JSON marshalling with type not exported

Question

I'm want to use a custom type which is not exported and define an interface to use it. The custom type will implement the interface and everything works fine except for unmarshaling from JSON.

I created an example to explain it better:

type (
    Value interface {
        Set(k, v string)
    }

    value map[string]string
)

func New() Value {
    return value{}
}

func (val value) Set(k, v string) {
    val[k] = v
}

This approach will not provide direct access to the map and force the usage of New() function to create objects of type "Value".

When trying to unmarshal in an object created with New() I'm getting the error:

Failed to unmarshal value json: cannot unmarshal object into Go value of type main.Value

That can be fixed by making New() return "value" instead of "Value" but will still be a problem when trying to unmarshal an object like:

Storage struct {
        Val Value `json:"val"`
}

Any suggestions? Implementing Unmarshaler doesn't help in this case. I create a go snippet with this example: https://play.golang.org/p/IEalgBCsTVR

Answer 1

You need to give json.Unmarshal() a pointer to a concrete type. So change (from your linked playground):

unm := New()

to

unm := New().(value)