Merging a struct from another struct using reflection in Go. https://go.dev/play/p/m4dXDahCgyW?v=goprev

merge.go

// Merging a struct from another struct using reflection.
package main

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

type A struct {
	Foo     int
	Bar     string
	B       *B
	C       *C
	D       *C
	private bool
}

type B struct {
	Age     int
	Color   string
	C       *C
	private int
}

type C struct {
	Metric float64
	Name   string
}

// merges b into a based on rules
func merge(a, b interface{}) error {
	as := reflect.ValueOf(a)
	bs := reflect.ValueOf(b)

	if as.Kind() != reflect.Ptr || bs.Kind() != reflect.Ptr {
		return errors.New("must pass a pointer to merge")
	}

	return mergef(as.Elem(), bs.Elem())
}

func mergef(as, bs reflect.Value) error {

	if as.Kind() != reflect.Struct || bs.Kind() != reflect.Struct {
		return errors.New("must pass a struct value")
	}

	if as.Type() != bs.Type() {
		return errors.New("can't merge different types")
	}

fields:
	for i := 0; i < as.NumField(); i++ {
		af := as.Field(i)
		bf := bs.Field(i)

		if !af.CanSet() {
			continue
		}

		for af.Kind() == reflect.Ptr {
			if bf.IsNil() {
				// bf is nil so stop processing
				continue fields
			}

			if af.IsNil() {
				if af.Type().Elem().Kind() != reflect.Struct {
					// nil pointer to a non-struct; leave it alone
					break
				}

				// nil pointer to struct, create a zero instance
				af.Set(reflect.New(af.Type().Elem()))
			}
			af = af.Elem()
			bf = bf.Elem()
		}

		// If the field is a struct, recursively merge
		if af.Kind() == reflect.Struct {
			if err := mergef(af, bf); err != nil {
				return err
			}
			continue fields
		}

		// Otherwise set the af field value from the bf field
		if af.IsZero() {
			af.Set(bf)
		}

	}

	return nil
}

func main() {
	a1 := &A{
		Foo: 32,
		B: &B{
			Color: "red",
			C: &C{
				Metric: 0.314,
			},
			private: 21,
		},
		private: true,
	}

	a2 := &A{
		Foo: 18,
		Bar: "hello",
		B: &B{
			Age: 23,
			C: &C{
				Metric: 0.2222,
				Name:   "duece",
			},
			private: 22222,
		},
		C: &C{
			Metric: 44.1231111,
			Name:   "balance",
		},
		private: false,
	}

	if err := merge(a1, a2); err != nil {
		fmt.Println(err)
		return
	}

	data, _ := json.MarshalIndent(a1, "", "  ")
	fmt.Println(string(data))
}