TIL blog post on pointers, null and UnmarshalJSON() in Go

[dhermes]

If null is the JSON value being deserialized onto a Go pointer, then encoding/json will just skip over the type's custom UnmarshalJSON() altogether. "Obviously null in JSON means nil pointer."

https://go.dev/play/p/adfSt9EbqCp

---

package main

import (
	"encoding/json"
	"fmt"
	"os"
)

type T struct {
	RawJSON []byte
}

func (t *T) UnmarshalJSON(data []byte) error {
	t.RawJSON = data
	return nil
}

func (t T) MarshalJSON() ([]byte, error) {
	return t.RawJSON, nil
}

type S struct {
	Value   T  `json:"value"`
	Pointer *T `json:"pointer"`
}

func run() error {
	inputs := []string{
		`{}`,
		`{"value": 123}`,
		`{"pointer": 123}`,
		`{"value": null}`,
		`{"pointer": null}`,
		`{"value": null, "pointer": null}`,
		`{"value": 123, "pointer": null}`,
		`{"value": null, "pointer": 123}`,
	}

	for _, input := range inputs {
		s := S{}
		err := json.Unmarshal([]byte(input), &s)
		if err != nil {
			return err
		}

		fmt.Printf("--------------------\ninput:\n  %s\ns:\n  %#v\n", input, s)
	}

	return nil
}

func main() {
	err := run()
	if err != nil {
		fmt.Fprintf(os.Stderr, "%v\n", err)
		os.Exit(1)
	}
}