Why does json.Marshal with json.RawMessage return a Base64 encoded string?

Marshalling json.RawMessage Returns Base64 Encoded String

When calling json.Marshal with a json.RawMessage value, the result is unexpected. Instead of the desired JSON string, a base64 encoded string is returned.

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    raw := json.RawMessage(`{"foo":"bar"}`)
    j, err := json.Marshal(raw)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(j))  // Output: "eyJmb28iOiJiYXIifQ=="
}

The issue lies in the usage of json.RawMessage in json.Marshal. The json.RawMessage type, designed to store raw JSON data without decoding it, has a MarshalJSON method that simply returns the byte slice.

func (m *RawMessage) MarshalJSON() ([]byte, error) {
    return *m, nil
}

However, for json.Marshal to function correctly with json.RawMessage, the value passed must be a pointer to the json.RawMessage instance.

j, err := json.Marshal(&raw)

By passing a pointer to json.RawMessage, the MarshalJSON method is invoked on the pointer, ensuring that the byte slice is returned without base64 encoding.

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    raw := json.RawMessage(`{"foo":"bar"}`)
    j, err := json.Marshal(&raw)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(j))  // Output: "{"foo":"bar"}"
}

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