重写局部变量赋值

Question

我想用 Scala 写这个：

var b: Map[String, Int] = Map()
def method() = {
  def a_=(i: Int) = b += "method" -> i
  // ...
  a = 2
}

但这会抱怨说 a 没有定义。这是为什么？我以为 a = ... 被重写为 a_=(...).

解决方案：感谢 Jörg，我的工作，必须提供 getter 并使方法成为顶级：

var b: Map[String, Int] = Map()
def a_=(i: Int) = b += "method" -> i
def a: Int = ??? // Dummy
def method() = {
  // ...
  a = 2
}

编译成功。

Answer 1

您不能覆盖赋值运算符，因为它是一个保留字。你可以做的是：

object BAndA {
  var b: Map[String, Int] = Map()

  def a = b // getter
  def a_=(i: Int) = b += "method" -> i // setter
}

object Test extends App {
  import BAndA._

  a = 1
  println(b)
}

getter 需要

Answer 2

Why is that? I thought a = ... was rewritten to a_=(...).

没有

您需要 getter 和一个 setter 才能使重写生效。
只有当有东西要重写时才会重写，即 object、class 或 trait.

参见section 6.15 Assignments of the Scala Language Specifiation（粗体强调我的）：

If x is a parameterless function defined in some template, and the same template contains a setter function x_= as member, then the assignment x = e is interpreted as the invocation x_=(e) of that setter function.

重写局部变量赋值

Rewrite local var assignment in

scala

variable-assignment

rewriting