Passing interfaces into functions - json

Question

I'm trying to write a configuration package that takes a json filename and a configuration struct. It should unmarshal the json into the passed-in struct and return it. I'm trying to work with interfaces so I can pass any struct I want
The error is:
panic: interface conversion: interface {} is map[string]interface {}, not *main.ConfigurationData
I'm not quite sure how to solve this issue.
Here is my main package
package main
import (
"config"
"commons"
)
type ConfigurationData struct {
S3ARN string `json:"S3ARN"`
SQSQueueUrl string `json:"SQSQueueUrl"`
}
var configData *ConfigurationData
func main(){
configData=config.Load("aws.config.json",configData).(*ConfigurationData)
commons.Dump(configData)
}
Here is my config package
package config
import (
"os"
"encoding/json"
"sync"
"commons"
)
var configLock = new(sync.RWMutex)
func Load(filename string,config interface{})interface{} {
file, err := os.Open(filename)
commons.CheckErrorf(err, "Config Open Error")
defer file.Close()
decoder := json.NewDecoder(file)
configLock.Lock()
err = decoder.Decode(&config)
commons.CheckErrorf(err, "Config Decode Error")
configLock.Unlock()
return config
}

Answer 1

This answer explains well why you get the exception.
What you should do:
When the encoding/json package runs into a type that implements the Marshaler interface, it uses that type’s MarshalJSON() method instead of the default marshaling code to turn the object into JSON. Similarly, when decoding a JSON object it will test to see if the object implements the Unmarshaler interface, and if so it will use the UnmarshalJSON() method instead of the default unmarshaling behavior.
Mine solution for this would be to implement UnmarshalJSON method on *ConfigurationData and method Load should accept Unmarshaler interface instead of interface{}.
You can read more about technics here: https://blog.gopheracademy.com/advent-2016/advanced-encoding-decoding/
Then you simple would do json.Unmarshal(b, &config) inside the Load method where b is []byte read from file.

Answer 2

According to the documentation on json.Unmarshall unmarshalling into an interface value will unmarshall to a new struct of one of a predefined list of types:
To unmarshal JSON into an interface value, Unmarshal stores one of
these in the interface value:
bool, for JSON booleans
float64, for JSON numbers
string, for JSON
strings []interface{}, for JSON arrays
map[string]interface{}, for JSON objects
nil for JSON null
In your case, the type that was chosen was map[string]interface{}. Within your function, a pointer to that newly unmarshalled struct is stored in the config parameter and returned. The panic occurs as the type of the value returned is not the type you're asserting it is.