CODEWITHSUNDEEP
jsongo
meissnersd
meissnersd

Reputation: 1312

Why does encoding JSON struct members not invoking custom MarshalJSON?

In Golang, I have a struct whose member is a custom int type with constant values. Basically, the custom type is a logical enum. 

type Flavor int

const (
    Vanilla Flavor = iota
    Chocolate
    Strawberry
)

func (f *Flavor) MarshalJSON() ([]byte, error) {
    return []byte(strconv.Quote(f.String())), nil
}

The custom type has defined MarshalJSON and UnmarshalJSON functions so when I serialize the custom type to JSON, I expect to get the string of the value in the serialized output, not the int value.

My issue is that if I have a pointer to a containing type, then the containing type marshals using the custom function but if try to marshal with just a struct value, the custom MarshalJSON is not invoked by the JSON package

type Dessert struct {
    Flavor Flavor `json:"flavor"`
    Count  int
}
....
    d := Dessert{Strawberry, 13}

    b, err = json.Marshal(d) // !! does not invoke members Marshal !!
    b, err = json.Marshal(&d) // works as expected
....

produces 

{"flavor":2,"Count":13}
{"flavor":"Strawberry","Count":13}

I expected the second output in both case.
Why does passing a struct value not invoke MarshalJSON on the member but it does encode otherwise correct JSON?

see https://play.golang.org/p/mOl1GHhgynf for full working code

Upvotes: 1

Views: 1226

Answers (2)

meissnersd
meissnersd

Reputation: 1312

oh huh. I think you had it Volker and Leon. I had assumed that I needed a pointer receiver for MarshalJSON since UnmarshalJSON definitely needs a pointer receiver. But 

func (f Flavor) MarshalJSON() ([]byte, error) {
...
func (f *Flavor) UnmarshalJSON(b []byte) error {
...

and mixing the receivers causes the expected output for both json.Marshal(d) and json.Marshal(&d)

Upvotes: -1

Volker
Volker

Reputation: 42431

In your code Flavor does not have a method MarshalJSON as you defined the method for *Flavor only.

If you want type Flavor to have the MarshalJSON method you must define it on Flavor not *Flavor.

Upvotes: 8

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