在Golang中返回接口

Question

I am trying to write a method on a struct that takes in a interface type and returns that interface type but converted to the appropriate type.

type Model interface {
    GetEntity()
}

type TrueFalseQuestions struct {
   //some stuff
}

func (q *TrueFalseQuestions) GetEntity() {
   //some stuff
}

type MultiQuestions struct {
    //some stuff
}

func (q *MultiQuestions) GetEntity() {
    //some stuff
}


type Manager struct {
}


func (man *Manager) GetModel(mod Model) Model {
    mod.GetEntity()
    return mod
}

func main() {
    var man Manager

    q := TrueFalseQuestions {}
    q = man.GetModel(&TrueFalseQuestions {})
}

So when I call GetModel() with type TrueFalseQuestions I want to automatically return a TrueFalseQuestions type. I figured that would mean that my GetModel() method should return a Model type. That way if I pass a MultiQuestion type a MultiQuestion struct is returned.

Answer 1

You can't directly return a TrueFalseQuestions when the return type is Model. It will always be implicitly wrapped in a Model interface.

To get the TrueFalseQuestions back, you need to use a type-assertion. (you also need watch out for pointers vs values)

// this should be a pointer, because the interface methods all have pointer receivers
q := &TrueFalseQuestions{}
q = man.GetModel(&TrueFalseQuestions{}).(*TrueFalseQuestions)

That of course can panic if you got a MultiQuestions, so you should check the ok value, or use a type switch

switch q := man.GetModel(&TrueFalseQuestions{}).(type) {
case *TrueFalseQuestions:
    // q isTrueFalseQuestions
case *MultiQuestions:
    // q is MultiQuestions
default:
    // unexpected type
}