Need to pass empty slice in [] this format and not as ""-CodePudding

I have a api with one attribute of body as

type Reg struct{ Env []string json:"env" }

while calling the api I need to pass value of "env" as optional parameter. i.e. in this format env: [], and not as env: "" Can anyone help pls.

I have tried using len(env)==0 but it take both "" and [] as empty. I have also tried using reflect.ValueOf(Reg.Env).Kind() == reflect.Slice to differentiate between "" and [] but it takes both the values as slice only.

CodePudding user response：

If you check the error of json.Unmarshal, for example, you will know if the user passed invalid input.

var r struct { Env []string `json:"env"` }

if err := json.Unmarshal([]byte(`{"env": ""}`), &r); err != nil {
    fmt.Println(err)
}

Produces this error message:

json: cannot unmarshal string into Go struct field .env of type []string

https://go.dev/play/p/LozhJ14C7zj

The same happens when you use a decoder. I.e. from a http requests body.

var r struct { Env []string `json:"env"` }

dec := json.NewDecoder(strings.NewReader(`{"env": ""}`))

if err := dec.Decode(&r); err != nil { fmt.Println(err) }

https://go.dev/play/p/_XpY3jquxVx

CodePudding user response：

There are multiple options depending on what do you need to send:

Case A: Reg{} will be json-serialized to {"env": null}

Case B: Reg{Env: []string{} } will be json-serialized to {"env": []}

Case C: If ,omitempty is added to the struct tag like this:

type Reg struct {
    Env []string `json:"env,omitempty"`
}

then Reg{} will be json-serialized to {}

See code example here: https://go.dev/play/p/lKI91VCoI0V