覆盖继承的构造函数字段时有什么区别？ [英] Differences when overriding inherited constructor fields?

问题描述

考虑这个简单的Scala类：

  A类（val d：Int）
  
 
 
 
 
 
 
 <$ p在Scala中是否有差异（在行为或生成的字节码中） $ p>  class B（d：Int）extends A（d）
  

  class B（override val d：Int）extends A（d）
  

或两者是否相同？

如果 A 会有所不同，如果它们不同，定义为 class A（var d：Int）？

解决方案

对于vals，没有语义差异。但是，生成的字节码可能有差异。特别地，如果在派生类中定义的方法引用 d ，它引用构造函数参数 d ，而不是到相同名称的 val 。

对于vars， 是行为的差异强>。如果没有重载，引用类中引用 d 的任何方法都将引用构造函数参数，而引用 d 从外面的类就会得到字段。在这种情况下，两个值可能不同（如果值自从构建以来已更改）。

这是一个演示var的行为的会话：

  scala> class A（var d：Int）
定义类A 
 
 scala> class B（d：Int）extends A（d）{override def toString =d：+ d} 
定义类B 
 
 scala> val b = new B（1）
 b：B = d：1 
 
 scala> b.d = 2 
 
 scala> b.d 
 res1：Int = 2 
 
 scala> b 
 res2：B = d：1 
  

此问题相关： Idiomatic Sc​​ala方式处理基础vs派生类字段名称？。

Consider this simple Scala class:

class A(val d: Int)

Is there a difference in Scala (either in behaviour or generated bytecode) between

class B(d: Int) extends A(d)

and

class B(override val d: Int) extends A(d)

or are both equivalent? If they are different, what would be the specific usecase for each of them?

Would it be different if A was defined as class A(var d: Int)?

解决方案

For vals, there is no semantic difference. However, there may be a difference in the generated bytecode. In particular, if a method defined in the derived class refers to d, it refers to the constructor parameter d rather than to the val of the same name. This is implemented via an additional private field generated for the derived class.

For vars, there is a difference in behavior. Without an override, any methods that refer to d from within the derived class will be referring to the constructor parameter, while callers referencing d from outside the class will get the field. In this case, the two values may differ (if the value has changed since construction).

Here's a session that demonstrates the behavior with a var:

scala> class A(var d: Int)
defined class A

scala> class B(d: Int) extends A(d) { override def toString = "d: " + d }
defined class B

scala> val b = new B(1)
b: B = d: 1

scala> b.d = 2

scala> b.d
res1: Int = 2

scala> b
res2: B = d: 1

This question is related: Idiomatic Scala way to deal with base vs derived class field names?.

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