How to Unmarshal JSON into Concrete Structs in Go When Using `interface{}`?

Unmarshaling JSON into Concrete Structs

When working with complex data structures, it's often necessary to unmarshal JSON into a concrete struct instead of an interface. However, the default behavior of json.Unmarshal is to convert the JSON data into a map when the destination is an interface.

Problem Explanation

In the provided example, the getFoo function returns an interface{} value that wraps a concrete Foo struct. When json.Unmarshal is called with this value, it creates a map instead of using the underlying Foo struct because the interface{} type alone doesn't provide enough information for unmarshaling.

Solution: Explicitly Passing Struct Reference

To fix this issue, it's necessary to explicitly pass a pointer to the concrete struct to json.Unmarshal. This ensures that the decoder can identify the correct struct type:

func getFoo() interface{} {
    return &Foo{"bar"}
}

By returning a pointer to the struct, the interface{} wrapper now contains a reference to the concrete type, allowing json.Unmarshal to unmarshal the data correctly.

Note:

It's important to note that this solution applies when the specific struct type is not known at compile time. If the struct type is known, it's preferable to pass it directly to json.Unmarshal for better type safety and performance.

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