Question

在阅读一些关于Scala的文章时，我发现了一些带有好奇语法的例子，我可能会错误地理解它

class Child[C <: Child[C]] {
  some_name : C =>                   // here, what does it mean?
   var roomie : Option[C] = None

   def roomWith(aChild : C)= { 
     roomie = Some(aChild)
     aChild.roomie = Some(this) 
   }
}
class Boy extends Child[Boy]

我发现了类似特征的例子。

这是否意味着我在类范围内将this对象声明为C的类型？

Answer 1

这是一种自我类型的注释。

这意味着类Child必须是C类型，即创建必须满足给定类的继承依赖项。

一个小例子：

scala> trait Baz
defined trait Baz


scala> class Foo {
     | self:Baz => 
     | }
defined class Foo


scala> val a = new Foo
<console>:9: error: class Foo cannot be instantiated because it does not conform to its self-type Foo with Baz
       val a = new Foo
               ^

scala> val a = new Foo with Baz
a: Foo with Baz = $anon$1@199de181


scala> class Bar extends Foo with Baz
defined class Bar

在这种情况下，Foo也必须是Baz。满足该要求，可以创建Foo实例。此外，定义一个新类（在本例中为Bar）也需要它Baz。

请参阅： http://www.scala-lang.org/node/124

Answer 2

自我类型的一个非常有用的应用是CRTP（http://en.wikipedia.org/wiki/Curiously_recurring_template_pattern）的简洁实现，例如。

abstract class Base[Sub] {
  self:Sub =>
  def add(s:Sub) : Sub
}

case class Vec3(x:Double,y:Double,z:Double) extends Base[Vec3] {
  def add(that:Vec3) = Vec3(this.x+that.x, this.y+that.y, this.z+that.z)
}

试图“欺骗”继承是行不通的：

class Foo extends Base[Vec3] {
  add(v:Vec3) = v
}

//error: illegal inheritance;
//self-type Foo does not conform to Base[Vec3]'s selftype Base[Vec3] with Vec3
//      class Foo extends Base[Vec3] {

Answer 3

除了JaimeJorge的响应中的继承要求之外，如果要从内部类引用外部实例，则可以使用self类型为外部实例指定名称：

scala> class Company(name: String) {
     |   company =>
     |   class Department(name: String) {
     |     override def toString = "Department "+ name +" of "+ company.name
     |   }
     | }
defined class Company

scala> val c = new Company("ACME")
c: Company = Company@56a57bb2

scala> val d = new c.Department("Marketing")
d: c.Department = Department Marketing of ACME

“`class declaration head` {val_name：Type =＆gt;`class body`}”的语法含义是什么？

3 个答案: