デフォルト実装である必要はない
class Object {} protocol SomeChild where Self : Object { func doSomething() } class ChildObject: Object, SomeChild { func doSomething() { print("hello") } } class ParentObject: Object { var children: [SomeChild] = [ChildObject()] var child: SomeChild = ChildObject() } let parent = ParentObject() parent.child.doSomething() parent.children.forEach { $0.doSomething() }