Scala：不能写setter没有getter？

发布时间：2020-12-16 09:48:19 所属栏目：安全来源：网络整理

导读：这工作： class ButtonCountObserver { private var cnt = 0 // private field def count = cnt // reader method def count_=(newCount: Int) = cnt = newCount // writer method // ...}val b = new ButtonCountObserver b.count = 0 但这不是 class Butto

这工作：

class ButtonCountObserver {
  private var cnt = 0  // private field
  def count = cnt      // reader method
  def count_=(newCount: Int) = cnt = newCount  // writer method
 // ...
}

val b = new ButtonCountObserver 
b.count = 0

但这不是

class ButtonCountObserver {
  private var cnt = 0  // private field
  def count_=(newCount: Int) = cnt = newCount  // writer method
 // ...
}

val b = new ButtonCountObserver 
b.count = 0

我得到：错误：值计数不是ButtonCountObserver的成员

是否有可能创建一个setter(使用语法糖)没有getter？

解决方法

规范要求setter和getter都被定义为能够使用语法糖来调用setter：

The interpretation of an assignment to
a simple variable x = e depends on the
definition of x. If x denotes a
mutable variable,then the assignment
changes the current value of x to be
the result of evaluating the
expression e. The type of e is
expected to conform to the type of x.
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.
Analogously,an assignment f .x = e to
a parameterless function x is
interpreted as the invocation f .x_=(e
). An assignment f (args) = e with a
function application to the left of
the ‘=’ operator is interpreted as f
.update(args,e ),i.e. the invocation
of an update function defined by f .

此外，为了使用设定器，吸气剂必须是可见的。我不知道如果这是指定

Getter不可见#1

// error: method x cannot be accessed in x.Test
object x {
  class Test { 
    private[this] var x0: Int = 0
    private[Test] def x = x0
    def x_=(a: Int) = x0 = a 
  }
  val t = new Test
  t.x = 1
}

Getter不可见#2

//<console>:11: error: type mismatch; found   : x.Test required: ?{val x: ?}
object x {
  class Test { 
    private[this] var x0: Int = 0
    private[this] def x = x0
    def x_=(a: Int) = x0 = a 
  }
  val t = new Test
  t.x = 1
}

Getter可见

object x {
  class Test { 
    private[this] var x0: Int = 0
    private[x] def x = x0
    def x_=(a: Int) = x0 = a 
  }
  val t = new Test
  t.x = 1
}

（编辑：李大同）

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