Why Does `json.Unmarshal` Throw an `InvalidUnmarshalError` When Using an Uninitialized Pointer?

JSON Unmarshal: Pointer vs. Reference

Recently, users have noticed unexpected behavior using json.Unmarshal when passing pointers. Let's investigate why the difference exists.

In the official documentation, we encounter a statement that suggests Unmarshal auto-allocates a new value to point to if the pointer is initially nil:

Unmarshal unmarshals the JSON into the value pointed at by the pointer. If the pointer is nil, Unmarshal allocates a new value for it to point to.

However, when attempting to use Unmarshal with an uninitialized pointer, we encounter an InvalidUnmarshalError:

// An InvalidUnmarshalError describes an invalid argument passed to Unmarshal.

// (The argument to Unmarshal must be a non-nil pointer.)

This error conflicts with the documentation's suggestion of automatic memory allocation.

To resolve this discrepancy, it's crucial to address the subtle difference between using a reference and a pointer. In the first example where the code works, the usage of &animals creates a reference to the variable, ensuring it's always non-nil. This reference behavior aligns with the expected functionality.

In contrast, the second example uses an uninitialized pointer (*animals), which represents an invalid argument in the eyes of Unmarshal. The error it generates reflects this issue accurately.

As a final note, the correct spelling of the word is "unmarshaling," per the official documentation.

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