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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>
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<hr>
<h2 id="712-通过类型断言询问行为"><a class="header" href="#712-通过类型断言询问行为">7.12. 通过类型断言询问行为</a></h2>
<p>下面这段逻辑和net/http包中web服务器负责写入HTTP头字段（例如：&quot;Content-type:text/html&quot;）的部分相似。io.Writer接口类型的变量w代表HTTP响应；写入它的字节最终被发送到某个人的web浏览器上。</p>
<pre><code class="language-go">func writeHeader(w io.Writer, contentType string) error {
if _, err := w.Write([]byte(&quot;Content-Type: &quot;)); err != nil {
return err
}
if _, err := w.Write([]byte(contentType)); err != nil {
return err
}
// ...
}
</code></pre>
<p>因为Write方法需要传入一个byte切片而我们希望写入的值是一个字符串，所以我们需要使用[]byte(...)进行转换。这个转换分配内存并且做一个拷贝，但是这个拷贝在转换后几乎立马就被丢弃掉。让我们假装这是一个web服务器的核心部分并且我们的性能分析表示这个内存分配使服务器的速度变慢。这里我们可以避免掉内存分配么？</p>
<p>这个io.Writer接口告诉我们关于w持有的具体类型的唯一东西：就是可以向它写入字节切片。如果我们回顾net/http包中的内幕，我们知道在这个程序中的w变量持有的动态类型也有一个允许字符串高效写入的WriteString方法；这个方法会避免去分配一个临时的拷贝。（这可能像在黑夜中射击一样，但是许多满足io.Writer接口的重要类型同时也有WriteString方法，包括<code>*bytes.Buffer</code>，<code>*os.File</code>和<code>*bufio.Writer</code>。）</p>
<p>我们不能对任意io.Writer类型的变量w，假设它也拥有WriteString方法。但是我们可以定义一个只有这个方法的新接口并且使用类型断言来检测是否w的动态类型满足这个新接口。</p>
<pre><code class="language-go">// writeString writes s to w.
// If w has a WriteString method, it is invoked instead of w.Write.
func writeString(w io.Writer, s string) (n int, err error) {
type stringWriter interface {
WriteString(string) (n int, err error)
}
if sw, ok := w.(stringWriter); ok {
return sw.WriteString(s) // avoid a copy
}
return w.Write([]byte(s)) // allocate temporary copy
}
func writeHeader(w io.Writer, contentType string) error {
if _, err := writeString(w, &quot;Content-Type: &quot;); err != nil {
return err
}
if _, err := writeString(w, contentType); err != nil {
return err
}
// ...
}
</code></pre>
<p>为了避免重复定义，我们将这个检查移入到一个实用工具函数writeString中，但是它太有用了以致于标准库将它作为io.WriteString函数提供。这是向一个io.Writer接口写入字符串的推荐方法。</p>
<p>这个例子的神奇之处在于，没有定义了WriteString方法的标准接口，也没有指定它是一个所需行为的标准接口。一个具体类型只会通过它的方法决定它是否满足stringWriter接口，而不是任何它和这个接口类型所表达的关系。它的意思就是上面的技术依赖于一个假设，这个假设就是：如果一个类型满足下面的这个接口，然后WriteString(s)方法就必须和Write([]byte(s))有相同的效果。</p>
<pre><code class="language-go">interface {
io.Writer
WriteString(s string) (n int, err error)
}
</code></pre>
<p>尽管io.WriteString实施了这个假设，但是调用它的函数极少可能会去实施类似的假设。定义一个特定类型的方法隐式地获取了对特定行为的协约。对于Go语言的新手，特别是那些来自有强类型语言使用背景的新手，可能会发现它缺乏显式的意图令人感到混乱，但是在实战的过程中这几乎不是一个问题。除了空接口interface{}，接口类型很少意外巧合地被实现。</p>
<p>上面的writeString函数使用一个类型断言来获知一个普遍接口类型的值是否满足一个更加具体的接口类型；并且如果满足，它会使用这个更具体接口的行为。这个技术可以被很好的使用，不论这个被询问的接口是一个标准如io.ReadWriter，或者用户定义的如stringWriter接口。</p>
<p>这也是fmt.Fprintf函数怎么从其它所有值中区分满足error或者fmt.Stringer接口的值。在fmt.Fprintf内部，有一个将单个操作对象转换成一个字符串的步骤，像下面这样：</p>
<pre><code class="language-go">package fmt
func formatOneValue(x interface{}) string {
if err, ok := x.(error); ok {
return err.Error()
}
if str, ok := x.(Stringer); ok {
return str.String()
}
// ...all other types...
}
</code></pre>
<p>如果x满足这两个接口类型中的一个，具体满足的接口决定对值的格式化方式。如果都不满足，默认的case或多或少会统一地使用反射来处理所有的其它类型；我们可以在第12章知道具体是怎么实现的。</p>
<p>再一次的，它假设任何有String方法的类型都满足fmt.Stringer中约定的行为，这个行为会返回一个适合打印的字符串。</p>
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