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< h2 id = "122-reflecttype-和-reflectvalue" > < a class = "header" href = "#122-reflecttype-和-reflectvalue" > 12.2. reflect.Type 和 reflect.Value< / a > < / h2 >
< p > 反射是由 reflect 包提供的。它定义了两个重要的类型， ， ， （ < / p >
< p > 函数 reflect.TypeOf 接受任意的 interface{} 类型，并以 reflect.Type 形式返回其动态类型：< / p >
< pre > < code class = "language-Go" > t := reflect.TypeOf(3) // a reflect.Type
fmt.Println(t.String()) // " int"
fmt.Println(t) // " int"
< / code > < / pre >
< p > 其中 TypeOf(3) 调用将值 3 传给 interface{} 参数。回到 7.5节 的将一个具体的值转为接口类型会有一个隐式的接口转换操作，它会创建一个包含两个信息的接口值：操作数的动态类型（这里是 int） （ ） < / p >
< p > 因为 reflect.TypeOf 返回的是一个动态类型的接口值，它总是返回具体的类型。因此，下面的代码将打印 " *os.File" 而不是 " io.Writer" 。稍后，我们将看到能够表达接口类型的 reflect.Type。< / p >
< pre > < code class = "language-Go" > var w io.Writer = os.Stdout
fmt.Println(reflect.TypeOf(w)) // " *os.File"
< / code > < / pre >
< p > 要注意的是 reflect.Type 接口是满足 fmt.Stringer 接口的。因为打印一个接口的动态类型对于调试和日志是有帮助的， fmt.Printf 提供了一个缩写 %T 参数，内部使用 reflect.TypeOf 来输出：< / p >
< pre > < code class = "language-Go" > fmt.Printf(" %T\n" , 3) // " int"
< / code > < / pre >
< p > reflect 包中另一个重要的类型是 Value。一个 reflect.Value 可以装载任意类型的值。函数 reflect.ValueOf 接受任意的 interface{} 类型，并返回一个装载着其动态值的 reflect.Value。和 reflect.TypeOf 类似， < / p >
< pre > < code class = "language-Go" > v := reflect.ValueOf(3) // a reflect.Value
fmt.Println(v) // " 3"
fmt.Printf(" %v\n" , v) // " 3"
fmt.Println(v.String()) // NOTE: " < int Value> "
< / code > < / pre >
< p > 和 reflect.Type 类似， < / p >
< p > 对 Value 调用 Type 方法将返回具体类型所对应的 reflect.Type： < / p >
< pre > < code class = "language-Go" > t := v.Type() // a reflect.Type
fmt.Println(t.String()) // " int"
< / code > < / pre >
< p > reflect.ValueOf 的逆操作是 reflect.Value.Interface 方法。它返回一个 interface{} 类型，装载着与 reflect.Value 相同的具体值：< / p >
< pre > < code class = "language-Go" > v := reflect.ValueOf(3) // a reflect.Value
x := v.Interface() // an interface{}
i := x.(int) // an int
fmt.Printf(" %d\n" , i) // " 3"
< / code > < / pre >
< p > reflect.Value 和 interface{} 都能装载任意的值。所不同的是，一个空的接口隐藏了值内部的表示方式和所有方法，因此只有我们知道具体的动态类型才能使用类型断言来访问内部的值（就像上面那样），内部值我们没法访问。相比之下，一个 Value 则有很多方法来检查其内容，无论它的具体类型是什么。让我们再次尝试实现我们的格式化函数 format.Any。< / p >
< p > 我们使用 reflect.Value 的 Kind 方法来替代之前的类型 switch。虽然还是有无穷多的类型， ： ； ； ； ） < / p >
< p > < u > < i > gopl.io/ch12/format< / i > < / u > < / p >
< pre > < code class = "language-Go" > package format
import (
" reflect"
" strconv"
)
// Any formats any value as a string.
func Any(value interface{}) string {
return formatAtom(reflect.ValueOf(value))
}
// formatAtom formats a value without inspecting its internal structure.
func formatAtom(v reflect.Value) string {
switch v.Kind() {
case reflect.Invalid:
return " invalid"
case reflect.Int, reflect.Int8, reflect.Int16,
reflect.Int32, reflect.Int64:
return strconv.FormatInt(v.Int(), 10)
case reflect.Uint, reflect.Uint8, reflect.Uint16,
reflect.Uint32, reflect.Uint64, reflect.Uintptr:
return strconv.FormatUint(v.Uint(), 10)
// ...floating-point and complex cases omitted for brevity...
case reflect.Bool:
return strconv.FormatBool(v.Bool())
case reflect.String:
return strconv.Quote(v.String())
case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Slice, reflect.Map:
return v.Type().String() + " 0x" +
strconv.FormatUint(uint64(v.Pointer()), 16)
default: // reflect.Array, reflect.Struct, reflect.Interface
return v.Type().String() + " value"
}
}
< / code > < / pre >
< p > 到目前为止，我们的函数将每个值视作一个不可分割没有内部结构的物品，因此它叫 formatAtom。对于聚合类型（ ） ， ， （ ） ， ， ， ， < / p >
< pre > < code class = "language-Go" > var x int64 = 1
var d time.Duration = 1 * time.Nanosecond
fmt.Println(format.Any(x)) // " 1"
fmt.Println(format.Any(d)) // " 1"
fmt.Println(format.Any([]int64{x})) // " []int64 0x8202b87b0"
fmt.Println(format.Any([]time.Duration{d})) // " []time.Duration 0x8202b87e0"
< / code > < / pre >
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