Why Does `json.Unmarshal` Fail When Using a Pointer but Succeed with a Reference?

Unmarshaling JSON: Unveiling the Difference Between Reference and Pointer

In the realm of JSON parsing, json.Unmarshal is an indispensable tool. However, its behavior can sometimes be puzzling. Specifically, users may wonder why unmarshaling works smoothly when using a reference but fails mysteriously when employing a pointer. Let's delve into the nitty-gritty of this issue.

Reference vs. Pointer

In the first example, we define "animals" as an Animal type variable. When we pass "&animals" to Unmarshal, we are providing a reference to this variable. This allowsUnmarshal to directly manipulate the original structure.

In contrast, the second example declares "animals" as a pointer to an Animal type. Pointers hold the address of another variable, not the variable itself. Passing "animals" directly to Unmarshal means we are passing a nil pointer since it has not been initialized.

The Paradox

Oddly enough, the documentation for Unmarshal states that it can allocate a new value for the pointer in case it is nil. So, why does the second example fail?

Unveiling the Error

The error message in the second example is actually an InvalidUnmarshalError. This error is thrown when the argument passed to Unmarshal is an invalid pointer. Despite the documentation's statement, Unmarshal does require a non-nil pointer as an argument.

The Bottom Line

To properly Unmarshal JSON with a pointer, you must initialize it first. As the documentation implies, this can be done by simply assigning a valid value to the pointer before calling Unmarshal.

Unmarshalling vs. Unmarshaling

Moreover, you might have noticed that the documentation uses both "unmarshalling" and "unmarshaling." While both spellings are considered correct, "unmarshaling" is the more common and preferred usage in the Go community.

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