How to Omit Empty Nested Structs in Go's JSON Marshaling?

Omitting Empty Nested Stucts in JSON Marshaling

Context:

JSON marshaling in Go enables converting structs into JSON objects. By default, all exported struct fields are included in the JSON output. However, empty fields can be omitted by specifying :",omitempty" as the field tag.

Problem:

If a nested struct is empty (i.e., has zero values for all its fields), it may still appear in the JSON output even though it has the :",omitempty" tag.

Solution:

To omit empty nested structs, it is necessary to use pointers to the structs. By doing so, the empty value of the nested struct will be considered a nil pointer.

Explanation:

From the Go documentation:

• Struct values encode as JSON objects. Empty values include nil pointers.
• The omitempty tag applies to empty values, including nil pointers.

Therefore, by using a pointer to the nested struct, we ensure that it is considered an empty value and thus omitted from the JSON output when the :",omitempty" tag is used.

Example:

type ColorGroup struct {
    ID     int `json:",omitempty"`
    Name   string
    Colors []string
}

type Total struct {
    A *ColorGroup `json:",omitempty"`
    B string     `json:",omitempty"`
}

// Empty nested struct (zero values)
group := Total{
    B: "abc",
}

// Marshal group without nested struct (only "B" field included)
b, err := json.Marshal(group)
if err != nil {
    fmt.Println("error:", err)
}
os.Stderr.Write(b)

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