How to list all classes conforming to protocol in Swift?

Since you're using the Objective-C runtime to get the type introspection you need to add @objc to your code in this manner:

@objc protocol Animal {  func speak()}class Cat:Animal {  @objc func speak() {    print("meow")  }}class Dog: Animal {  @objc func speak() {    print("Av Av!")  }}class Horse: Animal {  @objc func speak() {    print("Hurrrr")  }}

Note that this kind of type introspection might be very slow.

In terms of speed, you can do it in a single for, and avoid having to iterate through all classes like so:

func getClassesConformingProtocol(p: Protocol)-> [AnyClass]{        let expectedClassCount = objc_getClassList(nil, 0)        let allClasses = UnsafeMutablePointer<AnyClass>.allocate(capacity: Int(expectedClassCount))        let autoreleasingAllClasses = AutoreleasingUnsafeMutablePointer<AnyClass>(allClasses)        let actualClassCount:Int32 = objc_getClassList(autoreleasingAllClasses, expectedClassCount)        var classes = [AnyClass]()        for i in 0 ..< actualClassCount {            let currentClass = allClasses[Int(i)]            if class_conformsToProtocol(currentClass, p) {                classes.append(currentClass)            }        }        return classes}