如何从interface {}值（反射）为显式类型的结构成员设置新值？ Golang

Question

I want to understand some subtle moments of using reflect package. Please, see example below, it describes better what I want to know:

type Robot struct {
    id    int
    model string
}

func change(i interface{}, fields ...string) {
        v := reflect.ValueOf(i).Elem()
        // here I emulate function by slice that could return any value,
        // so here I need to check if I can store incoming values to existing struct
        returns := []interface{}{100, "Something"}
        for i, name := range fields {
            x := reflect.ValueOf(&returns[i]).Elem()
            //check if value representing x is the same of struct member
            v.FieldByName(name).Set(x)
            // ^ here I want to store 100 to Robot.id when i = 0,
            // and "Something" to Robot.model when i = 1
        }


}

func main() {
    robot := &Robot{id: 1, model: "T310"}
    change(robot, "model", "id")
    // now robot become as follows: &Robot{100, "Something"}
}

Why does it need for?

    // It is need for retrieving values from sql DB into struct members 
    // (only for training purposes :))
    // Example:
    f := func(q string, structs interface{}, fields ...string) {
        rows, _ := db.Query(q)
        for i := 0; rows.Next(); i++ {
            rows.Scan(&structs[i])
            // very dirty here! it's hard to understand how to implement it
        }
    }
    var robots = []*Robot
    f("select id, model from robots", robots, "id", "model")
    // now each member of robots var should contain values from DB

I tried to be descriptive and explain as short as possible. I hope you understand me..

Answer 1

You can only set exported fields via reflection, so capitalize those first. Otherwise, if you're counting on positional values, make sure they are properly aligned.

Something like this for example: http://play.golang.org/p/ItnjwwJnxe

type Robot struct {
    ID    int
    Model string
}

func change(i interface{}, fields ...string) {
    returns := []interface{}{100, "Something"}

    v := reflect.ValueOf(i).Elem()
    for i, name := range fields {
        val := reflect.ValueOf(returns[i])
        v.FieldByName(name).Set(val)
    }
}

func main() {
    robot := &Robot{ID: 1, Model: "T310"}
    fmt.Println(robot)
    change(robot, "ID", "Model")
    fmt.Println(robot)
}