Does dart support operator overloading

Question

I read that Dart does not support function overloading. Does it support operator overloading? If yes, can you show me how it's done in a simple example? And what are some advantages etc?

Answer 1

The chosen answer is no longer valid when you try overloads using the == operator in the new version. Now you need to do this:

class MyClass {
  @override
  bool operator ==(other) {
    // compare this to other
  }
}

But it's not safe. other is not specified as a type, Something unexpected may happened. For example:

void main() {
  var a = A(1);
  var b = B(1);
  var result = a == b;
  print(result); //result is true
}

class A {
  A(this.index);

  final int index;

  @override
  bool operator ==(other) => other.index == index;
}

class B {
  B(this.index);

  final int index;
}

So you could do like this:

class A {
  A(this.index);

  final int index;

  @override
  bool operator ==(covariant A other) => other.index == index;
}

You need to use covariant. Because Object overloads the == operator.

or you can

Test Object Type:
visit: hash_and_equals

class A {
  A(this.index);

  final int index;

  @override
  bool operator ==(other) => other is A && (other.index == index);

  @override
  int get hashCode => index;
}

Answer 2

Dart does support operator overloading using the operator keyword followed by the operator you want to overload. The following example overloads the == operator for the MyClass object:

class MyClass {
  operator ==(MyClass other) {
    // compare this to other
  }
}

almost all Darts built-in operators can be overloaded with a few notable exceptions being the assignment operator = and reference equivalence operator === (doesn't exist anymore).

As for the advantage of operator overloading, it allows you to reuse operators that have a well known semantic meaning such as == or + for operations on your objects. For example, if you have a Matrix class that overloads the + operator then you can add two matrices using the syntax m1 + m2 instead of the more cumbersome m1.plus(m2)

Answer 3

To extend Lars' answer, you can also overload operators using the inline function syntax.

class MyClass {
  operator ==(MyClass o) => id == o.id;
}