On Thursday, 10 May 2018 at 14:48:23 UTC, rikki cattermole wrote:
module foo;
class A { }
final class B : A { }
in one module, he wants to be able to create a new
final class C : B { }
and keep class B as final so that no one else can extend it in
another module.
Not entirely. Let's use code to explain. We have:
module pack.foo;
sealed class A {}
class B : A {} // Legal
final class C : A {} // Legal
sealed(pack) class D {}
class E : D {}
===================
module pack.bar;
class F : A {} // Illegal, A is sealed
class B1 : B {} // Legal, B is not sealed - A is
class C1 : C {} // Illegal of course, as C is final
class D1 : D {} // OK, D is package-sealed
class E1 : E {} // OK, E is neither sealed nor final
===================
module anotherpack.baz;
class G : A {} // Illegal, A is sealed
class B2 : B {} // Legal, B is not sealed
class C2 : C {} // Illegal, C is final
class D2 : D {} // Illegal, D is package-sealed for pack
class E2 : E {} // Legal, E is neither sealed nor final
In general: sealed means a special case of neither private,
protected nor final and it probably could not be achieved with a
combination of these two.
Sealed does not mean private, as access to the class is not
restricted in any way. It is also not transitive - class
extending a sealed class can be extended freely unless they are
sealed as well or final. Given those classes:
sealed class Pet {}
class Cat : Pet {}
class Dog : Pet {}
we can implement a different types of Cats and Dogs - but
creating a Ferret or Parrot is illegal. So it may be used to
limit one level of inheritance, but retain the freedom to extend
the few defined types. Making the base class private would not be
OK, as we don't want to restrict the access to that class.
