转到：工厂返回的指针和接口{}类型

Question

Consider the following factory: Go playground

type TypeA struct {
    placeholder int
}

func NewTypeA() *TypeA {
    return new(TypeA)
}

func main() {
  factory := make(map[string]func() interface{})
  factory["TypeA"] = NewTypeA
}

This gives me the following error: cannot use NewTypeA (type func() *TypeA) as type func() interface {} in assignment which is pretty clear.

I thought that interface{} was able to be matched with a pointer.

I found this solution: Go playground

type Entity interface {}

type TypeA struct {
    Entity
    placeholder int
}

func NewTypeA() Entity {
    return new(TypeA)
}

func main() {
  var tmp interface{}
  factory := make(map[string]func() Entity)
  factory["TypeA"] = NewTypeA
  tmp = factory["TypeA"]()
  log.Println(reflect.TypeOf(tmp))
}

Which gives me the expected type *TypeA. Is there an other way to do this using the interface{} type directly instead of a "dumb" interface (Entity)?

Answer 1

In your 2nd example NewTypeA() returns Entity not *TypeA, so it is no different from a solution where NewTypeA() would return interface{} which renders the Entity type completely needless/redundant:

type TypeA struct {
    placeholder int
}

func NewTypeA() interface{} {
    return new(TypeA)
}

func main() {
    factory := make(map[string]func() interface{})
    factory["TypeA"] = NewTypeA

    tmp := factory["TypeA"]()
    fmt.Printf("%T", tmp)
}

Output (try it on the Go Playground):

*main.TypeA

Keeping the *TypeA return type

For convenience if you want your NewTypeA() factory function to still return *TypeA, you can do so by putting an anonymous wrapper function in your factory map:

func NewTypeA() *TypeA {
    return new(TypeA)
}

func main() {
    factory := make(map[string]func() interface{})
    factory["TypeA"] = func() interface{} { return NewTypeA() }

    tmp := factory["TypeA"]()
    fmt.Printf("%T", tmp)
}

Output again (Go Playground):

*main.TypeA