golang reflect如何判断零值

How to judge zero value of golang reflect

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())
        }
    }
}

这个程序的输出是:

···

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, 0x0)
    /usr/local/go/src/reflect/value.go:897 +0x62

···

怎么回事?为什么v == nilfalse

v 不是零,它包含一个 *string.

但是,如果你想检查一个反射值是否有效(非零),你可以使用val.IsValid()来检查。

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

一个小演示:

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)
}

playground