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<title>接口是合约 - 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" class="active"><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" ><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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<h1 class="menu-title"><a href="../index.html">Go语言圣经</a></h1>
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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>7.1. 接口约定</h2>
<p>目前为止，我们看到的类型都是具体的类型。一个具体的类型可以准确的描述它所代表的值，并且展示出对类型本身的一些操作方式：就像数字类型的算术操作，切片类型的取下标、添加元素和范围获取操作。具体的类型还可以通过它的内置方法提供额外的行为操作。总的来说，当你拿到一个具体的类型时你就知道它的本身是什么和你可以用它来做什么。</p>
<p>在Go语言中还存在着另外一种类型：接口类型。接口类型是一种抽象的类型。它不会暴露出它所代表的对象的内部值的结构和这个对象支持的基础操作的集合；它们只会表现出它们自己的方法。也就是说当你有看到一个接口类型的值时，你不知道它是什么，唯一知道的就是可以通过它的方法来做什么。</p>
<p>在本书中，我们一直使用两个相似的函数来进行字符串的格式化：fmt.Printf，它会把结果写到标准输出，和fmt.Sprintf，它会把结果以字符串的形式返回。得益于使用接口，我们不必可悲的因为返回结果在使用方式上的一些浅显不同就必需把格式化这个最困难的过程复制一份。实际上，这两个函数都使用了另一个函数fmt.Fprintf来进行封装。fmt.Fprintf这个函数对它的计算结果会被怎么使用是完全不知道的。</p>
<pre><code class="language-go">package fmt
func Fprintf(w io.Writer, format string, args ...interface{}) (int, error)
func Printf(format string, args ...interface{}) (int, error) {
return Fprintf(os.Stdout, format, args...)
}
func Sprintf(format string, args ...interface{}) string {
var buf bytes.Buffer
Fprintf(&amp;buf, format, args...)
return buf.String()
}
</code></pre>
<p>Fprintf的前缀F表示文件（File），也表明格式化输出结果应该被写入第一个参数提供的文件中。在Printf函数中的第一个参数os.Stdout是<code>*os.File</code>类型；在Sprintf函数中的第一个参数&amp;buf是一个指向可以写入字节的内存缓冲区。然而它并不是一个文件类型，尽管它在某种意义上和文件类型相似。</p>
<p>即使是Fprintf函数中的第一个参数，它也不是一个文件类型。它是io.Writer类型，这是一个接口类型定义如下：</p>
<pre><code class="language-go">package io
// Writer is the interface that wraps the basic Write method.
type Writer interface {
// Write writes len(p) bytes from p to the underlying data stream.
// It returns the number of bytes written from p (0 &lt;= n &lt;= len(p))
// and any error encountered that caused the write to stop early.
// Write must return a non-nil error if it returns n &lt; len(p).
// Write must not modify the slice data, even temporarily.
//
// Implementations must not retain p.
Write(p []byte) (n int, err error)
}
</code></pre>
<p>io.Writer类型定义了函数Fprintf和这个函数调用者之间的约定。一方面这个约定需要调用者提供具体类型的值就像<code>*os.File</code>和<code>*bytes.Buffer</code>，这些类型都有一个特定签名和行为的Write的函数。另一方面这个约定保证了Fprintf接受任何满足io.Writer接口的值都可以工作。Fprintf函数可能没有假定写入的是一个文件或是一段内存，而是写入一个可以调用Write函数的值。</p>
<p>因为fmt.Fprintf函数没有对具体操作的值做任何假设，而是仅仅通过io.Writer接口的约定来保证行为，所以第一个参数可以安全地传入一个只需要满足io.Writer接口的任意具体类型的值。一个类型可以自由地被另一个满足相同接口的类型替换，被称作可替换性（LSP里氏替换）。这是一个面向对象的特征。</p>
<p>让我们通过一个新的类型来进行校验，下面<code>*ByteCounter</code>类型里的Write方法，仅仅在丢弃写向它的字节前统计它们的长度。（在这个+=赋值语句中，让len(p)的类型和<code>*c</code>的类型匹配的转换是必须的。）</p>
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch7/bytecounter<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
<pre><code class="language-go">type ByteCounter int
func (c *ByteCounter) Write(p []byte) (int, error) {
*c += ByteCounter(len(p)) // convert int to ByteCounter
return len(p), nil
}
</code></pre>
<p>因为*ByteCounter满足io.Writer的约定，我们可以把它传入Fprintf函数中；Fprintf函数执行字符串格式化的过程不会去关注ByteCounter正确的累加结果的长度。</p>
<pre><code class="language-go">var c ByteCounter
c.Write([]byte(&quot;hello&quot;))
fmt.Println(c) // &quot;5&quot;, = len(&quot;hello&quot;)
c = 0 // reset the counter
var name = &quot;Dolly&quot;
fmt.Fprintf(&amp;c, &quot;hello, %s&quot;, name)
fmt.Println(c) // &quot;12&quot;, = len(&quot;hello, Dolly&quot;)
</code></pre>
<p>除了io.Writer这个接口类型，还有另一个对fmt包很重要的接口类型。Fprintf和Fprintln函数向类型提供了一种控制它们值输出的途径。在2.5节中，我们为Celsius类型提供了一个String方法以便于可以打印成这样&quot;100°C&quot; ，在6.5节中我们给*IntSet添加一个String方法，这样集合可以用传统的符号来进行表示就像&quot;{1 2 3}&quot;。给一个类型定义String方法，可以让它满足最广泛使用之一的接口类型fmt.Stringer：</p>
<pre><code class="language-go">package fmt
// The String method is used to print values passed
// as an operand to any format that accepts a string
// or to an unformatted printer such as Print.
type Stringer interface {
String() string
}
</code></pre>
<p>我们会在7.10节解释fmt包怎么发现哪些值是满足这个接口类型的。</p>
<p><strong>练习 7.1：</strong> 使用来自ByteCounter的思路，实现一个针对单词和行数的计数器。你会发现bufio.ScanWords非常的有用。</p>
<p><strong>练习 7.2：</strong> 写一个带有如下函数签名的函数CountingWriter，传入一个io.Writer接口类型，返回一个把原来的Writer封装在里面的新的Writer类型和一个表示新的写入字节数的int64类型指针。</p>
<pre><code class="language-go">func CountingWriter(w io.Writer) (io.Writer, *int64)
</code></pre>
<p><strong>练习 7.3：</strong> 为在gopl.io/ch4/treesort（§4.4）中的*tree类型实现一个String方法去展示tree类型的值序列。</p>
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