Question

Does Dart support == and === ? What is the difference between equality and identity?

Answer 1

Dart supports == for equality and identical(a, b) for identity. Dart no longer supports the === syntax.

Use == for equality when you want to check if two objects are "equal". You can implement the == method in your class to define what equality means. For example:

class Person {
  String ssn;
  String name;

  Person(this.ssn, this.name);

  // Define that two persons are equal if their SSNs are equal
  bool operator ==(other) {
    return (other is Person && other.ssn == ssn);
  }
}

main() {
  var bob =  Person('111', 'Bob');
  var robert =  Person('111', 'Robert');

  print(bob == robert); // true

  print(identical(bob, robert)); // false, because these are two different instances
}

Note that the semantics of a == b are:

• If either a or b are null, return identical(a, b)
• Otherwise, return a.==(b)

Use identical(a, b) to check if two variables reference the same instance. The identical function is a top-level function found in dart:core.