golang json元帅：如何省略空的嵌套结构

Question

go playground

As shown in the code above, one can use json:",omitempty" to omit certain fields in a struct to appear in json.

For example

type ColorGroup struct {
    ID     int `json:",omitempty"`
    Name   string
    Colors []string
}
type Total struct {
    A ColorGroup`json:",omitempty"`
    B string`json:",omitempty"`
}
group := Total{
       A: ColorGroup{},

}

In this case, B won't show up in json.Marshal(group)

However, if

group := Total{
       B:"abc",

}

A still shows up in json.Marshal(group)

{"A":{"Name":"","Colors":null},"B":"abc"}

Question is how do we get only

{"B":"abc"}

EDIT: After some googling, here is a suggestion use pointer, in other words, turn Total into

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

Answer 1

From the documentation:

Struct values encode as JSON objects. Each exported struct field becomes a member of the object unless

• the field's tag is "-", or
• the field is empty and its tag specifies the "omitempty" option.

The empty values are false, 0, any nil pointer or interface value, and any array, slice, map, or string of length zero.

In your declaration of group, it's implicit that group.A will be the zero value of the ColorGroup struct type. And notice that zero-values-of-struct-types is not mentioned in that list of things that are considered "empty values".

As you found, the workaround for your case is to use a pointer. This will work if you don't specify A in your declaration of group. If you specify it to be a pointer to a zero-struct, then it will show up again.

playground link:

package main

import (
    "encoding/json"
    "fmt"
    "os"
)

func main() {
    type colorGroup struct {
        ID     int `json:",omitempty"`
        Name   string
        Colors []string
    }
    type total struct {
        A *colorGroup `json:",omitempty"`
        B string     `json:",omitempty"`
    }

    groupWithNilA := total{
        B: "abc",
    }
    b, err := json.Marshal(groupWithNilA)
    if err != nil {
        fmt.Println("error:", err)
    }
    os.Stderr.Write(b)

    println()

    groupWithPointerToZeroA := total{
        A: &colorGroup{},
        B: "abc",
    }
    b, err = json.Marshal(groupWithPointerToZeroA)
    if err != nil {
        fmt.Println("error:", err)
    }
    os.Stderr.Write(b)
}