Golang - Hiding empty struct from JSON Response

Question

I'm trying to make the Error and Success struct disappear if either one of them is empty

package main

import (
    "encoding/json"
    "net/http"
)

type appReturn struct {
    Suc *Success `json:"success,omitempty"`
    Err *Error   `json:"error,omitempty"`
}

type Error struct {
    Code    int    `json:"code,omitempty"`
    Message string `json:"message,omitempty"`
}

type Success struct {
    Code    int    `json:"code,omitempty"`
    Message string `json:"message,omitempty"`
}

func init() {
    http.HandleFunc("/", handler)
}

func handler(w http.ResponseWriter, r *http.Request) {
    j := appReturn{&Success{}, &Error{}}

    js, _ := json.Marshal(&j)
    w.Header().Set("Content-Type", "application/json")
    w.Write(js)
}

Output:

{
    success: { },
    error: { }
}

How can I hide the Error or Success struct from the JSON output? I thought that sending the pointer as an argument would do the trick.

dave · Accepted Answer · 2014-09-02 17:32:36Z

2

You can pass in nil for either argument to make it disappear:

http://play.golang.org/p/9Say6mVzCg

j := appReturn{&Success{}, nil}
js, _ := json.Marshal(&j)
fmt.Println(string(js))
j = appReturn{nil, &Error{}}
js, _ = json.Marshal(&j)
fmt.Println(string(js))
j = appReturn{nil, nil}
js, _ = json.Marshal(&j)
fmt.Println(string(js))

If for whatever reason you can't do that, you could also write a custom JSON marshaler to check for the empty struct:

http://play.golang.org/p/W0UhB4qtXH

func (j appReturn) MarshalJSON() ([]byte, error) {
    suc, _ := json.Marshal(j.Suc)
    err, _ := json.Marshal(j.Err)
    if (string(err) == "{}") {
       return []byte("{\"success\":" + string(suc) + "}"), nil
    } else {
        return []byte("{\"error\":" + string(err) + "}"), nil
    }
}

answered Sep 2, 2014 at 17:32

dave

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Mr_Pink · Accepted Answer · 2014-09-02 17:27:01Z

2

This is because appReturn.Suc and appReturn.Err are not empty; they contain pointers to initialized structs, which just happen to have nil pointers inside. The only empty pointer is a nil pointer.

Initialize appReturn as j := appReturn{}

edited Sep 2, 2014 at 17:27

answered Sep 2, 2014 at 17:21

Mr_Pink

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