Why Does Unmarshaling into an Uninitialized Pointer Fail in Go?

Unmarshaling into References and Pointers

In the process of deserializing JSON data into Go structs, the json.Unmarshal function plays a crucial role. However, users may encounter discrepancies when attempting to unmarshal into references and pointers.

Consider the following code snippets:

// Unmarshaling into a reference variable
var animals Animal
err := json.Unmarshal(jsonBlob, &animals)

This code successfully unmarshals the JSON data into the animals reference variable.

However, a different outcome arises when attempting to unmarshal into a pointer variable:

// Unmarshaling into a pointer variable
var animals *Animal
err := json.Unmarshal(jsonBlob, animals)

In this case, Unmarshal fails with an obscure error: "json: Unmarshal(nil *main.Animal)". This error is encountered because animals is an uninitialized pointer.

The documentation for Unmarshal states that "If the pointer is nil, Unmarshal allocates a new value for it to point to." However, this behavior does not seem to apply in the case of uninitialized pointers, as evidenced by the error message.

To resolve this issue, it is recommended to initialize the pointer variable animals before attempting to unmarshal into it:

animals = &Animal{}
err := json.Unmarshal(jsonBlob, animals)

With this modification, the unmarshaling process should succeed.

Note that in the documentation, the term "unmarshaling" is used consistently.

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