pre>
<p>Display(&quot;strangelove&quot;, strangelove)调用将显示（strangelove电影对应的中文名是《奇爱博士》）：</p>
<pre><code class="language-Go">Display strangelove (display.Movie):
strangelove.Title = &quot;Dr. Strangelove&quot;
strangelove.Subtitle = &quot;How I Learned to Stop Worrying and Love the Bomb&quot;
strangelove.Year = 1964
strangelove.Color = false
strangelove.Actor[&quot;Gen. Buck Turgidson&quot;] = &quot;George C. Scott&quot;
strangelove.Actor[&quot;Brig. Gen. Jack D. Ripper&quot;] = &quot;Sterling Hayden&quot;
strangelove.Actor[&quot;Maj. T.J. \&quot;King\&quot; Kong&quot;] = &quot;Slim Pickens&quot;
strangelove.Actor[&quot;Dr. Strangelove&quot;] = &quot;Peter Sellers&quot;
strangelove.Actor[&quot;Grp. Capt. Lionel Mandrake&quot;] = &quot;Peter Sellers&quot;
strangelove.Actor[&quot;Pres. Merkin Muffley&quot;] = &quot;Peter Sellers&quot;
strangelove.Oscars[0] = &quot;Best Actor (Nomin.)&quot;
strangelove.Oscars[1] = &quot;Best Adapted Screenplay (Nomin.)&quot;
strangelove.Oscars[2] = &quot;Best Director (Nomin.)&quot;
strangelove.Oscars[3] = &quot;Best Picture (Nomin.)&quot;
strangelove.Sequel = nil
</code></pre>
<p>我们也可以使用Display函数来显示标准库中类型的内部结构，例如<code>*os.File</code>类型：</p>
<pre><code class="language-Go">Display(&quot;os.Stderr&quot;, os.Stderr)
// Output:
// Display os.Stderr (*os.File):
// (*(*os.Stderr).file).fd = 2
// (*(*os.Stderr).file).name = &quot;/dev/stderr&quot;
// (*(*os.Stderr).file).nepipe = 0
</code></pre>
<p>可以看出，反射能够访问到结构体中未导出的成员。需要当心的是这个例子的输出在不同操作系统上可能是不同的，并且随着标准库的发展也可能导致结果不同。（这也是将这些成员定义为私有成员的原因之一！）我们甚至可以用Display函数来显示reflect.Value 的内部构造（在这里设置为<code>*os.File</code>的类型描述体）。<code>Display(&quot;rV&quot;, reflect.ValueOf(os.Stderr))</code>调用的输出如下，当然不同环境得到的结果可能有差异：</p>
<pre><code class="language-Go">Display rV (reflect.Value):
(*rV.typ).size = 8
(*rV.typ).hash = 871609668
(*rV.typ).align = 8
(*rV.typ).fieldAlign = 8
(*rV.typ).kind = 22
(*(*rV.typ).string) = &quot;*os.File&quot;
(*(*(*rV.typ).uncommonType).methods[0].name) = &quot;Chdir&quot;
(*(*(*(*rV.typ).uncommonType).methods[0].mtyp).string) = &quot;func() error&quot;
(*(*(*(*rV.typ).uncommonType).methods[0].typ).string) = &quot;func(*os.File) error&quot;
...
</code></pre>
<p>观察下面两个例子的区别：</p>
<pre><code class="language-Go">var i interface{} = 3
Display(&quot;i&quot;, i)
// Output:
// Display i (int):
// i = 3
Display(&quot;&amp;i&quot;, &amp;i)
// Output:
// Display &amp;i (*interface {}):
// (*&amp;i).type = int
// (*&amp;i).value = 3
</code></pre>
<p>在第一个例子中，Display函数调用reflect.ValueOf(i)，它返回一个Int类型的值。正如我们在12.2节中提到的，reflect.ValueOf总是返回一个具体类型的 Value，因为它是从一个接口值提取的内容。</p>
<p>在第二个例子中，Display函数调用的是reflect.ValueOf(&amp;i)，它返回一个指向i的指针，对应Ptr类型。在switch的Ptr分支中，对这个值调用 Elem 方法，返回一个Value来表示变量 i 本身，对应Interface类型。像这样一个间接获得的Value，可能代表任意类型的值，包括接口类型。display函数递归调用自身，这次它分别打印了这个接口的动态类型和值。</p>
<p>对于目前的实现，如果遇到对象图中含有回环，Display将会陷入死循环，例如下面这个首尾相连的链表：</p>
<pre><code class="language-Go">// a struct that points to itself
type Cycle struct{ Value int; Tail *Cycle }
var c Cycle
c = Cycle{42, &amp;c}
Display(&quot;c&quot;, c)
</code></pre>
<p>Display会永远不停地进行深度递归打印：</p>
<pre><code class="language-Go">Display c (display.Cycle):
c.Value = 42
(*c.Tail).Value = 42
(*(*c.Tail).Tail).Value = 42
(*(*(*c.Tail).Tail).Tail).Value = 42
...ad infinitum...
</code></pre>
<p>许多Go语言程序都包含了一些循环的数据。让Display支持这类带环的数据结构需要些技巧，需要额外记录迄今访问的路径；相应会带来成本。通用的解决方案是采用 unsafe 的语言特性，我们将在13.3节看到具体的解决方案。</p>
<p>带环的数据结构很少会对fmt.Sprint函数造成问题，因为它很少尝试打印完整的数据结构。例如，当它遇到一个指针的时候，它只是简单地打印指针的数字值。在打印包含自身的slice或map时可能卡住，但是这种情况很罕见，不值得付出为了处理回环所需的开销。</p>
<p><strong>练习 12.1：</strong> 扩展Display函数，使它可以显示包含以结构体或数组作为map的key类型的值。</p>
<p><strong>练习 12.2：</strong> 增强display函数的稳健性，通过记录边界的步数来确保在超出一定限制后放弃递归。（在13.3节，我们会看到另一种探测数据结构是否存在环的技术。）</p>
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