#11

Protocol Defaults Through Class Inheritance

★★★

protocols
dispatch
inheritance

Verified quizzes have their expected answer checked by compiling and running the code with the listed Swift toolchain during the site build.

protocol P { func foo() -> String }
extension P {
    func foo() -> String {
        "P"
    }
}

class A: P {
    func foo() -> String {
        "A"
    }
}

class B: P {}
class C: B {
    func foo() -> String {
        "C"
    }
}

print(A().foo())
print((A() as P).foo())
print(B().foo())
print((B() as P).foo())
print(C().foo())
print((C() as P).foo())

Hint

When a class conforms to a protocol but doesn't implement a requirement, the compiler fills the witness slot with the protocol's default. That choice is frozen at the point of the conformance declaration. What does that mean for a subclass that later defines its own foo()?

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