What is the difference between protocols and classes in the context of abstract classes?

A protocol defines a set of requirements for a type (methods, properties, initializers), but does not provide their implementation. A class can implement one or more protocols.

An abstract class (in Swift, this is emulated through classes with unimplemented methods or protocols) is a base class from which other classes inherit. It can contain both abstract (unimplemented) and implemented methods and properties.

Differences:

Attribute	Protocol	Abstract class (emulation in Swift)
Multiple inheritance/implementation	A type can implement multiple protocols	A class can inherit from only one class
State storage	Cannot have stored properties	Can have stored properties
Method implementation	Does not provide implementation	Can contain implemented methods
Instance creation	Cannot create an instance of a protocol	Can create an instance of an abstract class (if there are no abstract methods requiring implementation)