What elements can be extended using extension in programming language?

In Swift language, extensions can be used to extend the following elements:

• Classes
• Structures
• Enumerations (enum)
• Protocols

Extensions allow adding new methods, computed properties, initializers, subscripts, and protocol conformance without modifying the original source code of the type.

Example of an extension to add a method to the String type:

extension String {
    func reversedString() -> String {
        return String(self.reversed())
    }
}

let text = "hello"
print(text.reversedString()) // output: "olleh"