CODEWITHSUNDEEP
dartmixins
Joe Lapp
Joe Lapp

Reputation: 2965

Should subclasses inherit private mixin variables in Dart?

Should I get the following error:

class.dart:11:11: Error: The getter '_privateID' isn't defined for the class 'Y'. - 'Y' is from 'class.dart'. Try correcting the name to the name of an existing getter, or defining a getter or field named '_privateID'.

From the following code?

mixin.dart:

class Mixin {
  static int _nextID = 0;

  int publicID = _nextID++; // I only need one of these lines
  int _privateID = _nextID++; // but this variable is inaccessible
}

class.dart:

import 'mixin.dart';

class X with Mixin {
  void run() {
    print(publicID); // no error here
  }
}

class Y with Mixin {
  void run() {
    print(_privateID); // Error: _privateID not defined
  }
}

void main() {
  Y().run();
}

Or is this a bug? If it's not a bug, I'd like to understand why this behavior is reasonable.

When I instead define the mixin in the same file as the above classes, I get no error.

(Dart SDK 2.4.1.)

Upvotes: 4

Views: 2695

Answers (1)

lrn
lrn

Reputation: 71713

It is not a bug. The private field is inherited, but you cannot access it because its name is private to a different library.

Dart's notion of "privacy" is library private names. The name _privateID in the mixin.dart library introduces a library private name. This name is special in that it can only be written inside the same library. If someone writes _privateID in a different library, it is a different name, one unique to that library instead.

It is as if private names includes the library URI of the library it is written in, so what you really declare is a name [email protected]. When you try to read that field in class.dart, you write ._privateID, but because it is in a different library, what you really write is [email protected], a completely different name, and the classs does not have any declarations with that name.

So, if one class needs to access a private member of another class (or mixin, or anything), then the two needs to be declared in the same library, because otherwise they cannot even write the name of that variable.

That is why the code works if you write the mixin in the same library. If you want to move the mixin to a separate file, but not necessarily a separate library, you can use a part file.

Upvotes: 7

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