嵌入式结构的多态JSON解组

Question

Here is an example (see also https://play.golang.org/p/or7z4Xc8tN):

package main

import (
    "encoding/json"
    "fmt"
)

type A struct {
    X string
    Y int
}

type B struct {
    A
    Y string 
}

func main() {
    data := []byte(`{"X": "me", "Y": "hi"}`)
    b := &B{}
    json.Unmarshal(data, b)
    fmt.Println(b)
    fmt.Println(b.A)

    b = &B{}
    data = []byte(`{"X": "me", "Y": 123}`)
    json.Unmarshal(data, b)
    fmt.Println(b)
    fmt.Println(b.A)
}

Which outputs:

&{{me 0} hi}
{me 0}
&{{me 0} }
{me 0}

Is there a way to polymorphically unmarshal the field Y to either an int or a string? Or even unmarshal into A.Y at all since B.Y is defined?

I know some might suggest unmarshalling with something like json.Unmarshall(data, &b.A), but I don't know if I can fit that into my current design.

Answer 1

Go's only polymorphism is interfaces. Embedding does not offer polymorphism.

If you're trying to unmarshal JSON where you can't assume what type one of the fields is going to be, you can use a field of type interface{} along with type assertions, fmt.Sprint, or reflection. Which you should use depends on the particular use case - once you've got the value, what are you going to do with it? At some point you have to care if it's an int or a string, which will determine how you handle the value.