setting a nil pointer to a slice in a struct using reflection

Question

I'm practicing reflections using go and I'm trying to achieve the following, to have a struct type, with a field that is a pointer to a slice of strings.

now that pointer is nil, i want to create the slice, add a value and set that pointer in the struct to point to the newly created slice, and to do all of that using reflections.

I created the following sample code to demonstrate what I'm doing:

package main

import (
    "log"
    "reflect"
)

type UserInfo struct {
    Name   string
    Roles  *[]string
    UserId int
}


func main() {
    var myV UserInfo
    myV.Name="moshe"
    myV.UserId=5
    v := reflect.ValueOf(&myV.Roles)
    t := reflect.TypeOf(myV.Roles)
    myP := reflect.MakeSlice(t.Elem(),1,1)
    myP.Index(0).SetString("USER")
    v.Elem().Set(reflect.ValueOf(&myP)) <-- PANIC HERE
    log.Print(myV.Roles)
}

this panics the message

panic: reflect.Set: value of type *reflect.Value is not assignable to type *[]string

of course the slice does not create a pointer so if i do this:

v.Elem().Set(myP.Convert(v.Elem().Type()))

I get

panic: reflect.Value.Convert: value of type []string cannot be converted to type *[]string

but when I try to convert the address with

v.Elem().Set(reflect.ValueOf(&myP).Convert(v.Elem().Type()))

I get

panic: reflect.Value.Convert: value of type *reflect.Value cannot be converted to type *[]string

what am I still missing?

thanks!

Mr_Pink · Accepted Answer · 2020-10-05 17:06:15Z

1

You are attempting to set the value using a reflect.Value of a pointer to a reflect.Value, which is definitely not the same as a *[]string

Build the value up in steps and work your way outwards:

// create the []string, and set the element
slice := reflect.MakeSlice(t.Elem(), 1, 1)
slice.Index(0).SetString("USER")

// create the *[]string pointer, and set its value to point to the slice
ptr := reflect.New(slice.Type())
ptr.Elem().Set(slice)

// set the pointer in the struct
v.Elem().Set(ptr)

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

answered Oct 5, 2020 at 17:06

Mr_Pink

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