downcast

I have three classes: class A {}; class B : virtual public A {}; class C : virtual public A {}; class D: public B, public C {}; Attempting a static cast from A* to B* I get the below error: cannot convert from base A to derived type B via virtual base A In order to understand the cast system you need to dive in the object model. The classic representation of a simple hierarchy model is containment: that if B derives from A then the B object will in fact contain a A subobject alongside its own attributes. With this model, downcasting is a simple pointer manipulation, by an offset known at

pub struct WidgetWrap { // ... widget: RefCell<Box<Any>>, } At some point I want to cast Box<Any> to Box<WidgetTrait> let mut cell = widget.borrow_mut(); let w = cell.downcast_mut::<Box<WidgetTrait>>(); This gives me an error of this kind: error: instantiating a type parameter with an incompatible type `Box<WidgetTrait>`, which does not fulfill `'static` [E0144] What does this really mean? I've looked at How to fix: value may contain references; add `'static` bound to `T` and did try adding + 'static everywhere. pub struct WidgetWrap { // ... widget: RefCell<Box<Any + 'static>>, } let mut cell

Given the following in Swift: var optionalString: String? let dict = NSDictionary() What is the practical difference between the following two statements: optionalString = dict.objectForKey("SomeKey") as? String vs optionalString = dict.objectForKey("SomeKey") as! String? 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

I am new to C# (and OOP ). When I have some code like the following: class Employee { // some code } class Manager : Employee { //some code } Question 1 : If I have other code that does this: Manager mgr = new Manager(); Employee emp = (Employee)mgr; Here Employee is a Manager , but when I cast it like that to an Employee it means I am upcasting it? Question 2 : When I have several Employee class objects and some but not all of them are Manager 's, how can I downcast them where possible? RCIX That is correct. When you do that you are casting it it into an employee object, so that means you

I have come across this example on http://www.javabeginner.com/learn-java/java-object-typecasting and in the part where it talks about explicit type casting there is one example which confuses me. The example: class Vehicle { String name; Vehicle() { name = "Vehicle"; } } class HeavyVehicle extends Vehicle { HeavyVehicle() { name = "HeavyVehicle"; } } class Truck extends HeavyVehicle { Truck() { name = "Truck"; } } class LightVehicle extends Vehicle { LightVehicle() { name = "LightVehicle"; } } public class InstanceOfExample { static boolean result; static HeavyVehicle hV = new HeavyVehicle();