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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.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-hidd
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<!-- 头部 -->
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<li><em>KusonStack一站式可编程配置技术栈: <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
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<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<h2 id="123-display一个递归的值打印器"><a class="header" href="#123-display一个递归的值打印器">12.3. Display，一个递归的值打印器</a></h2>
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<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>
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<pre><code class="language-Go">e, _ := eval.Parse("sqrt(A / pi)")
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Display("e", e)
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</code></pre>
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<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>
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<pre><code class="language-Go">Display e (eval.call):
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e.fn = "sqrt"
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e.args[0].type = eval.binary
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e.args[0].value.op = 47
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e.args[0].value.x.type = eval.Var
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e.args[0].value.x.value = "A"
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e.args[0].value.y.type = eval.Var
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e.args[0].value.y.value = "pi"
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</code></pre>
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<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>
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<p><u><i>gopl.io/ch12/display</i></u></p>
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<pre><code class="language-Go">func Display(name string, x interface{}) {
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fmt.Printf("Display %s (%T):\n", name, x)
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display(name, reflect.ValueOf(x))
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}
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</code></pre>
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<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>
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<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>
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<pre><code class="language-Go">func display(path string, v reflect.Value) {
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switch v.Kind() {
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case reflect.Invalid:
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fmt.Printf("%s = invalid\n", path)
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case reflect.Slice, reflect.Array:
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for i := 0; i < v.Len(); i++ {
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display(fmt.Sprintf("%s[%d]", path, i), v.Index(i))
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}
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case reflect.Struct:
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for i := 0; i < v.NumField(); i++ {
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fieldPath := fmt.Sprintf("%s.%s", path, v.Type().Field(i).Name)
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display(fieldPath, v.Field(i))
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}
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case reflect.Map:
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for _, key := range v.MapKeys() {
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display(fmt.Sprintf("%s[%s]", path,
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formatAtom(key)), v.MapIndex(key))
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}
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case reflect.Ptr:
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if v.IsNil() {
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fmt.Printf("%s = nil\n", path)
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} else {
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display(fmt.Sprintf("(*%s)", path), v.Elem())
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}
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case reflect.Interface:
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if v.IsNil() {
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fmt.Printf("%s = nil\n", path)
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} else {
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fmt.Printf("%s.type = %s\n", path, v.Elem().Type())
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display(path+".value", v.Elem())
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}
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default: // basic types, channels, funcs
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fmt.Printf("%s = %s\n", path, formatAtom(v))
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}
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}
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</code></pre>
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<p>让我们针对不同类型分别讨论。</p>
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<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>
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<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>
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<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: "Dr. Strangelove",
|
|||
|
|
Subtitle: "How I Learned to Stop Worrying and Love the Bomb",
|
|||
|
|
Year: 1964,
|
|||
|
|
Color: false,
|
|||
|
|
Actor: map[string]string{
|
|||
|
|
"Dr. Strangelove": "Peter Sellers",
|
|||
|
|
"Grp. Capt. Lionel Mandrake": "Peter Sellers",
|
|||
|
|
"Pres. Merkin Muffley": "Peter Sellers",
|
|||
|
|
"Gen. Buck Turgidson": "George C. Scott",
|
|||
|
|
"Brig. Gen. Jack D. Ripper": "Sterling Hayden",
|
|||
|
|
`Maj. T.J. "King" Kong`: "Slim Pickens",
|
|||
|
|
},
|
|||
|
|
|
|||
|
|
Oscars: []string{
|
|||
|
|
"Best Actor (Nomin.)",
|
|||
|
|
"Best Adapted Screenplay (Nomin.)",
|
|||
|
|
"Best Director (Nomin.)",
|
|||
|
|
"Best Picture (Nomin.)",
|
|||
|
|
},
|
|||
|
|
}
|
|||
|
|
</code></pre>
|
|||
|
|
<p>Display("strangelove", 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 = "Dr. Strangelove"
|
|||
|
|
strangelove.Subtitle = "How I Learned to Stop Worrying and Love the Bomb"
|
|||
|
|
strangelove.Year = 1964
|
|||
|
|
strangelove.Color = false
|
|||
|
|
strangelove.Actor["Gen. Buck Turgidson"] = "George C. Scott"
|
|||
|
|
strangelove.Actor["Brig. Gen. Jack D. Ripper"] = "Sterling Hayden"
|
|||
|
|
strangelove.Actor["Maj. T.J. \"King\" Kong"] = "Slim Pickens"
|
|||
|
|
strangelove.Actor["Dr. Strangelove"] = "Peter Sellers"
|
|||
|
|
strangelove.Actor["Grp. Capt. Lionel Mandrake"] = "Peter Sellers"
|
|||
|
|
strangelove.Actor["Pres. Merkin Muffley"] = "Peter Sellers"
|
|||
|
|
strangelove.Oscars[0] = "Best Actor (Nomin.)"
|
|||
|
|
strangelove.Oscars[1] = "Best Adapted Screenplay (Nomin.)"
|
|||
|
|
strangelove.Oscars[2] = "Best Director (Nomin.)"
|
|||
|
|
strangelove.Oscars[3] = "Best Picture (Nomin.)"
|
|||
|
|
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("os.Stderr", os.Stderr)
|
|||
|
|
// Output:
|
|||
|
|
// Display os.Stderr (*os.File):
|
|||
|
|
// (*(*os.Stderr).file).fd = 2
|
|||
|
|
// (*(*os.Stderr).file).name = "/dev/stderr"
|
|||
|
|
// (*(*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("rV", 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) = "*os.File"
|
|||
|
|
|
|||
|
|
(*(*(*rV.typ).uncommonType).methods[0].name) = "Chdir"
|
|||
|
|
(*(*(*(*rV.typ).uncommonType).methods[0].mtyp).string) = "func() error"
|
|||
|
|
(*(*(*(*rV.typ).uncommonType).methods[0].typ).string) = "func(*os.File) error"
|
|||
|
|
...
|
|||
|
|
</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("i", i)
|
|||
|
|
// Output:
|
|||
|
|
// Display i (int):
|
|||
|
|
// i = 3
|
|||
|
|
|
|||
|
|
Display("&i", &i)
|
|||
|
|
// Output:
|
|||
|
|
// Display &i (*interface {}):
|
|||
|
|
// (*&i).type = int
|
|||
|
|
// (*&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(&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, &c}
|
|||
|
|
Display("c", 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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