What is the difference between 'is' and '==' in Dart?

Question

Let\'s say I have:

class Test {
  void method() {
    if (T is int) {
      // T is int
    } 

    if (T == int) {
      // T is int
    }
  }
}
         


        

Answer 1

• identical(x, y) checks if x is the same object as y.

• x == y checks whether x should be considered equal to y. The default implementation for operator == is the same as identical(), but operator == can be overridden to do deep equality checks (or in theory could be pathological and be implemented to do anything).

• x is T checks whether x has type T. x is an object instance.

class MyClass {
  MyClass(this.x);

  int x;

  @override
  bool operator==(dynamic other) {
    return runtimeType == other.runtimeType && x == other.x;
  }

  @override
  int get hashCode => x.hashCode;
}

void main() {
  var c1 = MyClass(42);
  var c2 = MyClass(42);
  var sameC = c1;

  print(identical(c1, c2));    // Prints: false
  print(identical(c1, sameC)); // Prints: true

  print(c1 == c2);    // Prints: true
  print(c1 == sameC); // Prints: true

  print(c1 is MyClass);      // Prints: true
  print(c1 is c1);           // Illegal.  The right-hand-side must be a type.
  print(MyClass is MyClass); // Prints: false
}

Note the last case: MyClass is MyClass is false because the left-hand-side is a type, not an instance of MyClass. (MyClass is Type would be true, however.)

In your code, T is int is incorrect because both sides are types. You do want T == int in that case.