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!

score 5 · Accepted Answer · 2019-02-08 14:37:02Z

5

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

edited Feb 8, 2019 at 14:37

answered Feb 8, 2019 at 14:34

user5728991

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2 Comments

pmrs Over a year ago

Many thanks! In this case I know the field type. What if I wanted to initialize the field with a Empty/Zero value of the type pointed (i.e. empty string in this case)? Any thoughts on that?

user5728991 Over a year ago

valueField.Set(reflect.New(valueField.Type().Elem()))

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