How do I read the value of a pointer to a slice using reflect?

Question

package main

import (
    "fmt"
    "reflect"
)

func main() {

    type tu struct {
        N int
    }

    type t struct {
        ARRAY []tu
        NESTED *tu
        NESTED_ARRAY []*tu
    }

    var n = t{[]tu{{4}}, &tu{5}, []*tu{{6}}}

    //print value
    fmt.Printf("value2: %v\n", reflect.ValueOf(&n).Elem().Field(1).Slice(0, 1))
    fmt.Printf("value3: %v\n", reflect.ValueOf(&n).Elem().Field(2).Elem().Field(0))
    fmt.Printf("value4: %v\n", reflect.ValueOf(&n).Elem().Field(3).Slice(0, 1).Elem().Field(0))

    return
}

I'm trying to access the value of a slice pointer using reflection.

reflect.ValueOf(&n).Elem().Field(3).Slice(0, 1).Elem().Field(0) should work but it doesn't.

How should I go around doing this?

Answer 1

Field index starts with 0, not 1. So decrease all indices by 1.

And in your last example the Value.Slice() call returns a slice, not a pointer (not an element of the slice, but a subslice). If you need an element, call Value.Index() on that, then you can call Elem() and Field():

fmt.Printf("value2: %v\n", reflect.ValueOf(&n).Elem().Field(0).Slice(0, 1))
fmt.Printf("value3: %v\n", reflect.ValueOf(&n).Elem().Field(1).Elem().Field(0))
fmt.Printf("value4: %v\n", reflect.ValueOf(&n).Elem().Field(2).Slice(0, 1).Index(0).Elem().Field(0))

This will output (try it on the Go Playground):

value2: [{4}]
value3: 5
value4: 6

Also note that the Value.Slice() method is equivalent to a slicing expression. If you need an element, just call Value.Index(), you don't need the Slice() method:

fmt.Printf("value2: %v\n", reflect.ValueOf(&n).Elem().Field(0).Index(0))
fmt.Printf("value3: %v\n", reflect.ValueOf(&n).Elem().Field(1).Elem().Field(0))
fmt.Printf("value4: %v\n", reflect.ValueOf(&n).Elem().Field(2).Index(0).Elem().Field(0))

This will output (try this one on the Go Playground):

value2: {4}
value3: 5
value4: 6