Dart - Get current type of the class

Question

I would like to create a class and refer to the "type of the current class".

So for example in this small snippet, I create a class A which has .child.

As it is written, the type of .child (A?) is "hardcoded", what I mean is, if I create B that extends A, .child will be of type A?. I would like it to automatically be B? "because the current class is B".

class A {
  A? child;
}

class B extends A {
  
}

B? f(B b) {
  return b.child;
}

As it is right now, it shows the error

line 10 • A value of type 'A?' can't be returned from the function 'f' because it has a return type of 'B?'. 

Is there a way to do what I am trying to do? If yes, how?

Answer 1

One approach would be to use generics and borrow the curiously-recurring template pattern from C++ so that the base class knows the derived class's type:

class A<Derived extends A<Derived>> {
  Derived? child;
}

class B extends A<B> {
}

Alternatively you could make A an abstract class and have B override child:

abstract class A {
  A? get child;
}

class B extends A {
  @override
  B? child;
}

If A needs a setter for child, you can use covariant to appease static analysis and allow the setter's value to be checked at runtime:

abstract class A {
  A? get child;
  set child(A? value);
}

class B extends A {
  @override
  covariant B? child;
}