Overriding fields in strong mode

[Samuel Rawlins]

Hey Dart strong mode experts,

I am wondering why we cannot override fields in strong mode (without changing type). Take this example:

class A {

  int a;

}

class B extends A {

  @deprecated

  int a;

}

https://dartpad.dartlang.org/69bf3a7802e742e87a9c82b7a641a30a

yields the error: "Field declaration A.a cannot be overridden in B."

I think there are probably many reasons to want to annotate a field from your super class (mark it @deprecated, or to observe it, or declare it non-null, etc), and the only way to annotate it is to re-declare it.

A) Why can we not re-declare a field, if we don't change it's type or access?

B) How else might you annotate a field in your super class?

[Frank Pepermans]

Never used the annotation myself, but did you try @virtual? https://github.com/dart-lang/sdk/issues/27384

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[Samuel Rawlins]

That is almost what I'm looking for, Frank. Thanks for the tip.

However, I'm looking at the issue where you don't control/own the super class. In particular if you are extending dart:html's Element, or something really distant from your code, in a separate package.

Here's the announcement: https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md#strong-mode-1

[Vijay Menon]

FYI - this is slated to be supported again in 1.23 (overriding without virtual):

https://github.com/dart-lang/sdk/issues/28117

That said, in your example below, instances of type B will end up with two field slots for 'a', which might not be what you want.