带有选项类型的 class 参数是否不需要传递值？

Question

this code:

class B(d:Int,val b:Option[Int])

object A extends App  {
  val c=new B(1)
}

is wrong at line B(1) , I have to give a value of b , like

new B(1,Some(2))

I am studying slick, the example code is:

final class MessageTable1(tag: Tag) extends Table[Message](tag, "message") {

class Table is in file main\scala\slick\relational\RelationalProfile.scala line 119:

abstract class Table[T](_tableTag: Tag, _schemaName: Option[String], _tableName: String) 
  extends AbstractTable[T](_tableTag, _schemaName, _tableName)

So the _schemaName have the type Option[String] , but when the example code create Table , it seems not pass a value to it, only give the other two parameters: tag and "message" , why?

Answer 1

Three lines down , you can see that Table has an auxiliary constructor with two parameters of type Tag and String (note: no Option here):

def this(_tableTag: Tag, _tableName: String) = this(_tableTag, None, _tableName)

This is the constructor that is called in the example code you showed, not the primary constructor.

You can also see the two constructors in the documentation .