Note: In this answer I will use the dyn Trait syntax to make it more clear when a type is a trait object. The older way to write Rc<dyn Trait> is Rc<Trait>. See What does “dyn” mean in a type? No, you can’t downcast Rc<dyn Trait> to Rc<Concrete>, because trait objects like dyn Trait don’t … Read more
The practical difference is this: var optionalString = dict[“SomeKey”] as? String optionalString will be a variable of type String?. If the underlying type is something other than a String this will harmlessly just assign nil to the optional. var optionalString = dict[“SomeKey”] as! String? This says, I know this thing is a String?. This too … Read more
That is correct. When you do that you are casting it it into an employee object, so that means you cannot access anything manager specific. Downcasting is where you take a base class and then try and turn it into a more specific class. This can be accomplished with using is and an explicit cast … Read more
[*] The technical problem is that there’s no way to work out from a Base* what the offset is between the start of the Base sub-object and the start of the Derived object. In your example it appears OK, because there’s only one class in sight with a Base base, and so it appears irrelevant … Read more
Casting to change the object’s type is not possible in PHP (without using a nasty extension). Once you instantiate a object, you can’t change the class (or other implementation details) anymore… You can simulate it with a method like so: public function castAs($newClass) { $obj = new $newClass; foreach (get_object_vars($this) as $key => $name) { … Read more
(I shall ignore the ‘static part as it’s comparatively irrelevant for the parts I’m explaining.) Box<Trait> for a given trait Trait is stored as two pieces of data: a pointer to the actual data in memory and a pointer to the vtable for its type’s implementation of Trait. From that, you may see that you … Read more
No, there’s no built-in way to convert a class like you say. The simplest way to do this would be to do what you suggested: create a DerivedClass(BaseClass) constructor. Other options would basically come out to automate the copying of properties from the base to the derived instance, e.g. using reflection. The code you posted … Read more
Reference vs Object vs Types The key, for me, is understanding the difference between an object and its references, or put in other words the difference between an object and its types. When we create an object in Java, we declare its true nature, which will never change (e.g. new Truck()). But any given object … Read more
No. A reference to a derived class must actually refer to an instance of the derived class (or null). Otherwise how would you expect it to behave? For example: object o = new object(); string s = (string) o; int i = s.Length; // What can this sensibly do? If you want to be able … Read more
Upcasting is casting to a supertype, while downcasting is casting to a subtype. Upcasting is always allowed, but downcasting involves a type check and can throw a ClassCastException. In your case, a cast from a Dog to an Animal is an upcast, because a Dog is-a Animal. In general, you can upcast whenever there is … Read more