dart是否支持运算符重载

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

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 2

The chosen answer is no longer valid when you try overloads using the == operator in the new version.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 3

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

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

Answer 4

A amazing example to learn how to use operator overloading is a class to handle complex numbers in dart :

import 'dart:core';

class Complex {
  final double real;
  final double imaginary;

  Complex({this.real = 0, this.imaginary = 0});

  Complex.ri(this.real, this.imaginary);

  Complex operator +(Complex b) {
    return Complex(
        real: this.real + b.real, imaginary: this.imaginary + b.imaginary);
  }

  Complex operator -(Complex b) {
    return Complex(
        real: this.real - b.real, imaginary: this.imaginary - b.imaginary);
  }

  Complex operator *(Complex b) {
    return Complex(
        real: this.real * b.real - this.imaginary * b.imaginary,
        imaginary: this.real * b.imaginary + this.imaginary * b.real);
  }

  Complex operator /(Complex b) {
    // https://stackoverflow.com/a/41146661/6846888
    var conjugation = b.conjugate();
    var denominatorRes = b * conjugation;

    // denominator has only real part
    var denominator = denominatorRes.real;
    var nominator = this * conjugation;

    return Complex(
        real: nominator.real / denominator,
        imaginary: nominator.imaginary / denominator);
  }

  bool operator ==(b) {
    return b.real == this.real && b.imaginary == this.imaginary;
  }

  @override
  String toString() {
    return 'Complex(real: ${real}, imaginary: ${imaginary})';
  }
}

Answer 5

Since Dart version 2.7 you can add operators to existing classes, for example:

extension Contains on String {
  bool operator <<(Pattern other) => contains(other);
  bool operator >>(String other) => other.contains(this);
}

Answer 6

Using the Complex numbers as sample, I can implement "Complex * num" like:

Complex operator *(dynamic b) {
  if (b is Complex) {
    return Complex( real: ... );
  } else if (b is num) {
    return Complex.ri(real*b, imaginary*b);
  }
}
...

But how implement "num * Complex"? Is possible?