Covariant return type for subclasses

December 23, 2023

Beginner in Dart here.

abstract class Supertype {
  WithQuantity<Supertype> newQuantity(num quantity) =>
    WithQuantity(this, quantity)
}

class Subtype extends Supertype {
}

class WithQuantity<S extends Supertype> {
  final S thing;
  final num quantity;
  WithQuantity(this.thing, this.quantity);
}

void main() {
  WithQuantity<Subtype> quantityOfSubtype = Subtype().newQuantity(5);  
}

I’d like for the last line to compile.

In this version of course it doesn’t, because newQuantity returns WithQuantity<Supertype>, not WithQuantity<Subtype>.

I can override the newQuantity method to make the return type narrower, but I’d have to do that for every subclass separately; seems duplicative.

In Java I could have done something like this:

class Supertype<S extends Supertype> {
  WithQuantity<S> newQuantity(...);
}

But Dart doesn’t like this recursion.

Can I somehow tell the compiler that I need to return WithQuantity<whateverMyRuntimeTypeIs>?

Thanks!

>Solution :

You can declare Supertype as a generic whose type parameter is the subclass and then explicitly cast this to be that subclass type:

abstract class Supertype<S extends Supertype<S>> {
  WithQuantity<S> newQuantity(num quantity) =>
      WithQuantity(this as S, quantity);
}

class Subtype extends Supertype<Subtype> {}

class WithQuantity<S extends Supertype<S>> {
  final S thing;
  final num quantity;
  WithQuantity(this.thing, this.quantity);
}

void main() {
  WithQuantity<Subtype> listOfSubtype = Subtype().newQuantity(5);
}