How to pass struct as a parameter to a function - json

Question

How can I do something Like this?
I am trying to pass a struct as a parameter to function in Go.
func handleEntityProperties(w http.ResponseWriter, r *http.Request) {
const sliceSize = 100
var entityProperties struct {
Instance string `json:"instance"`
Entities []struct {
Id string `json:"id"`
Properties map[string]string `json:"properties"`
Type string `json:"type"`
} `json:"entities"`
}
body, err := ioutil.ReadAll(r.Body)
if err != nil {
panic(err)
}
if !json.Valid([]byte(body)) {
fmt.Fprintf(w, "invalid json")
return
}
err = json.Unmarshal(body, &entityProperties)
sendData(entityProperties.Entities[0:100])
return
}
func sendData(entities struct) {
log.Println("Doing things with entities ", entities)
}
as you can see in code I am trying to send first 100 elements of entityProperties.Entities struct to a sendData. I know this is syntactically wrong.

Answer 1

Just declare your type outside of the functions:
type entity struct {
Id string `json:"id"`
Properties map[string]string `json:"properties"`
Type string `json:"type"`
}
And reuse it in handleEntityProperties() and in the signature of sendData():
func handleEntityProperties(w http.ResponseWriter, r *http.Request) {
const sliceSize = 100
var entityProperties struct {
Instance string `json:"instance"`
Entities []entity `json:"entities"`
}
body, err := ioutil.ReadAll(r.Body)
if err != nil {
panic(err)
}
if !json.Valid([]byte(body)) {
fmt.Fprintf(w, "invalid json")
return
}
err = json.Unmarshal(body, &entityProperties)
sendData(entityProperties.Entities[0:sliceSize])
return
}
func sendData(entities []entity) {
log.Println("Doing things with entities ", entities)
}
Also note that there is no guarantee that the client will send at least 100 entities, so you should check that else the slicing expression might result in a runtime panic:
max := 100
if len(entityProperties.Entities) < max {
max = len(entityProperties.Entities)
}
sendData(entityProperties.Entities[:max])
Also that check for invalid JSON is unnecessary: if the JSON is invalid, json.Unmarshal() will report a (non-nil) error and you'll know it.
Going further, you don't even have to read the complete body into memory (into a byte slice), you may use json.Decoder to read from it directly (without the intermediate memory buffer) like this:
dec := json.NewDecoder(r.Body)
if err := dec.Decode(&entityProperties); err != nil {
// handle error
}
And the final return statement is also unnecessary.
So an improved version may look like this:
func handleEntityProperties(w http.ResponseWriter, r *http.Request) {
var entityProperties struct {
Instance string `json:"instance"`
Entities []entity `json:"entities"`
}
dec := json.NewDecoder(r.Body)
if err := dec.Decode(&entityProperties); err != nil {
// handle error
http.Error(w, "invalid json", http.StatusBadRequest)
return
}
max := 100
if len(entityProperties.Entities) < max {
max = len(entityProperties.Entities)
}
sendData(entityProperties.Entities[:max])
}