CODEWITHSUNDEEP
databasereflectioninterfacego
Timur Fayzrakhmanov
Timur Fayzrakhmanov

Reputation: 19577

How to set new value to struct member of explicit type from interface{} value (reflection)? Golang

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..

Upvotes: 0

Views: 1254

Answers (1)

JimB
JimB

Reputation: 109332

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)
}

Upvotes: 2

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