Extending class in a singleton object

Question

I wrote this example and was pretty wondered:

class TestMatch(val i: Int)

object TestMatch extends TestMatch(10){ // <-- Here
  def apply(i: Int) = new TestMatch(i)
  def unapply(tm : TestMatch): Option[Int] = Some(tm.i)
}

What's going on here? We extend TestMatch(10) . How can we extend an instance f test match created with i = 10 ? It doesn't make much sense to me. Or TestMatch(val i: Int) defines a set of types instead of a single type. Like template in C++:

template<int i>
class TestMatch{
   //...
}

I was consufed by the line object

TestMatch extends TestMatch(10)

It looks like we extend TestMatch(10) which I thought was an object of type TestMatch created with a construcotr parameter i = 10 . If I wrote

TestMatch extends TestMatch

it would not compile.

Answer 1

You are not extending an instance of TestMatch class. The below syntax just passes in constructor parameters to the base class.

 object TestMatch extends TestMatch(10)