Scala:是否可以将对trait函数定义的访问限制为其直接调用方?

A. protected 只能从定义成员的类的子类访问成员:

scala> trait LoggerHelper {
     |   protected def log() = ???
     | }
defined trait LoggerHelper

scala> class A extends LoggerHelper {
     |   log()
     | }
defined class A

scala> class B(a: A) {
     |   a.log()
     | }
<console>:13: error: method log in trait LoggerHelper cannot be accessed in A
 Access to protected method log not permitted because
 enclosing class B is not a subclass of
 trait LoggerHelper where target is defined
         a.log()
           ^

使用保护的成员 protected[this] 只能从访问 this 类及其子类的实例。

class Base{
    protected val alpha ="Alpha";
    protected[this] def sayHello = "Hello";
    def foo = Console println(new Base().sayHello) // won't compile
    def bar = Console println(this.sayHello)
}
class Derived extends Base{
    def hello = println(this.sayHello) ;
    //def hello2 = println((new Derived() .sayHello) // won't compile
}

使用保护的成员 protected 可以从定义成员的类的任何实例及其子类访问。

class Base{
    protected val alpha ="Alpha";
    protected def sayHello = "Hello";
}
class Derived extends Base{
    def hello = println(this.sayHello);
    def hello2 = println((new Derived()).sayHello); // has access to sayHello() in the original instance
}

使用[此]限制受保护的方法,如

trait LoggerHelper {
  protected[this] def log() = ???
}