Dart中“ is”和“ ==”有什么区别？ [英] What is the difference between 'is' and '==' in Dart?

问题描述

让我说：

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

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

我知道我可以覆盖 == 运算符，但是 == 和<$ c之间的主要区别是什么如果我不重写任何运算符，则Dart中的$ c>是。

I know I can override == operator but what's the main difference between == and is in Dart if I don't override any operator.

编辑：

假设我有

extension MyIterable<T extends num> on Iterable<T> {
  T sum() {
    T total = T is int ? 0 : 0.0; // setting `T == int` works
    for (T item in this) {
      total += item;
    }
    return total;
  }
}

当我使用扩展方法时，例如：

And when I use my extension method with something like:

var addition = MyIterable([1, 2, 3]).sum();

我收到此错误：

类型'double'不是类型'int'

type 'double' is not a subtype of type 'int'

推荐答案

• same（x，y） 检查 x 是否与 y 。

  x == y 检查是否 x 应该被认为等于 y 。 默认实现> operator == 与 identical（）相同，但 operator == 进行深度平等检查（或从理论上讲可以是病理性的，可以执行任何操作）。

  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是T 检查 x 是否具有类型 T 。 x 是一个对象实例。

  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
  }
  

  请注意最后一种情况： MyClass是MyClass 是 false ，因为左侧是 type ，而不是 instance 为 MyClass 。 （但是 MyClass是Type 会是 true 。）

  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.)

  在您的代码中， T是int 是错误的，因为双方都是类型。在这种情况下，您要做要 T == int 。

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

  这篇关于Dart中“ is”和“ ==”有什么区别？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！