Why Does `json.Unmarshal` with a Non-Pointer `interface{}` Parameter Produce Unexpected Results in Go?

Interface{} Type Misunderstanding: Unveiling the Complexities

In Go, the interface{} type offers both flexibility and complexity. However, misunderstanding its nature can lead to unexpected consequences, as showcased by a puzzling example.

When attempting to pass a non-pointer type as an interface{} parameter to a function and use json.Unmarshal with it, the output can be surprising. The example below demonstrates this behavior:

package main

import (
    "encoding/json"
    "fmt"
)

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)
}

type Test struct {
    Foo string
}

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

Output:

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

Unveiling the Mystery

The confusion stems from the nature of interface{}. It's not simply a typeless container but rather a wrapper for a (value, type) pair. The interface stores a reference to the concrete value and its type.

json.Unmarshal takes an interface{} value as its input, so we can directly pass i to it. Attempting to take its address (as with &i) is unnecessary.

Pointers and Interfaces

However, if i contains a non-pointer value, json.Unmarshal faces a challenge. Since the package can't unmarshal into a non-pointer, it creates a new value of type interface{}. This allows it to choose the type, which defaults to map[string]interface{}.

The Correct Scenario

For a pointer scenario, passing &i as an argument works because json.Unmarshal dereferences the pointer to find the interface{} value containing the *Test value. The pointer ensures that the unmarshaling occurs into the correct type.

Conclusion

To avoid these confusions, refrain from using pointers to interfaces. Instead, "place" pointers within the interfaces and pass them along directly. By understanding the interplay between interface{} and pointers, developers can avoid unexpected results and effectively leverage Go's type system.

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