在参数列表和初始化程序列表中分配值之间的区别 [英] Difference between assigning the values in parameter list and initialiser list

问题描述

class A {
  A(int value);
}

class B extends A{
  final int foo;

  B.one(this.foo) : super(foo); // Works

  B.two(int foo) : foo = foo, super(this.foo); // Doesn't work
}

在 B.one ，我可以轻松地将 foo 的值传递给 super 但在 B中。两个，我做不到。在这两种情况下，都在调用 super 之前分配了 foo 字段，在一种情况下它起作用，而在另一种情况下则失败。因此，问题在于在构造函数中创建字段的时间。

In B.one, I can easily pass the value of foo to super but in B.two, I can't do that. In both cases, the field foo is assigned before calling super, in one case it works and in another it fails. So, the question is at what point the fields are created in the constructor.

推荐答案

Dart分为两个阶段构造对象：

Dart constructs objects in two phases: the first outside-in and then inside-out.

初始化列表是从外到内执行的（从派生类到基类）。完成此操作后，应初始化该对象的成员，该对象被认为是构造的，并且 this 存在。 （这就是为什么您不能在初始化列表中使用 this ；它尚不存在的原因。）

Initialization lists are executed outside-in (from derived class to base class). After this is done, the object's members should be initialized, the object is considered to be "constructed", and this exists. (That's why you can't use this in an initialization list; it doesn't exist yet.)

然后构造函数体从内部类（从基类到派生类）执行。

Constructor bodies are then executed inside-out (from base class to derived class).

此方法保证在基类构造函数体执行时初始化对象的成员，从而允许在构造函数中进行虚拟分派。 （相比之下，C ++纯粹由内而外构造对象，并不允许在构造函数和析构函数中进行虚拟派发。或者与Java相反，IIRC中，该类主要定义了自己的构造顺序，并且这些类负责确保执行任何虚函数调用

This approach guarantees that the object's members are initialized when the base class constructor body executes, allowing virtual dispatch to occur in the constructor. (In contrast, C++ constructs objects purely inside-out and disallows virtual dispatch in constructors and destructors. Or contrast to Java where, IIRC, the class mostly defines its own construction order, and the classes are responsible to ensure that any virtual function calls performed by the constructor are safe.)

没有这种方法，Dart构造函数主体要么不能保证执行虚拟函数时的安全性（Java方法），要么必须禁止虚拟调度（C ++方法）。

Without this approach, a Dart constructor body either cannot guarantee safety when executing virtual functions (the Java approach) or must disallow virtual dispatch (the C++ approach).

另请参见：构造函数中的虚拟函数，语言为何有所不同？

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