If I have a class C defined as
class C[A]
is there any way to create a new instance of A
within C? Something like
You can demand an implicit parameter, like so:
class A[T](implicit newT : T) {
val t = newT
}
All you need then is to have an implicit factory of the desired type in scope when you instanciate A
, e.g. the following works:
implicit def newSeq[T] = Seq[T]()
val a = new A[Seq[String]]
As shown by:
scala> a.t
res22: Seq[String] = List()
You could use a type class to abstract instantiation:
trait Makeable[T] {
def make: T
}
class C[T: Makeable] {
def f(): T = implicitly[Makeable[T]].make
}
For example,
implicit object StringIsMakeable extends Makeable[String] {
def make: String = "a string"
}
val c = new C[String]
c.f // == "a string"
When you instantiate C, you'll need to provide, explicitly or implicitly, a Makeable that will act as a factory of the appropriate type. That factory, of course, would be responsible for supplying any constructor arguments when it invokes the constructor.
Alternatively, you could use a Manifest, but be warned that this approach relies on reflection and is not type safe:
class C[T: Manifest] {
def f(): T = manifest[T].erasure.newInstance.asInstanceOf[T]
}
For completeness, you can also easily extend this approach to pass some or all of the constructor parameters in to the make method:
trait Makeable[Args, T] { def make(a: Args): T }
class C[Args, T](implicit e: Makeable[Args, T]) {
def f(a: Args): T = e.make(a)
}
// some examples
case class Person(firstName: String, lastName: String)
implicit val personFactory1 = new Makeable[(String, String), Person] {
def make(a: (String, String)): Person = Person(a._1, a._2)
}
implicit val personFactory2 = new Makeable[String, Person] {
def make(a: String): Person = Person(a, "Smith")
}
val c1 = new C[String, Person]
c1.f("Joe") // returns Person("Joe", "Smith")
val c2 = new C[(String, String), Person]
c2.f("John", "Smith") // returns Person("John", "Smith")
The same as @Raphael's answer with a case class's apply
method:
class Container[A](contained: A)
case class Person(name: String)
case class PersonContainer(person: Person) extends Container[Person](person)
implicit def _ = PersonContainer.apply _
class Creator {
def deserializeAndPackage[A, B <: Container[A]](data: Array[Byte])
(implicit containerCreator: (A => B)): B = {
val p = /* deserialize data as type of A */
containerCreator(p)
}
}