Access unexported fields in golang/reflect?

问题

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
}

回答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 the the documentation for the unsafe package, and go vets cleanly.

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.

回答2:

reflect.DeepEqual() can do it because it has access to unexported features of the reflect package, in this case namely for the valueInterface() function, which takes a safe argument, which denies access to unexported field values via the Value.Interface() method if safe=true. reflect.DeepEqual() will (might) call that passing safe=false.

You can still do it, but you cannot use Value.Interface() for unexported fields. Instead you have to use type-specific methods, such as Value.String() for string, Value.Float() for floats, Value.Int() for ints etc. These will return you a copy of the value (which is enough to inspect it), but will not allow you to modify the field's value (which might be "partly" possible if Value.Interface() would work and the field type would be a pointer type).

If a field happens to be an interface type, you may use Value.Elem() to get to the value contained / wrapped by the interface value.

To demonstrate:

type Foo struct {
    s string
    i int
    j interface{}
}

func main() {
    x := Foo{"hello", 2, 3.0}
    v := reflect.ValueOf(x)

    s := v.FieldByName("s")
    fmt.Printf("%T %v\n", s.String(), s.String())

    i := v.FieldByName("i")
    fmt.Printf("%T %v\n", i.Int(), i.Int())

    j := v.FieldByName("j").Elem()
    fmt.Printf("%T %v\n", j.Float(), j.Float())
}

Output (try it on the Go Playground):

string hello
int64 2
float64 3

来源：https://stackoverflow.com/questions/42664837/access-unexported-fields-in-golang-reflect

标签

reflection

private-members