Modifying struct value using reflection and loop

Question

I wanted to loop through a struct and modify fields value using reflection. How can I Set it?

func main() {
    x := struct {
        Foo string
        Bar int
    }{"foo", 2}
    StructCheck(Checker, x)
}

func Checker(s interface{}) interface{} {
    log.Println(s)
    return s
}

func StructCheck(check func(interface{}) interface{}, x interface{}) interface{} {
    v := reflect.ValueOf(x)
    for i := 0; i < v.NumField(); i++ {
        r := check(v.Field(i))
        w := reflect.ValueOf(&r).Elem()

        log.Println(w.Type(), w.CanSet())

        // v.Field(i).Set(reflect.ValueOf(w))

    }
    return v
}

Running Set() causes panic and shows :reflect.Value.Set using unaddressable value

2 revs user12258482 · Accepted Answer · 2021-04-27 19:40:06Z

4

You must pass an addressable value to the function.

StructCheck(Checker, &x)

There's

Dereference the value in the StructCheck:

v := reflect.ValueOf(x).Elem() // Elem() gets value of ptr

There were some other issues. Here's the updated code:

func StructCheck(check func(interface{}) interface{}, x interface{}) {
    v := reflect.ValueOf(x).Elem()
    for i := 0; i < v.NumField(); i++ {
        r := check(v.Field(i).Interface())
        v.Field(i).Set(reflect.ValueOf(r))

    }
}

Run it on the Playground.

edited Apr 27, 2021 at 19:40

community wiki

2 revs
user12258482

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2 Comments

ned Over a year ago

But when I try to cast it to x's struct it panics again any way to fix this? link

user12258482 Over a year ago

@ned I edited the answer to remove the return value from StructCheck. Because the caller already has the value, there's no need to get the value again from the return value.

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