How to restrict interface{} to a specific type

Question content

I am using go and have a factory function that returns different types of objects based on the requested identifier:

func newobject(id string) interface{} {
    switch id {
    case "truck":
        return truck{}
    case "car":
        return car{}
    ....
    }
}

The following are the respective structure types:

type Truck struct {
    Foo string
}

type Car struct {
    Bar string
}

As you can see, trucks and cars have nothing in common. The problem arises because I now have to deal with the overly broad type interface{} when calling newobject(..). I know there are generics, but this requires keeping a list of all supported types in a type constraint, which complicates things in my code base.

Basically I'm looking for a way how to use inheritance here, which of course go doesn't support. What's the alternative?

Correct answer

newobject(..)Function can be implemented with the support of generics. You don't need to keep a list of all supported types in a type constraint.

func NewObject[T any](id string) T {
 var vehicle any

 switch id {

 case "truck":
  vehicle = Truck{
   Foo: "foo",
  }
 case "car":
  vehicle = Car{
   Bar: "bar",
  }
 }

 if val, ok := vehicle.(T); ok {
  return val
 }

 var otherVehicle T
 fmt.Printf("Not implemented. Returning with default values for \"%v\"\n", id)
 return otherVehicle
}

You can see the full example here.

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