Class modifiers reference

This page contains reference information for class modifiers.

Valid combinations

The valid combinations of class modifiers and their resulting capabilities are:

Declaration	Construct?	Extend?	Implement?	Mix in?	Exhaustive?
class	Yes	Yes	Yes	No	No
base class	Yes	Yes	No	No	No
interface class	Yes	No	Yes	No	No
final class	Yes	No	No	No	No
sealed class	No	No	No	No	Yes
abstract class	No	Yes	Yes	No	No
abstract base class	No	Yes	No	No	No
abstract interface class	No	No	Yes	No	No
abstract final class	No	No	No	No	No
mixin class	Yes	Yes	Yes	Yes	No
base mixin class	Yes	Yes	No	Yes	No
abstract mixin class	No	Yes	Yes	Yes	No
abstract base mixin class	No	Yes	No	Yes	No
mixin	No	No	Yes	Yes	No
base mixin	No	No	No	Yes	No

Invalid combinations

Certain combinations of modifiers are not allowed:

Combination	Reasoning
base, interface, and final	All control the same two capabilities (extend and implement), so are mutually exclusive.
sealed and abstract	Neither can be constructed, so are redundant together.
sealed with base, interface, or final	sealed types already cannot be mixed in, extended or implemented from another library, so are redundant to combine with the listed modifiers.
mixin and abstract	Neither can be constructed, so are redundant together.
mixin and interface, final, or sealed	A mixin or mixin class declaration is intended to be mixed in, which the listed modifiers prevent.
enum and any modifiers	enum declarations can't be extended, implemented, mixed in, and can always be instantiated, so no modifiers apply to enum declarations.
extension type and any modifiers	extension type declarations can't be extended or mixed in, and can only be implemented by other extension type declarations.