在Go中基于字符串动态创建某些类型的变量

Question

Simple version
How can you create a variable of a certain type based upon the value of a string?

type ta struct { a int }
type tb struct { b float }
type tc struct { c string }

t := "tb"
v := MagicVarFunc(t) // Returns a new allocated var of type interface{}
v.(tb).b = 8.3

The true example
In my, surprisingly enough, working example below, I am dynamically creating variables based on a string. This is done by registering each struct type in a map with the string being the key and a nil-pointer of the type being the value.
Each type implements an interface with the method New() which returns a new variable of that specific type.

The example below is very close to what I wish to do, where each action has a set of JSON encoded data which will populate the corresponding struct. The way I've structured it is also because I wish to be able to create new stand alone actions that I register to the map.

I am not sure if am abusing the language now.
May anyone give me any pointers if I am completely out of my mind? Is there an obviously easier way?

package main

import (
    "fmt"
    "encoding/json"
)

// All I require of an action is that it may be executed
type ActionHandler interface {
    Exec()
    New() ActionHandler
}

// My list of actions
var mActions = make(map[string]ActionHandler)

// Action Exit (leaving the program)
type aExit struct {}
func (s *aExit) Exec() { fmt.Println("Good bye") }
func (s *aExit) New() ActionHandler { return new(aExit) }
func init() {
    var a *aExit
    mActions[`exit`] = a
}

// Action Say (say a message to someone)
type aSay struct {
    To  string
    Msg string
}
func (s *aSay) Exec() { fmt.Println(`You say, "` + s.Msg + `" to ` + s.To) }
func (s *aSay) New() ActionHandler { return new(aSay) }
func init() {
    var a *aSay
    mActions[`say`] = a
}

func inHandler(action string, data []byte) {
    a := mActions[action].New()
    json.Unmarshal(data, &a)
    a.Exec()
}

func main(){
    inHandler(`say`, []byte(`{"to":"Sonia","msg":"Please help me!"}`))
    inHandler(`exit`, []byte(`{}`))
}

Answer 1

You can use reflection to get the zero value of, or to allocate a new value (like new) of a type using reflection, if you can get the Type value at runtime. However, I don't think there is a way to get the Type from a string. You would need to have a value of that type to get the type itself.

I adopted your idea, of using a map. I map the string to the type itself, which you can get using reflect.TypeOf, which gets the type out of an interface value. Then I used reflect.Zero to get the zero value of that type (a convenient value that exists for every type). Then I got the value out as an interface.

package main
import "reflect"

type ta struct { a int }
type tb struct { b float64 }
type tc struct { c string }

var mActions map[string]reflect.Type = make(map[string]reflect.Type)
func init() {
  var a ta
  mActions[`ta`] = reflect.TypeOf(a)
  var b tb
  mActions[`tb`] = reflect.TypeOf(b)
  var c ta
  mActions[`tc`] = reflect.TypeOf(c)
}

func MagicVarFunc(action string) interface{} {
  return reflect.Zero(mActions[action]).Interface()
}

func main() {
  t := "tb"
  v := MagicVarFunc(t) // Returns a new allocated var of type interface{}
  x := v.(tb)
  x.b = 8.3
}