Why Does json.Unmarshal Sometimes Return a Map Instead of a Struct?

Unraveling the Discrepancy in json.Unmarshal Output

json.Unmarshal is a versatile function that populates a data structure with values from a JSON string. However, a common puzzlement arises when the expected output differs from the actual, with the resulting structure being a map instead of the intended struct. To unravel this discrepancy, we dive into the code and explore the underlying mechanism.

In the given example, the "bad" function assigns an interface{} named ping to a Ping struct. However, when json.Unmarshal is invoked, it encounters an abstract interface with no concrete type information. As a result, it interprets the incoming JSON as a map, which explains the observed behavior.

To rectify this, we need to explicitly provide json.Unmarshal with the expected struct. This can be achieved by assigning an interface{} to a pointer to the Ping struct. By making this subtle change, we convey to the function the underlying type it should be working with. Alternatively, we can use reflection to create a new pointer to the Ping struct and deserialize into it, subsequently copying the value back to ping.

With these modifications, the json.Unmarshal function successfully marshals the JSON into the desired Ping struct, eliminating the discrepancy and matching our expectations. By understanding the intricacies of the function, we can harness its power to populate data structures reliably and accurately.

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