What are the differences between asInstanceOf[T] and (o: T) in Scala?

Question

I saw that there are two methods to cast an object in Scala:

foo.asInstanceOf[Bar]
(foo: Bar)

When I tried, I found that asInstanceOf

Answer 1

There is example of the difference:

1. Type cast (asInstanceOf) is a run-time operation with potentially run-time exception.
2. Ascription is basically just an up-cast performed at compile-time.

Example:

class Parent() { def method() {} }
class Child1 extends Parent() { def method1() {} }
class Child2 extends Parent() { def method2() {} }

// we return Parent type
def getChild1() : Parent = new Child1()
def getChild2() : Parent = new Child2()
def getChild()  : Child1 = new Child1()

(getChild1().asInstanceOf[Child1]).method1() // OK
(getChild1().asInstanceOf[Child2]).method2() // runtime ClassCastException

(getChild1() : Child2).method2() // compile-time error
(getChild2() : Child2).method2() // compile-time error
(getChild() : Parent).method1() // compile-time error
(getChild()).method()  // OK

// with asInstanceOf, we can cast to anything without compile-time error
getChild1().asInstanceOf[String] // runtime ClassCastException
getChild1().asInstanceOf[Int] // runtime ClassCastException

We can also call method using multiple-dispatch:

def prt(p: Parent) = println("parent")
def prt(ch: Child1) = println("child")

prt(new Parent()) // prints "parent"
prt((new Child1()) : Parent) // prints "parent"
prt(new Child1()) // prints "child"

prt(new Parent().asInstanceOf[Child1]) // runtime ClassCastException
prt(new Child1().asInstanceOf[Parent]) // prints "parent"

We can define implicit conversion:

// after definition of implicit conversions
implicit def toChild1(p: Parent) : Child1 = new Child1()
implicit def toChild2(p: Parent) : Child2 = new Child2()

(getChild1() : Child2).method2() // OK - implicit conversion to Child2 in ascription
(getChild2() : Child2).method2() // OK - implicit conversion to Child2 in ascription
(getChild2()).method1() // OK - implicit conversion to Child1 when calling method1()
(getChild2()).method2() // OK - implicit conversion to Child2 when calling method2()
(getChild2() : Parent).method() // OK - no implicit conversion
(getChild() : Parent).method1() // OK - implicit conversion to Child1 when calling method()

getChild1().asInstanceOf[Int] // still runtime ClassCastException (no implicit conversion)