Why Does `json.Unmarshal` Unexpectedly Change My Struct When Using `interface{}`?

Misconceptions About Go's interface{} Type

Background

The interface{} type, a significant aspect of Go programming, allows for dynamic handling of values, but its usage can often lead to misunderstandings.

An Unexpected Bug with json.Unmarshal

A common misconception arises when using interface{} as a function parameter and encountering an issue with json.Unmarshal. The following code snippet demonstrates this bug:

func test(i interface{}) {
    j := []byte(`{ "foo": "bar" }`)
    fmt.Printf("%T\n", i)
    fmt.Printf("%T\n", &i)
    json.Unmarshal(j, &i)
    fmt.Printf("%T\n", i)
}

func main() {
    test(Test{})
}

type Test struct {
    Foo string
}

When run, this code unexpectedly transforms the struct Test{} into a map[string]interface{} after being unmarshalled, as seen in the output:

main.Test
*interface {}
map[string]interface {}

Understanding the Misconception

The misunderstanding lies in the nature of the interface{} type. It's not an empty container as some may assume, but rather a wrapper for a value-type pair. When the non-pointer Test{} is passed to test(), the json.Unmarshal function expects a pointer, causing it to create a new map[string]interface{} value instead.

To resolve this issue, one must pass a pointer to the interface{} parameter.

func test(i interface{}) {
    j := []byte(`{ "foo": "bar" }`)
    fmt.Printf("%T\n", i)
    fmt.Printf("%T\n", &i)
    json.Unmarshal(j, i)
    fmt.Printf("%T\n", i)
    fmt.Println(i)
}

func main() {
    test(&Test{})
}

This modification ensures that a pointer to a pointer is passed to the function, allowing json.Unmarshal to unmarshal the JSON data correctly.

Conclusion

It's crucial to remember that interface{} requires careful usage to avoid unexpected behavior. Understanding its underlying implementation and its implications on functions like json.Unmarshal is essential for effective Go development.

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