How to show empty object instead of empty struct or nil in Go json marshal

Question

I need to show json's empty object {} when do json.Marshal() for a struct pointer. I can only output either null value or empty struct value.

If the person key is filled with &Person{} or new(Person), it will show empty struct like below:

{
    "data": {
        "person": {
            "name": "",
            "age": 0
        },
        "created_date": "2009-11-10T23:00:00Z"
    }
}

And if we don't initialize it at all, it will show null.

{
    "data": {
        "person": null,
        "created_date": "2009-11-10T23:00:00Z"
    }
}

I want to show "person": {}. Is it possible?

Go Playground for the complete code: https://play.golang.org/p/tT15G2ESPVc

use omitempty for the Person's fields. play.golang.org/p/o3jWdru_8bC — mkopriva
– mkopriva, Commented May 2, 2018 at 14:22
another way you could handle this is by using a non-pointer wrapper type that embeds your Person pointer type, like so: play.golang.org/p/EKQc7uf1_Vk — mkopriva
– mkopriva, Commented May 2, 2018 at 14:37
@mkopriva using omitempty for the Person's fields has worked and by far the simplest. But it will affect my Person's fields encoding when I want to keep showing, for example, Person.name in the marshal result when it is empty — tama
– tama, Commented May 2, 2018 at 14:39
You can use the alternative in my second comment or you'll have to implement the Marshaler interface on the Response type, I think that's pretty much all the options you're left with. — mkopriva
– mkopriva, Commented May 2, 2018 at 14:42
this one's a bit ugly but works without having to compromise your original structure. play.golang.org/p/1qkSCWZ225j — mkopriva
– mkopriva, Commented May 2, 2018 at 14:48

mkopriva · Accepted Answer · 2018-05-02 15:09:41Z

12

Option A, use the omitempty tag option on all of the Person's fields and make sure the response's field is allocated before marshaling.

type Person struct {
    Name string `json:"name,omitempty"`
    Age  int    `json:"age,omitempty"`
}

// ...

resp.Person = new(Person)

https://play.golang.org/p/o3jWdru_8bC

Option B, use a non-pointer wrapper type that embeds the Person pointer type.

type PersonJSON struct {
    *Person
}

type Response struct {
    Person      PersonJSON `json:"person"`
    CreatedDate time.Time   `json:"created_date"`
}

https://play.golang.org/p/EKQc7uf1_Vk

Option C, have the Reponse type implement the json.Marshaler interface.

func (r *Response) MarshalJSON() ([]byte, error) {
    type tmp Response
    resp := (*tmp)(r)

    var data struct {
        Wrapper struct {
            *Person
        } `json:"person"`
        *tmp
    }
    data.Wrapper.Person = resp.Person
    data.tmp = resp
    return json.Marshal(data)
}

https://play.golang.org/p/1qkSCWZ225j

There may be other options...

answered May 2, 2018 at 15:09

mkopriva

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1 Comment

kenswdev Over a year ago

Great answer! Option B was just what I needed.

Metatrix · Accepted Answer · 2018-05-02 14:25:08Z

0

In Go, an empty struct by definition assigns zero values to field elements. Eg: for int 0, "" for string, etc.

For your case, simply comparing to null would work out. Or, you could define an emptyPerson as:

 var BAD_AGE = -1
 emptyPerson := &Person{"", BAD_AGE} // BAD_AGE indicates no person
 if person[age] == BAD_AGE {
    // handle case for emptyPerson}

edited May 2, 2018 at 14:25

answered May 2, 2018 at 14:15

Metatrix

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