CODEWITHSUNDEEP
dart
user603749
user603749

Reputation: 1763

Why does Dart not figure out the class instance variable in the code below

I want to understand why Dart can see that the object b in printBye() function knows it is an Instance of Bye, but it does not know how to figure out the instance variable a;

class Hello {
  void world<T>(T a) {}
}

class Bye {
  int a = 1;
}

class Something extends Hello {
  @override
  void world<T>(T a) {
    printBye(a);
  }
}

void printBye<T>(T b) {
  print(b);    // prints Instance of Bye
  print(b.a); // error
}

void main() {
  new Something()..world(new Bye());
}

https://dartpad.dartlang.org/527e6692846bc018f929db7aea1af583

Edit: here's a simplified version of the code that achieves the same effect: 

class Bye {
  int a = 1;
}

void printBye<T>(T b) {
  print(b);    // prints Instance of Bye
  print(b.a); // error
}

void main() {
  printBye(new Bye());
}

Upvotes: 2

Views: 64

Answers (1)

Gazihan Alankus
Gazihan Alankus

Reputation: 11984

Here's a simple way to look at it: you did call printBye with a Bye, but you could have called it with any other type. When you call printBye with an int, that int wont have a .a. Therefore, you have to assume that printBye could be called with anything. Object represents that anything as every class derives from it.

If you are sure that it will be called with a Bye or a subclass, you can do this to get rid of the error: 

class Bye {
  int a = 1;
}

void printBye<T extends Bye>(T b) {
  print(b);    // prints Instance of Bye
  print(b.a);  // not an error anymore
}

void main() {
  printBye(new Bye());
}

More info: https://dart.dev/guides/language/language-tour#restricting-the-parameterized-type

Upvotes: 2

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