I'd like to marshal a variety of objects to file, then unmarshal them, and convert them back to their original type by getting the type of the variables which were marshalled. The key point is that I'd like to convert the unmarshalled object to the type of a specified variable, without specifying the type.
The short pseudocode:
// Marshal this
item := Book{"The Myth of Sisyphus", "Albert Camus"}
// Then unmarshal and convert to the type of the item variable.
itemType := reflect.TypeOf(item)
newItem itemType = unmarshalledItem.(itemType) // This is the problem.
fmt.Println("Unmarshalled is:", reflect.TypeOf(newItem)) // Should print *main.Book
The full code:
package main
import (
"encoding/json"
"fmt"
"io/ioutil"
"os"
"reflect"
)
type Book struct {
Title string
Author string
}
func main() {
// Create objects to marshal.
book := Book{"The Myth of Sisyphus", "Albert Camus"}
box := make(map[string]interface{})
box["The Myth of Sisyphus"] = &book
itemType := reflect.TypeOf(box["The Myth of Sisyphus"])
fmt.Println("Book is:", itemType)
// Marshal objects to file.
err := Write(&book)
if err != nil {
fmt.Println("Unable to save store.", err)
return
}
// Unmarshal objects from file.
untyped := make(map[string]interface{})
bytes, err := ioutil.ReadFile("store.txt")
if err != nil {
fmt.Println("Unable to load store.", err)
return
}
err = json.Unmarshal(bytes, &untyped)
if err != nil {
fmt.Println("Err in store unmarshal.", err)
return
}
// Get Title property of unmarshalled object,
// and use that to get variable type from box map.
for k, v := range untyped {
if k == "Title" {
itemTitle := v.(string)
fmt.Println("Cast item having title:", itemTitle)
targetType := reflect.TypeOf(box[itemTitle])
fmt.Println("Type to cast to is:", targetType)
// Convert untyped to targetType.
// This is the problem.
typed targetType = untyped.(targetType)
fmt.Println("Unmarshalled is:", reflect.TypeOf(typed)) // Should print *main.Book
}
}
}
func Write(b *Book) error {
data, err := json.Marshal(b)
if err != nil {
return err
}
newFilename := "store.txt"
f, err := os.OpenFile(newFilename, os.O_CREATE|os.O_TRUNC, 0660)
if err != nil {
return err
}
_, err = f.WriteString(string(data) + "
")
if err != nil {
return err
}
return nil
}