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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../index.html">Go语言圣经</a></li><li class="chapter-item expanded affix "><a href="../preface-zh.html">译者序</a></li><li class="chapter-item expanded affix "><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><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><li class="chapter-item expanded "><a href="../ch2/ch2.html"><strong aria-hidden="true">2.</strong> 程序结构</a></li><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><li class="chapter-item expanded "><a href="../ch3/ch3.html"><strong aria-hidden="true">3.</strong> 基础数据类型</a></li><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><li class="chapter-item expanded "><a href="../ch4/ch4.html"><strong aria-hidden="true">4.</strong> 复合数据类型</a></li><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><li class="chapter-item expanded "><a href="../ch5/ch5.html"><strong aria-hidden="true">5.</strong> 函数</a></li><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><li class="chapter-item expanded "><a href="../ch6/ch6.html"><strong aria-hidden="true">6.</strong> 方法</a></li><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><li class="chapter-item expanded "><a href="../ch7/ch7.html"><strong aria-hidden="true">7.</strong> 接口</a></li><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" class="active"><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><li class="chapter-item expanded "><a href="../ch8/ch8.html"><strong aria-hidden="true">8.</strong> Goroutines和Channels</a></li><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><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><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><li class="chapter-item expanded "><a href="../ch10/ch10.html"><strong aria-hidden="true">10.</strong> 包和工具</a></li><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><li class="chapter-item expanded "><a href="../ch11/ch11.html"><strong aria-hidden="true">11.</strong> 测试</a></li><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><li class="chapter-item expanded "><a href="../ch12/ch12.html"><strong aria-hidden="true">12.</strong> 反射</a></li><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"><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><li class="chapter-item expanded "><a href="../ch13/ch13.html"><strong aria-hidden="true">13.</strong> 底层编程</a></li><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><li class="chapter-item expanded "><a href="../appendix/appendix.html"><strong aria-hidden="true">14.</strong> 附录</a></li><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 "><a href="../appendix/appendix-b-author.html"><strong aria-hidden="true">14.2.</strong> 附录B：作者译者</a></li><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></li><li class="chapter-item expanded "><a href="../appendix/appendix-d-translations.html"><strong aria-hidden="true">14.4.</strong> 附录D：其它语言</a></li></ol></li></ol> </div>
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<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
<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>
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<hr>
<h2 id="75--接口值"><a class="header" href="#75--接口值">7.5. 接口值</a></h2>
<p>概念上讲一个接口的值，接口值，由两个部分组成，一个具体的类型和那个类型的值。它们被称为接口的动态类型和动态值。对于像Go语言这种静态类型的语言，类型是编译期的概念；因此一个类型不是一个值。在我们的概念模型中，一些提供每个类型信息的值被称为类型描述符，比如类型的名称和方法。在一个接口值中，类型部分代表与之相关类型的描述符。</p>
<p>下面4个语句中，变量w得到了3个不同的值。（开始和最后的值是相同的）</p>
<pre><code class="language-go">var w io.Writer
w = os.Stdout
w = new(bytes.Buffer)
w = nil
</code></pre>
<p>让我们进一步观察在每一个语句后的w变量的值和动态行为。第一个语句定义了变量w:</p>
<pre><code class="language-go">var w io.Writer
</code></pre>
<p>在Go语言中，变量总是被一个定义明确的值初始化，即使接口类型也不例外。对于一个接口的零值就是它的类型和值的部分都是nil（图7.1）。</p>
<p><img src="../images/ch7-01.png" alt="" /></p>
<p>一个接口值基于它的动态类型被描述为空或非空，所以这是一个空的接口值。你可以通过使用w==nil或者w!=nil来判断接口值是否为空。调用一个空接口值上的任意方法都会产生panic:</p>
<pre><code class="language-go">w.Write([]byte(&quot;hello&quot;)) // panic: nil pointer dereference
</code></pre>
<p>第二个语句将一个<code>*os.File</code>类型的值赋给变量w:</p>
<pre><code class="language-go">w = os.Stdout
</code></pre>
