Go: encoding/json,xml: omitempty with non-pointer structs

In Go encoding an empty non-pointer struct to JSON,XML with the struct tag omitempty can be a challenge.

Preface of the overall context can be read here: https://golang.org/pkg/encoding/json/#Marshal

Given the following types:

type Chars []byte

type Route struct {
    Sum32 uint32
    Chars
}

type Field struct {
	ID Route
	ConfigPath Route `json:",omitempty"`
	Label Chars `json:",omitempty"`
	CanBeEmpty bool `json:",omitempty"`
	// Default can contain any default config value: float64, int64, string, bool
	Default interface{} `json:",omitempty"`
}

I’ve removed lots of functions and other fields just to focus on the context.

The Field type defines several fields with type Route, a non-pointer struct. The sruct tag json:",omitempty" says that an empty field can be omitted.

This example shows how the Field type looks encoded in JSON with all fields filled out:

But what happens when the ConfigPath field is empty?

Ups. The ConfigPath will also be rendered into JSON because the Marshaller has no clue to check if the field ConfigPath is empty.

Developers usually run into this problem when having a time.Time type in struct. The Time type gets always copied around as a non-pointer. If you JSON encode it you get "t":"0001-01-01T00:00:00Z" instead of “t” being absent.

The reason for that has been buried down deep in the JSON package:

func isEmptyValue(v reflect.Value) bool {
	switch v.Kind() {
	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
		return v.Len() == 0
	case reflect.Bool:
		return !v.Bool()
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return v.Int() == 0
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		return v.Uint() == 0
	case reflect.Float32, reflect.Float64:
		return v.Float() == 0
	case reflect.Interface, reflect.Ptr:
		return v.IsNil()
	}
	return false
}

As you can see there is no switch case for reflect.Struct. Therefore the encoder has no knowledge how to check if a struct is empty.

The issue is open since Nov. 2012 #4357.

There are several solutions:

  1. Sept 2013: golang-nuts to use a pointer.
  2. Using a switch case for reflect.Struct #11939:

    case reflect.Struct:
            return reflect.Zero(v.Type()).Interface() == v.Interface()
    
  3. Using an additional interface in the encoding package #11939 and in #4357:

    type IsZeroer interface {
            IsZero() bool
    }
    
  4. json|xml.Marshal() should be able to return nil, nil (which is currently an error)

  5. json|xml.Marshal() should be able to return nil, encoding.ErrIsEmpty

  6. I’m not in for a pointer because it increases the pressure on the garbage collector.

  7. It’s one way to go.

  8. Additional interface need to be maintained.

  9. Was my first try until I realised the panic error. This feature cannot work because the JSON/XML key has already been written.

  10. Same as 4.

As _rsc stated in CL13914:

I’d really like to stop adding to these packages. I think we need to leave well enough alone at some point.

So I was quite shocked. What to do?

I’ve implemented my own interface :-. For now this seems to be so far so good. But can be refactored in the future. Naming is hard ;-)

type SelfRouter interface {
	SelfRoute() Route
}

The new Field type looks like:

type Field struct {
	ID Route
	ConfigPath SelfRouter `json:",omitempty"`
	Label Chars `json:",omitempty"`
	CanBeEmpty bool `json:",omitempty"`
	Default interface{} `json:",omitempty"`
}

Can you spot or guess why type Chars []byte will be encoded to a human readable string instead of to something like {"ID":"aHR0cF9wb3J0","Default":8080}? ;-)

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