使用反射附加到结构中的切片字段

Question

I have a struct that looks like this:

type guitaristT struct {
    Surname  string   `required=true`
    Year     int64    `required=false`
    American bool     // example of missing tag
    Rating   float32  `required=true`
    Styles   []string `required=true,minsize=1`
}

I have an environment variable that looks like the following, and I'm using reflection to fill the struct based on the keys.

jimiEnvvar :="surname=Hendrix|year=1942|american=true|rating=9.99
  |styles=blues|styles=rock|styles=psychedelic"

I'm able to set the string, int64, bool and float32 fields using reflection, but I'm stuck on how to append to the slice field Styles . For example, based on the above jimiEnvvar I would like the field jimi.Styles to have the values ["blues","rock", "psychedelic"] .

I have the following (simplified) code:

result := guitaristT{}
// result.Styles = make([]string, 10) // XXX causes 10 empty strings at start
result.Styles = make([]string, 0)     // EDIT: Alessandro's solution
...
v := reflect.ValueOf(&result).Elem()
...
field := v.FieldByName(key) // eg key = "styles"
...
switch field.Kind() {

case reflect.Slice:
     // this is where I get stuck
     //
     // reflect.Append() has signature:
     //   func Append(s Value, x ...Value) Value

     // so I convert my value to a reflect.Value
     stringValue := reflect.ValueOf(value) // eg value = "blues"

     // field = reflect.Append(field, stringValue)  // XXX doesn't append
     field.Set(reflect.Append(field, stringValue))  // EDIT: Alessandro's solution

---

EDIT:

A second part (which I solved) was filling a map in the struct. For example:

type guitaristT struct {
    ...
    Styles   []string `required=true,minsize=1`
    Cities   map[string]int
}

Where jimiEnvvar looks like:

jimiEnvvar += "|cities=New York^17|cities=Los Angeles^14"

I wrote in this manner:

    case reflect.Map:
        fmt.Println("keyAsString", keyAsString, "is Map, has value:", valueAsString)
        mapKV := strings.Split(valueAsString, "^")
        if len(mapKV) != 2 {
            log.Fatalln("malformed map key/value:", mapKV)
        }
        mapK := mapKV[0]
        mapV := mapKV[1]
        thisMap := fieldAsValue.Interface().(map[string]int)
        thisMap[mapK] = atoi(mapV)
        thisMapAsValue := reflect.ValueOf(thisMap)
        fieldAsValue.Set(thisMapAsValue)

---

The final result was:

main.guitaristT{
    Surname:  "Hendrix",
    Year:     1942,
    American: true,
    Rating:   9.989999771118164,
    Styles:   {"blues", "rock", "psychedelic"},
    Cities:   {"London":11, "Bay Area":9, "New York":17, "Los Angeles":14},
}

---

If you're interested the full code is at https://github.com/soniah/reflect/blob/master/structs.go . Code is just some notes/exercises I'm writing.

---

EDIT2:

Chapter 11 of "Go in Practice" (Butcher and Farina) has detailed explanations of reflection, structs and tags.

Answer 1

You were not too far off. Just replace with

field.Set(reflect.Append(field, stringValue)) 

and you are done. Also, make sure you that you initialise the slice using

result.Styles = make([]string, 0)

or you will end up having 10 blank string at the top of the Styles array.

Hope this helps and good luck with your project.