struct_copy.go

// from::
// http://stackoverflow.com/questions/11527935/ddg#11544854

package main

import (
	"encoding/json"
	"fmt"
	"reflect"
)

type M map[string]interface{} // just an alias

var Record = []byte(`{ "bit_size": 8, "secret_key": false }`)

type DB struct {
	NumBits int  `json:"bit_size"`
	Secret  bool `json:"secret_key"`
}

type User struct {
	NumBits int `json:"num_bits"`
}

func main() {
	d := new(DB)
	e := json.Unmarshal(Record, d)
	if e != nil {
		panic(e)
	}
	m := mapFields(d)
	fmt.Println("Mapped fields: ", m)
	u := new(User)
	o := applyMap(u, m)
	fmt.Println("Applied map: ", o)
	j, e := json.Marshal(o)
	if e != nil {
		panic(e)
	}
	fmt.Println("Output JSON: ", string(j))
}

func applyMap(u *User, m M) M {
	t := reflect.TypeOf(u).Elem()
	o := make(M)
	for i := 0; i < t.NumField(); i++ {
		f := t.FieldByIndex([]int{i})
		// skip unexported fields
		if f.PkgPath != "" {
			continue
		}
		if x, ok := m[f.Name]; ok {
			k := f.Tag.Get("json")
			o[k] = x
		}
	}
	return o
}

func mapFields(x *DB) M {
	o := make(M)
	v := reflect.ValueOf(x).Elem()
	t := v.Type()
	for i := 0; i < v.NumField(); i++ {
		f := t.FieldByIndex([]int{i})
		// skip unexported fields
		if f.PkgPath != "" {
			continue
		}
		o[f.Name] = v.FieldByIndex([]int{i}).Interface()
	}
	return o
}