Modify null pointer field in struct through reflection in Go

Question

I'm trying to modify the indirect value of the null pointer field "MyField" through a reflection loop. I'm getting a panic: reflect: call of reflect.Value.Set on zero Value.

Any ideas on how to do it?

https://play.golang.org/p/IJvA_J_cD60

package main

import (
    "fmt"
    "reflect"
)

type MyStruct struct {
    MyField *string
}

func main() {
    s := MyStruct{}

    v := reflect.ValueOf(s)

    for i := 0; i < v.NumField(); i++ {
        valueField := v.Field(i)
        fieldName := v.Type().Field(i).Name
        if fieldName == "MyField" && valueField.Kind() == reflect.Ptr {
            valueField.Elem().Set(reflect.ValueOf("some changed name"))
            fmt.Printf("the elem after change: %v\n", valueField.Elem())
        }

    }
    fmt.Print(*s.MyField)
}

Thanks a lot!

Answer 1

There are two issues with the program.

The field cannot be set because v is not an addressable value. To get an addressable value, create a reflect.Value from a pointer to s:

s := MyStruct{}
v := reflect.ValueOf(&s).Elem()

The program attempts to set a value through a nil pointer. The reflect code is identical to *s.MyField = "some changed name". This statement will panic if MyField is nil as it is in the question. To fix this, set a pointer into the field:

t := "some changed name"
valueField.Set(reflect.ValueOf(&t))

Run it on the Playground