function - dart 是否支持运算符重载

Question

我读到 Dart 不支持函数重载。它是否支持运算符重载？如果是的话，你能用一个简单的例子告诉我它是如何完成的吗？还有什么优点等？

Answer 1

==当您在新版本中使用运算符尝试重载时，所选答案不再有效。现在你需要这样做：

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

但这并不安全。other未指定为类型，可能会发生意外情况。例如：

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;
}

所以你可以这样做：

class A {
  A(this.index);

  final int index;

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

你需要使用covariant. 因为 Object 重载了==操作符。

或者你可以

测试对象类型：
访问：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 确实支持使用operator关键字后跟要重载的运算符的运算符重载。以下示例重载了MyClass对象的==运算符：

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

几乎所有 Darts 内置运算符都可以重载，但有一些值得注意的例外是赋值运算符= 和引用等价运算符===（不再存在）。

至于运算符重载的优点，它允许您重用具有众所周知的语义含义的运算符，例如==或+用于对对象的操作。例如，如果您有一个重载+运算符的 Matrix 类，那么您可以使用语法m1 + m2添加两个矩阵，而不是更麻烦的m1.plus(m2)

Answer 3

要扩展 Lars 的答案，您还可以使用内联函数语法重载运算符。

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

Answer 4

学习如何使用运算符重载的一个惊人示例是在dart中处理复数的类：

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

从 Dart 2.7 版开始，您可以将运算符添加到现有类中，例如：

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