Check if struct is empty if struct contains json.RawMessage

Question

I'm trying to create a method extension to check if my struct was initialized but I'm getting this error:

invalid operation: myStruct literal == inStruct (struct containing json.RawMessage cannot be compared)

Here's my code:

package datamodels

import "encoding/json"

type myStruct struct {
        a string json:"a"
        b json.RawMessage json:"b"
        c json.RawMessage json:"c"
    }

func (m *myStruct ) IsEmpty() bool {
    return (myStruct {}) == m 
}

score 5 · Accepted Answer · edited May 23 '17 at 12:23

The reason is that json.RawMessage is a Alias for a []byte and maps, slices etc can not be compared normally.

You can compare slices with reflect.DeepEqual method.

See example below.

package main

import "encoding/json"
import "reflect"

type myStruct struct 
{
        a string `json:"a"`
        b json.RawMessage `json:"b"`
        c json.RawMessage `json:"c"`
}   

func (m myStruct ) IsEmpty() bool {
    return reflect.DeepEqual(myStruct{}, m)
}


func main() {
    var mystuff myStruct = myStruct{}

    mystuff.IsEmpty()
}

GOLANG Playground

Reference for comparing slices: How to compare struct, slice, map are equal?

See the RawMessage type. json.RawMessage type: https://golang.org/src/encoding/json/stream.go?s=6218:6240#L237

This does the trick. Clean and simple. Thanks for providing the actual code, the GOLANG Playground link and reference. — RAFJR, Oct 19 '16 at 13:02

score 1 · Answer 2 · answered Oct 18 '16 at 21:58

1

The zero value of myStruct is a struct where a, b and c are zero values of their type. The zero value of a string is "", of json.RawMessage it is nil (because it's just an alias for []byte). Combining this knowledge you get:

type myStruct struct {
    a string
    b json.RawMessage
    c json.RawMessage
}

func (m *myStruct ) IsEmpty() bool {
    return m.a == "" && m.b == nil && m.c == nil
}

There is no need for reflect.DeepEqual()

answered Oct 18 '16 at 21:58

fl0cke

2,624
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1

this does the job but I prefer not specifying every single field/property of my struct. There has to be a better way since my struct actually have twenty fields/properties. – RAFJR Oct 19 '16 at 05:48

Check if struct is empty if struct contains json.RawMessage

2 Answers2