go - golang json marshal: how to omit empty nested struct -

go playground

as shown in code above, 1 can use json:",omitempty" omit fields in struct appear in json.

for example

type colorgroup struct {     id     int `json:",omitempty"`     name   string     colors []string } type total struct {     colorgroup`json:",omitempty"`     b string`json:",omitempty"` } group := total{        a: colorgroup{},  } 

in case, b won't show in json.marshal(group)

however, if

group := total{        b:"abc",  } 

a still shows in json.marshal(group)

{"a":{"name":"","colors":null},"b":"abc"} 

question how only

{"b":"abc"} 

edit: after googling, here suggestion use pointer, in other words, turn total into

type total struct {     *colorgroup`json:",omitempty"`     b string`json:",omitempty"` } 

from the documentation:

struct values encode json objects. each exported struct field becomes member of object unless

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

the empty values false, 0, nil pointer or interface value, , array, slice, map, or string of length zero.

in declaration of group, it's implicit group.a 0 value of colorgroup struct type. , notice zero-values-of-struct-types not mentioned in list of things considered "empty values".

as found, workaround case use pointer. work if don't specify a in declaration of group. if specify pointer zero-struct, show 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 {         *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) }