How do you create a new instance of a struct from its type at run time in Go?

Question

In Go, how do you create the instance of an object from its type at run time? I suppose you would also need to get the actual type of the object first too?

I am trying to do lazy instantiation to save memory.

Answer 1

In order to do that you need reflect.

package main  import (     "fmt"     "reflect" )  func main() {     // one way is to have a value of the type you want already     a := 1     // reflect.New works kind of like the built-in function new     // We'll get a reflected pointer to a new int value     intPtr := reflect.New(reflect.TypeOf(a))     // Just to prove it     b := intPtr.Elem().Interface().(int)     // Prints 0     fmt.Println(b)      // We can also use reflect.New without having a value of the type     var nilInt *int     intType := reflect.TypeOf(nilInt).Elem()     intPtr2 := reflect.New(intType)     // Same as above     c := intPtr2.Elem().Interface().(int)     // Prints 0 again     fmt.Println(c) } 

You can do the same thing with a struct type instead of an int. Or anything else, really. Just be sure to know the distinction between new and make when it comes to map and slice types.

Answer 2

As reflect.New doesn't automatically make reference types used in struct fields, you could use something like the following to recursively initialize those field types (note the recursive struct definition in this example):

package main  import (     "fmt"     "reflect" )  type Config struct {     Name string     Meta struct {         Desc string         Properties map[string]string         Users []string     } }  func initializeStruct(t reflect.Type, v reflect.Value) {   for i := 0; i < v.NumField(); i++ {     f := v.Field(i)     ft := t.Field(i)     switch ft.Type.Kind() {     case reflect.Map:       f.Set(reflect.MakeMap(ft.Type))     case reflect.Slice:       f.Set(reflect.MakeSlice(ft.Type, 0, 0))     case reflect.Chan:       f.Set(reflect.MakeChan(ft.Type, 0))     case reflect.Struct:       initializeStruct(ft.Type, f)     case reflect.Ptr:       fv := reflect.New(ft.Type.Elem())       initializeStruct(ft.Type.Elem(), fv.Elem())       f.Set(fv)     default:     }   } }  func main() {     t := reflect.TypeOf(Config{})     v := reflect.New(t)     initializeStruct(t, v.Elem())     c := v.Interface().(*Config)     c.Meta.Properties["color"] = "red" // map was already made!     c.Meta.Users = append(c.Meta.Users, "srid") // so was the slice.     fmt.Println(v.Interface()) }