Why Does `json.Unmarshal` Fail When Using Interfaces Instead of Structs?

Unmarshaling JSON to Structs not Interfaces

When unmarshaling JSON data using json.Unmarshal, it's important to provide a concrete type for the destination variable to avoid unexpected conversions.

In your provided code, you define a function getFoo() that returns an interface containing a Foo struct. When you send this interface to json.Unmarshal, it interprets the value as a map, not a concrete struct.

To address this issue, you can either pass a pointer to the concrete struct within the interface or specify the concrete struct type directly in json.Unmarshal.

Here's an example using a pointer:

func getFoo() interface{} {
    return &Foo{"bar"}
}

func main() {
    fooInterface := getFoo()

    jsonBytes := []byte(`{"bar":"This is the new value of bar"}`)

    err := json.Unmarshal(jsonBytes, fooInterface)
    if err != nil {
        fmt.Println(err)
    }

    fmt.Println(fooInterface) // *main.Foo &{Bar:This is the new value of bar}
}

Alternatively, you can use type assertion to specify the concrete type:

func main() {
    fooInterface := getFoo()

    jsonBytes := []byte(`{"bar":"This is the new value of bar"}`)

    foo := fooInterface.(*Foo)
    err := json.Unmarshal(jsonBytes, foo)
    if err != nil {
        fmt.Println(err)
    }

    fmt.Println(foo) // &{Bar:This is the new value of bar}
}

By providing a concrete type, either through a pointer or type assertion, you ensure that json.Unmarshal directly unmarshals the data into the desired struct, preserving its type information and field values.

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