如何使用反射将值设置为包含nil的指针

Question

I'm trying to set a value to a nil pointer in a struct like so.

// https://play.golang.org/p/jPTMNC_ZQ9
package main

import (
    "fmt"
    "reflect"
)

type T struct {
    A *int
}

func main() {
    fmt.Println("Hello, playground")
    t := &T{}

    v := 1
    vptr := &v

    CopyValue(vptr, t.A) // I want to set t.A to contain 1
}

func CopyValue(src interface{}, dest interface{}) {
    srcRef := reflect.ValueOf(src)
    if srcRef.Kind() == reflect.Ptr {
        srcRef = srcRef.Elem()
    }

    destRef := reflect.New(srcRef.Type()).Elem()
    destRef.Set(srcRef)
    reflect.ValueOf(dest).Elem().Set(destRef)
}

However, I encounter the following error:

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

goroutine 1 [running]:
reflect.flag.mustBeAssignable(0x0, 0x1040a128)
    /usr/local/go/src/reflect/value.go:221 +0x260
reflect.Value.Set(0x0, 0x0, 0x0, 0xdefc0, 0x1040a128, 0x182)
    /usr/local/go/src/reflect/value.go:1339 +0x40
main.CopyValue(0xd7860, 0x1040a124, 0xd7860, 0x0)
    /tmp/sandbox487854080/main.go:30 +0x1a0
main.main()
    /tmp/sandbox487854080/main.go:19 +0x100

What am I doing wrong?

Answer 1

In order to be able to modify what t.A points to, you need to send a reference to it to your CopyValue function.

CopyValue(vptr, &t.A) // (note the &)

You can then assign the pointer to the new address:

func CopyValue(src interface{}, dest interface{}) {
    srcRef := reflect.ValueOf(src)
    vp := reflect.ValueOf(dest)
    vp.Elem().Set(srcRef)
}

See the 3rd "law of reflection" here: https://blog.golang.org/laws-of-reflection

Full working code:

package main

import (
    "fmt"
    "reflect"
)

type T struct {
    A *int
}

func main() {
    t := &T{}
    v := 1
    vptr := &v
    CopyValue(vptr, &t.A) // we pass a reference to t.A since we want to modify it
    fmt.Printf("%v
", *t.A)
}

func CopyValue(src interface{}, dest interface{}) {
    srcRef := reflect.ValueOf(src)
    vp := reflect.ValueOf(dest)
    vp.Elem().Set(srcRef)
}