在函数中传递接口切片并将其解编组

Question

Can i pass into function a slice of structs, converted to []interface{}, fill it and use after function end work?

Here is full example of the problem https://play.golang.org/p/iPijsawEEg

Short describe:

type DBResponse struct {
    Rows  int             `json:"rows"`
    Error string          `json:"error"`
    Value json.RawMessage `json:"value"`
}
type User struct {
    Id   int    `json:"id"`
    Name string `json:"name"`
}

func loadDBRows(p []interface{}) {
    var response DBResponse
    someDataFromDB := []byte("{\"rows\":1, \"error\": \"\", \"value\": {\"name\":\"John\", \"id\":2}}")
    json.Unmarshal(someDataFromDB, &response)
    json.Unmarshal(response.Value, &p[0])
    fmt.Println(p)//p[0] filled with map, not object
}

func main() {
    users := make([]User, 5)
    data := make([]interface{}, 5)
    for i := range users {
        data[i] = users[i]
    }
    loadDBRows(data)
}

This problem can be easily solved for single interface{}, u can test in at full example. Why i can't solve it for slice?

I want to do it without reflect! Is there any "true way" to write universal json parser to selected data struct without reflect and map[string]interface{}? Don't want complicated code and extra operations

Thank you for help!

Answer 1

Since p is a slice of interfaces, on this line json.Unmarshal(response.Value, &p[0]) you're passing a pointer to an interface{} not to a User and since json.Unmarshal allows interfaces as the destination to which to unmarshal the data, it doesn't look under the interface{} for another type and just decodes the json into a map.

What you can do is to have the interface{} already be a pointer to a concrete type e.g. data[i] = &users[i] then you just pass the interface{} without & to json.Unmarshal.

func loadDBRows(p []interface{}) {
    var response DBResponse
    someDataFromDB := []byte("{\"rows\":1, \"error\": \"\", \"value\": {\"name\":\"John\", \"id\":2}}")
    json.Unmarshal(someDataFromDB, &response)
    json.Unmarshal(response.Value, p[0]) // notice the missing &
    fmt.Println(p)
}

users := make([]User, 5)
data := make([]interface{}, 5)
for i := range users {
    data[i] = &users[i] // notice the added &
}

https://play.golang.org/p/GEbIq9febY