如何强制接口对象执行一些索引操作？

Question

I got log data source looks like:

{"LogtypeA":{"content":"listen_logs":[{"useful_key1":val1,"useful_key2":val2},{"useful_key1":val1}]}}

Then I use simplejson to parse them. The value of listen_logs is a slice that contain at least one map. The code is:

for _, v := range js.Get("LogTypeA").Get("content").Get("listen_logs").MustArray() {
        _obj := reflect.ValueOf(v)
        fmt.Println(_obj.Kind())
    }

If I replace MustArray() with Array(), it will report incorrect variable and constant declarations. Using reflect module I will find that the _obj.Kind() is map, but I can't use any indexing method to get values from v such as:

_val1 := v["useful_key1"]

or

for i, v := range v {...}

Because type interface{} does not support indexing. So how should I extract those useful keyX from those logs?

Answer 1

You need to perform a type assertion on the resulting map:

new_v := v.(map[string]interface{})
_val1 := new_v["useful_key1"]

Or, if you're not sure it will be a map:

new_v, ok := v.(map[string]interface{})
if ok {
    // it was a map from strings to something
    _val1 := new_v["useful_key1"]
} else {
    print("Could not interpret v as a map.")
}

Since it's initially an empty interface, i.e. interface{}, that means it can be any type. So, you need to specify that you want to treat it as a map before trying to access the items.