How to access unexported struct fields

Question

Is there a way to use reflect to access unexported fields in Go 1.8? This no longer seems to work: https://stackoverflow.com/a/17982725/555493

Note that reflect.DeepEqual works just fine (that is, it can access unexported fields) but I can't make heads or tails of that function. Here's a go playarea that shows it in action: https://play.golang.org/p/vyEvay6eVG. The src code is below

import (
"fmt"
"reflect"
)

type Foo struct {
  private string
}

func main() {
    x := Foo{"hello"}
    y := Foo{"goodbye"}
    z := Foo{"hello"}

    fmt.Println(reflect.DeepEqual(x,y)) //false
    fmt.Println(reflect.DeepEqual(x,z)) //true
}

Answer 1

If the struct is addressable, you can use unsafe.Pointer to access the field (read or write) it, like this:

rs := reflect.ValueOf(&MyStruct).Elem()
rf := rs.Field(n)
// rf can't be read or set.
rf = reflect.NewAt(rf.Type(), unsafe.Pointer(rf.UnsafeAddr())).Elem()
// Now rf can be read and set.

See full example on the playground.

This use of unsafe.Pointer is valid according to the documentation and running go vet returns no errors.

If the struct is not addressable this trick won't work, but you can create an addressable copy like this:

rs = reflect.ValueOf(MyStruct)
rs2 := reflect.New(rs.Type()).Elem()
rs2.Set(rs)
rf = rs2.Field(0)
rf = reflect.NewAt(rf.Type(), unsafe.Pointer(rf.UnsafeAddr())).Elem()
// Now rf can be read.  Setting will succeed but only affects the temporary copy.

See full example on the playground.