Scala 趣題 22 函數重寫，位置參數，參數名

標籤：scala

  class C {    def sum(x: Int = 1, y: Int = 2): Int = x + y  }  class D extends C {    override def sum(y: Int = 3, x: Int = 4): Int = super.sum(x, y)  }  val d: D = new D  val c: C = d  c.sum(x = 0)  d.sum(x = 0)

Explanation

Scala uses the static type of a variable to bind parameter names, but the defaults are determined by the runtime type:

    The binding of parameter names is done by the compiler and the only information the compiler can use is the static type of the variable.

    For parameters with a default value the compiler creates methods which compute the default argument expressions (SLS §4.6). In the above example, both classes C and D contain the methods sum$default$1 and sum$default$2 for the two default parameters. When a parameter is missing, the compiler uses the result of these methods, and these methods are invoked at run-time. 

Scala 趣題 22 函數重寫，位置參數，參數名