CODEWITHSUNDEEP
dartoop
Matt
Matt

Reputation: 484

Instantiating a subclass in a superclass?

abstract class A {
  A(this.x, this.y);

  // error: abstract classes cannot be instantiated
  //
  // another issue: even if you used a base concrete class
  // to perform this operation, it would lose type information.
  A copy({int? x, int? y}) => A(x ?? this.x, y ?? this.y);

  final int x;
  final int y;
}

class B extends A {
  // Forced to implement copy and similar
  // methods on all classes that extend A,
  // which is problematic when that number
  // is large or changes are necessary.
}

Is there a way to solve this problem or do I have to essentially rewrite the same code for all classes that extend A?

Upvotes: 0

Views: 471

Answers (1)

lrn
lrn

Reputation: 71623

You can, but it requires you to do quite a lot of the work you are asking to avoid:

class A<T extends A<T>> {
  final T Function(int, int) _constructor;
  final int x;
  final int y;

  A._(this._constructor, this.x, this.y);

  T copy({int? x, int? y}) => _constructor(x ?? this.x, y ?? this.y);
}
class B extends A<B> {
  B(int x, int y) : super._((int x, int y) => B(x, y), x, y);
}

(The code will get shorter when Dart gets constructor tear-offs, then it's just, super._(B, x, y);.)

You cannot, currently, inherit constructors, and you can't create an instance of a type that you don't know yet (because constructors are not inherited, so you don't know if the constructor exists). The only way to abstract over actual behavior (which code to run) is to capture it in a closure and pass it as a function.

Upvotes: 1

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