Cast to interface for implmentation with pointer receiver

June 26, 2024

I want to check whether an received item of type any implements encoding.TextUnmarshaler, so I can invoke UnmarshalText on it. The problem is, it’s method must be a pointer receiver, and this seems to be the source of quite some trouble.

import (
    "encoding"
    "github.com/stretchr/testify/assert"
    "reflect"
    "testing"
)

type MyStruct struct{ value string }

var _ encoding.TextMarshaler = (*MyStruct)(nil)

func (id MyStruct) MarshalText() (text []byte, err error) {
    return []byte(id.value), nil
}

var _ encoding.TextUnmarshaler = (*MyStruct)(nil)

func (id *MyStruct) UnmarshalText(data []byte) error {
    *id = MyStruct{value: string(data)}
    return nil
}

func TestIfICouldInvokeUnmarshalText(t *testing.T) {
    var ms any = MyStruct{value: "hello"}
    _, ok := (ms).(encoding.TextMarshaler)
    assert.True(t, ok)
    _, ok = (ms).(encoding.TextUnmarshaler)
    //assert.True(t, ok) // this is the first attempt, which will fail
    v := reflect.ValueOf(ms)
    if assert.True(t, v.Type().NumMethod() > 0 && v.CanInterface()) {
        if v.CanAddr() {
            _, ok := v.Addr().Interface().(encoding.TextUnmarshaler)
            assert.True(t, ok)
        } else {
            t.FailNow() // this is where I end up
        }
    }
}

Is there any (idiomatic) way to achieve that?

>Solution :

I think what you want to do is something like this:

var ms any = MyStruct{value: "hello"}
_, ok := (ms).(encoding.TextMarshaler)
if !ok {
    t.FailNow()
}
_, ok = (ms).(encoding.TextUnmarshaler)
//assert.True(t, ok) // this is the first attempt, which will fail
v := reflect.Indirect(reflect.New(reflect.TypeOf(ms)))
v.Set(reflect.ValueOf(ms))
sp := v.Addr().Interface()
_, ok = (sp).(encoding.TextUnmarshaler)
if !ok {
    t.FailNow()
}