反思 – 如何判断golang反射的零值

package main

import (
    "fmt"
    "reflect"
)

func main() {
    var p1 *string = nil
    var v interface{} = p1
    val := reflect.Indirect(reflect.ValueOf(v))
    if v == nil {
        fmt.Printf("NULL")
    } else {
        if val.CanInterface() {
            fmt.Printf("if is %v\n",val.Interface())
        }
    }
}

该程序的输出是：

···

panic: reflect: call of reflect.Value.CanInterface on zero Value

goroutine 1 [running]:
panic(0xd65a0,0xc82005e000)
    /usr/local/go/src/runtime/panic.go:464 +0x3e6
reflect.Value.CanInterface(0x0,0x0,0x0)
    /usr/local/go/src/reflect/value.go:897 +0x62

···

怎么回事？为什么v == nil是假的？

解决方法

v不是nil,它包含*字符串.

val.IsValid()

此外,如果你想检查它是否为零,你可以检查val.Kind()== reflect.Ptr&& val.IsNil(){}.

一个小小的演示：

func main() {
    var p1 *string = nil
    var v interface{} = p1
    // this will return an invalid value because it will return the Elem of a nil pointer.
    //val := reflect.Indirect(reflect.ValueOf(v))
    val := reflect.ValueOf(v) // comment this to see the difference.
    if !val.IsValid() {
        fmt.Printf("NULL")
    } else {
        if val.CanInterface() {
            fmt.Printf("if is %#+v (%v)\n",val.Interface(),val.Interface() == nil)
        }
    }
    fmt.Println(v.(*string) == nil)
}