Access private fields with reflection

With reflection it's possible to read, but not write, unexported fields of a struct defined in another package.

In this example, we access the unexported field len in the List struct in package container/list:

package list

type List struct {
    root Element
    len  int
}

This code reads the value of len with reflection.

package main

import (
    "container/list"
    "fmt"
    "reflect"
)

func main() {
    l := list.New()
    l.PushFront("foo")
    l.PushFront("bar")

    // Get a reflect.Value fv for the unexported field len.
    fv := reflect.ValueOf(l).Elem().FieldByName("len")
    fmt.Println(fv.Int()) // 2

    // Try to set the value of len.
    fv.Set(reflect.ValueOf(3)) // ILLEGAL
}

2
panic: reflect: reflect.Value.Set using value obtained using unexported field

goroutine 1 [running]:
reflect.flag.mustBeAssignable(0x1a2, 0x285a)
	/usr/local/go/src/reflect/value.go:225 +0x280
reflect.Value.Set(0xee2c0, 0x10444254, 0x1a2, 0xee2c0, 0x1280c0, 0x82)
	/usr/local/go/src/reflect/value.go:1345 +0x40
main.main()
	../main.go:18 +0x280

More code examples

Go blueprints: code for com­mon tasks is a collection of handy code examples.

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