<p>这个赋值过程调用了一个具体类型到接口类型的隐式转换，这和显式的使用io.Writer(os.Stdout)是等价的。这类转换不管是显式的还是隐式的，都会刻画出操作到的类型和值。这个接口值的动态类型被设为<code>*os.File</code>指针的类型描述符，它的动态值持有os.Stdout的拷贝；这是一个代表处理标准输出的os.File类型变量的指针（图7.2）。</p>
<p><img src="../images/ch7-02.png" alt="" /></p>
<p>调用一个包含<code>*os.File</code>类型指针的接口值的Write方法，使得<code>(*os.File).Write</code>方法被调用。这个调用输出“hello”。</p>
<pre><code class="language-go">w.Write([]byte(&quot;hello&quot;)) // &quot;hello&quot;
</code></pre>
<p>通常在编译期，我们不知道接口值的动态类型是什么，所以一个接口上的调用必须使用动态分配。因为不是直接进行调用，所以编译器必须把代码生成在类型描述符的方法Write上，然后间接调用那个地址。这个调用的接收者是一个接口动态值的拷贝，os.Stdout。效果和下面这个直接调用一样：</p>
<pre><code class="language-go">os.Stdout.Write([]byte(&quot;hello&quot;)) // &quot;hello&quot;
</code></pre>
<p>第三个语句给接口值赋了一个*bytes.Buffer类型的值</p>
<pre><code class="language-go">w = new(bytes.Buffer)
</code></pre>
<p>现在动态类型是*bytes.Buffer并且动态值是一个指向新分配的缓冲区的指针（图7.3）。</p>
<p><img src="../images/ch7-03.png" alt="" /></p>
<p>Write方法的调用也使用了和之前一样的机制：</p>
<pre><code class="language-go">w.Write([]byte(&quot;hello&quot;)) // writes &quot;hello&quot; to the bytes.Buffers
</code></pre>
<p>这次类型描述符是*bytes.Buffer，所以调用了(*bytes.Buffer).Write方法，并且接收者是该缓冲区的地址。这个调用把字符串“hello”添加到缓冲区中。</p>
<p>最后，第四个语句将nil赋给了接口值：</p>
<pre><code class="language-go">w = nil
</code></pre>
<p>这个重置将它所有的部分都设为nil值，把变量w恢复到和它之前定义时相同的状态，在图7.1中可以看到。</p>
<p>一个接口值可以持有任意大的动态值。例如，表示时间实例的time.Time类型，这个类型有几个对外不公开的字段。我们从它上面创建一个接口值：</p>
<pre><code class="language-go">var x interface{} = time.Now()
</code></pre>
<p>结果可能和图7.4相似。从概念上讲，不论接口值多大，动态值总是可以容下它。（这只是一个概念上的模型；具体的实现可能会非常不同）</p>
<p><img src="../images/ch7-04.png" alt="" /></p>
<p>接口值可以使用==和!＝来进行比较。两个接口值相等仅当它们都是nil值，或者它们的动态类型相同并且动态值也根据这个动态类型的==操作相等。因为接口值是可比较的，所以它们可以用在map的键或者作为switch语句的操作数。</p>
<p>然而，如果两个接口值的动态类型相同，但是这个动态类型是不可比较的（比如切片），将它们进行比较就会失败并且panic:</p>
<pre><code class="language-go">var x interface{} = []int{1, 2, 3}
fmt.Println(x == x) // panic: comparing uncomparable type []int
</code></pre>
<p>考虑到这点，接口类型是非常与众不同的。其它类型要么是安全的可比较类型（如基本类型和指针）要么是完全不可比较的类型（如切片，映射类型，和函数），但是在比较接口值或者包含了接口值的聚合类型时，我们必须要意识到潜在的panic。同样的风险也存在于使用接口作为map的键或者switch的操作数。只能比较你非常确定它们的动态值是可比较类型的接口值。</p>
<p>当我们处理错误或者调试的过程中，得知接口值的动态类型是非常有帮助的。所以我们使用fmt包的%T动作:</p>
<pre><code class="language-go">var w io.Writer
fmt.Printf(&quot;%T\n&quot;, w) // &quot;&lt;nil&gt;&quot;
w = os.Stdout
fmt.Printf(&quot;%T\n&quot;, w) // &quot;*os.File&quot;
w = new(bytes.Buffer)
fmt.Printf(&quot;%T\n&quot;, w) // &quot;*bytes.Buffer&quot;
</code></pre>
<p>在fmt包内部，使用反射来获取接口动态类型的名称。我们会在第12章中学到反射相关的知识。</p>
<h3 id="751--警告一个包含nil指针的接口不是nil接口"><a class="header" href="#751--警告一个包含nil指针的接口不是nil接口">7.5.1. 警告：一个包含nil指针的接口不是nil接口</a></h3>
<p>一个不包含任何值的nil接口值和一个刚好包含nil指针的接口值是不同的。这个细微区别产生了一个容易绊倒每个Go程序员的陷阱。</p>
<p>思考下面的程序。当debug变量设置为true时，main函数会将f函数的输出收集到一个bytes.Buffer类型中。</p>
<pre><code class="language-go">const debug = true
func main() {
var buf *bytes.Buffer
if debug {
buf = new(bytes.Buffer) // enable collection of output
}
f(buf) // NOTE: subtly incorrect!
if debug {
// ...use buf...
}
}
// If out is non-nil, output will be written to it.
func f(out io.Writer) {
// ...do something...
if out != nil {
out.Write([]byte(&quot;done!\n&quot;))
}
}
</code></pre>
<p>我们可能会预计当把变量debug设置为false时可以禁止对输出的收集，但是实际上在out.Write方法调用时程序发生了panic：</p>
<pre><code class="language-go">if out != nil {
out.Write([]byte(&quot;done!\n&quot;)) // panic: nil pointer dereference
}
</code></pre>
<p>当main函数调用函数f时，它给f函数的out参数赋了一个*bytes.Buffer的空指针，所以out的动态值是nil。然而，它的动态类型是*bytes.Buffer，意思就是out变量是一个包含空指针值的非空接口（如图7.5），所以防御性检查out!=nil的结果依然是true。</p>
<p><img src="../images/ch7-05.png" alt="" /></p>
<p>动态分配机制依然决定(*bytes.Buffer).Write的方法会被调用，但是这次的接收者的值是nil。对于一些如*os.File的类型，nil是一个有效的接收者（§6.2.1），但是*bytes.Buffer类型不在这些种类中。这个方法会被调用，但是当它尝试去获取缓冲区时会发生panic。</p>
<p>问题在于尽管一个nil的*bytes.Buffer指针有实现这个接口的方法，它也不满足这个接口具体的行为上的要求。特别是这个调用违反了(*bytes.Buffer).Write方法的接收者非空的隐含先觉条件，所以将nil指针赋给这个接口是错误的。解决方案就是将main函数中的变量buf的类型改为io.Writer，因此可以避免一开始就将一个不完整的值赋值给这个接口：</p>
<pre><code class="language-go">var buf io.Writer
if debug {
buf = new(bytes.Buffer) // enable collection of output
}
f(buf) // OK
</code></pre>
<p>现在我们已经把接口值的技巧都讲完了，让我们来看更多的一些在Go标准库中的重要接口类型。在下面的三章中，我们会看到接口类型是怎样用在排序，web服务，错误处理中的。</p>
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