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

问题描述

假设我有:

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，但是如果我不覆盖，Dart 中 == 和 is 之间的主要区别是什么任何运营商.

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

说我有

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;
  }
}

当我使用我的扩展方法时:

And when I use my extension method with something like:

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

我收到此错误:

double"类型不是int"类型的子类型

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

推荐答案

• identical(x, y) 检查 x 是否与 y 是同一个对象.

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

    x == y 检查是否应将 x 视为等于 y.默认 operator = 的实现= 与 identical() 相同，但 operator == 可以被覆盖以进行深度相等检查(或者理论上可能是病态的并被执行以做任何事情).

    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 检查 x 是否具有 T 类型.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
    }
    

    注意最后一种情况:MyClass is MyClass 是 false 因为左侧是 type，而不是 MyClass 的实例.(不过，MyClass is Type 将是 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 is int 不正确，因为双方都是类型.在这种情况下，您确实想要T == int.请注意，T == int 将检查 exact 类型，如果一个类型是另一个的派生类型(例如 int == num 将是错误的).

    In your code, T is int is incorrect because both sides are types. You do want T == int in that case. Note that T == int would check for an exact type and would not be true if one is a derived type of the other (e.g. int == num would be false).

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