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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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2022-08-24 10:48:15 +00:00
< li > < em > KusonStack一站式可编程配置技术栈(Go): < a href = "https://github.com/KusionStack/kusion" > https://github.com/KusionStack/kusion< / a > < / em > < / li >
< li > < em > KCL 配置编程语言(Rust): < a href = "https://github.com/KusionStack/KCLVM" > https://github.com/KusionStack/KCLVM< / a > < / em > < / li >
< li > < em > 凹语言™: < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2022-08-04 06:59:33 +00:00
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< hr >
< 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， < / a > < / h2 >
< p > 接下来， ， ： ， ， < / p >
< pre > < code class = "language-Go" > e, _ := eval.Parse(" sqrt(A / pi)" )
Display(" e" , e)
< / code > < / pre >
< p > 在上面的调用中， ： < / p >
< pre > < code class = "language-Go" > Display e (eval.call):
e.fn = " sqrt"
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 = " A"
e.args[0].value.y.type = eval.Var
e.args[0].value.y.value = " pi"
< / 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， ， < / p >
< p > < u > < i > gopl.io/ch12/display< / i > < / u > < / p >
< pre > < code class = "language-Go" > func Display(name string, x interface{}) {
fmt.Printf(" Display %s (%T):\n" , name, x)
display(name, reflect.ValueOf(x))
}
< / code > < / pre >
< p > 在display函数中， ， ， ， （ ） ， （ ） < / p >
< p > 因为我们不再模拟fmt.Sprint函数， < / p >
< pre > < code class = "language-Go" > func display(path string, v reflect.Value) {
switch v.Kind() {
case reflect.Invalid:
fmt.Printf(" %s = invalid\n" , path)
case reflect.Slice, reflect.Array:
for i := 0; i < v.Len(); i++ {
display(fmt.Sprintf(" %s[%d]" , path, i), v.Index(i))
}
case reflect.Struct:
for i := 0; i < v.NumField(); i++ {
fieldPath := fmt.Sprintf(" %s.%s" , path, v.Type().Field(i).Name)
display(fieldPath, v.Field(i))
}
case reflect.Map:
for _, key := range v.MapKeys() {
display(fmt.Sprintf(" %s[%s]" , path,
formatAtom(key)), v.MapIndex(key))
}
case reflect.Ptr:
if v.IsNil() {
fmt.Printf(" %s = nil\n" , path)
} else {
display(fmt.Sprintf(" (*%s)" , path), v.Elem())
}
case reflect.Interface:
if v.IsNil() {
fmt.Printf(" %s = nil\n" , path)
} else {
fmt.Printf(" %s.type = %s\n" , path, v.Elem().Type())
display(path+" .value" , v.Elem())
}
default: // basic types, channels, funcs
fmt.Printf(" %s = %s\n" , 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， ， ； ， ， < / p >
< p > 虽然reflect.Value类型带有很多方法， ， ， < / p >
< p > < strong > 结构体：< / strong > NumField方法报告结构体中成员的数量， ， ， < / p >
< p > < strong > Maps:< / strong > MapKeys方法返回一个reflect.Value类型的slice， ， （ ； ， < / p >
< p > < strong > 指针：< / strong > Elem方法返回指针指向的变量， ， ， ， ， < / p >
< p > < strong > 接口：< / strong > 再一次， ， ， < / p >
< p > 现在我们的Display函数总算完工了， < / 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， ： < / 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（ ） ： < / 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， < 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， ， （ ！ ） < 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， ， ， ， < / p >
< p > 在第二个例子中， & i)， ， ， ， ， ， ， ， < / p >
< p > 对于目前的实现， ， ， ： < / 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， ； ， < / p >
< p > 带环的数据结构很少会对fmt.Sprint函数造成问题， ， ， ， ， < / p >
< p > < strong > 练习 12.1： < / strong > 扩展Display函数， < / p >
< p > < strong > 练习 12.2： < / strong > 增强display函数的稳健性， （ < / p >
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