CODEWITHSUNDEEP
dart
atreeon
atreeon

Reputation: 24097

No compile time error in Dart when a method is given a parent class in place of its subclass

The following gives a runtime error on the last line but why do I not receive a compile time error?

Why is this? fnSub (last line) accepts a type of Sub but here I'm passing it a type of Parent and it compiles. Ok, I do get a runtime error but I'd have thought this should have given me a compile time error. Is this a bug in Dart or am I misunderstanding the limitations of the type system or have I just gone crazy?

class Parent {}

class Sub implements Parent {
  String get blah => "blah";
}

String fnSub(Sub sub) => sub.blah;

String aProblem(Parent parent) => fnSub(parent);

https://dartpad.dev/acd2767cd42371deae0644fa66e8c602

Upvotes: 2

Views: 251

Answers (1)

julemand101
julemand101

Reputation: 31219

The problem is that implicit-casts are enabled by default in Dart which is a feature trying to make it easier to work around types in Dart by automatically adding type casts in your code base.

This feature will no longer be available when NNBD (Non-nullable by default) are coming where implicit-dynamic also will be gone. Both features can already be disabled today by following this guide: https://dart.dev/guides/language/analysis-options#enabling-additional-type-checks

Personally, I think most projects should disable this two features already since I have seen a lot of people on Stackoverflow being confused about what Dart are doing with the types. So I cannot wait for NNBD so we can get are lot more clear type experience in Dart. And hopefully, the errors from the analyzer will be clear enough for most people so they don't need to get help.

If you disable implicit-casts you code will fail at the following line:

String aProblem(Parent parent) => fnSub(parent);

And with this error:

  error - The argument type 'Parent' can't be assigned to the parameter type 'Sub'. - bin\stackoverflow.dart:9:41 - argument_type_not_assignable

If you want to test with Dartpad you can try with the following edition based on a beta version of the next Dart version which has enabled null-safety (and therefore have no implicit-casts): https://nullsafety.dartpad.dev/

Upvotes: 2

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