How to set a function parameter's default value as an object of a class which extends an abstract class?

Question

Let's say I have several classes which extend an abstract class. Now I want to pass a default value to a function argument, where the type of the argument is the abstract class. Dart expects a const value, and I couldn't create a const constructor for an abstract class. How can I pass a default value of the abstract class?

Sample code is as following:

class Main {

  late A objOfA;


  Main({ A nObjOfA = const B() }); // <===== Error here

}

abstract class A {

  abstract String name;
  
  abstract int id;
}

class B extends A {

  @override
  String name = "B";

  @override
  int id = 1;
}

class C extends A {

  @override
  String name = "C";

  @override
  int id = 1;
}

Here, how can I pass a default value of nObjOfA in the constructor of Main?

Answer 1

Dart expects a const value, and I couldn't create a const constructor for an abstract class.

There is no rule that you cannot create a const constructor for an abstract class. Abstract classes can have constructors; you just can't call them directly to instantiate an abstract class. The following code is legal:

class Main {
  A objOfA;

  Main({this.objOfA = const B()});

}

abstract class A {
  const A();
  
  abstract final String name;

  abstract final int id;
}

class B extends A {
  const B();
  
  @override
  final String name = "B";

  @override
  final int id = 1;
}

class C extends A {
  @override
  String name = "C";

  @override
  int id = 1;
}

Note that I needed to add final qualifiers since const objects must be immutable. (Unrelated, but I also fixed the inappropriate use of the late keyword.)

In general, there will be cases where you cannot create a const constructor. If you want to use an instance of that type as a default argument in such cases, you can use null as a default value and assign a non-const value later.