How to Unmarshal JSON into a Specific Struct Using an Interface Variable in Go?

How to Unmarshal JSON into a Specific Struct Using Interface Variables

Problem:

When unmarshaling JSON into an interface variable, the encoding/json package interprets it as a map. How can we instruct it to use a specific struct instead?

Explanation:

The JSON package requires a concrete type to target when unmarshaling. Passing an interface{} variable doesn't provide sufficient information, and the package defaults to map[string]interface{} for objects and []interface{} for arrays.

Solution:

There is no built-in way to tell json.Unmarshal to unmarshal into a concrete struct type using an interface variable. However, there is a workaround:

1. Wrap a pointer to the desired struct in the interface variable.

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

2. Pass the wrapped pointer to json.Unmarshal.

err := json.Unmarshal(jsonBytes, fooInterface)

3. Access the unmarshaled data through the interface variable.

fmt.Printf("%T %+v", fooInterface, fooInterface)

Example:

The following code demonstrates the technique:

package main

import (
    "encoding/json"
    "fmt"
)

type Foo struct {
    Bar string `json:"bar"`
}  

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

func main() {

    fooInterface := getFoo() 
    
    myJSON := `{"bar":"This is the new value of bar"}`
    jsonBytes := []byte(myJSON)

    err := json.Unmarshal(jsonBytes, fooInterface)
    if err != nil {
        fmt.Println(err)
    }
    fmt.Printf("%T %+v", fooInterface, fooInterface) // prints: *main.Foo &{Bar:This is the new value of bar}
}

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