polymorphism

问题 Here's what I've got on a command button; it's just creating variables and attempting to output their ID (which should be an instance variable inherited from the base class.) Private Sub Command1_Click() Dim ball1 As Ball, ball2 As Ball Dim cube1 As Cube, cube2 As Cube Set ball1 = New Ball Set cube1 = New Cube Set cube2 = New Cube Set ball2 = New Ball MsgBoxTheID (ball1) 'errors; should be 0 MsgBoxTheID (ball2) 'errors; should be 3 MsgBoxTheID (cube1) 'errors; should be 1 MsgBoxTheID (cube2)

问题 I have a class Animal , and its subclass Dog . I have a List<Animal> and I want to add the contents of some List<Dog> to the List<Animal> . Is there a better way to do so, than just cast the List<Dog> to a List<Animal> , and then use AddRange ? 回答1: You don't need the cast if you're using C#4: List<Animal> animals = new List<Animal>(); List<Dog> dogs = new List<Dog>(); animals.AddRange(dogs); That's allowed, because AddRange() accepts an IEnumerable<T> , which is covariant. If you don't have

问题 I have a basic struct called Frame that is useful for a bunch of calculations:. pub struct Frame<T> { grid_val: Vec<T>, grid_space: Vec<[T; 2]>, calculated_result: Option<Vec<T>> } Frame can be used to describe most basic calculations, but sometimes there's more complicated issues that come up and I need to add some more geometric information. So I used composition for each geometry: pub struct Sphere<T> { grid: Frame<T>, radius: T } pub struct Hyperbola<T> { top_grid: Frame<T>, bottom_grid:

问题 I have tried to override val in Scala as this: trait T{ val str: String } sealed abstract class Test extends T{ val str: String = "str" } class Test1 extends Test{ val str = super.str + "test1" //super may not be used on value str } But this refused to compile. Why? I wanted to override value in the abstract class using the abstract class's value itself. How to do this? In my particular case I cannot pass it as a class parameter. 回答1: Scala compiler does not allow to use super on a val . If

问题 I'm using CRTP to add a clone method to inherited classes, for example: class Base { virtual ~Base() {}; virtual Base* clone() const = 0; }; template<class Derived> class BaseCopyable : Base { public: virtual Base* clone() const { return new Derived(static_cast<Derived const&>(*this)); } }; class A : public BaseCopyable<A>; class B : public BaseCopyable<B>; etc... But if I have a class that inherits from B, for example: class differentB : public B; Then clone() doesn't return an object of

问题 I'm studying java language for SCJP test. It is little bit hard to understand "polymorphic method". Could you explain it for me? or give me some links? 回答1: "Polymorphic" means "many shapes." In Java, you can have a superclass with subclasses that do different things, using the same name. The traditional example is superclass Shape , with subclasses Circle , Square , and Rectangle , and method area() . So, for example // note code is abbreviated, this is just for explanation class Shape {

问题 I am extending a base class (one which I do not control) in Swift. I want to provide a class function for creating an instance typed to a subclass. A generic function is required. However, an implementation like the one below does not return the expected subclass type. class Calculator { func showKind() { println("regular") } } class ScientificCalculator: Calculator { let model: String = "HP-15C" override func showKind() { println("scientific") } } extension Calculator { class func create<T

问题 I am extending a base class (one which I do not control) in Swift. I want to provide a class function for creating an instance typed to a subclass. A generic function is required. However, an implementation like the one below does not return the expected subclass type. class Calculator { func showKind() { println("regular") } } class ScientificCalculator: Calculator { let model: String = "HP-15C" override func showKind() { println("scientific") } } extension Calculator { class func create<T

问题 This question already has answers here : Get all derived types of a type (8 answers) Get all inherited classes of an abstract class [duplicate] (4 answers) Closed 5 years ago . During runtime , I would like to populate a drop down list with classes that have extended my base class. Currently I have an enum, and this is what I use to populate that list, but I want to add additional classes (and other people are adding classes) and do not want to have to maintain an enum for this purpose. I

问题 Lets say i have: class Base {...}; class Derived1 : public Base {...}; class Derived2 : public Base {...}; class Derived3 : public Base {...}; class Store { public: void Add(const Base&); //Adds mix of Derived1/2/3 private: vector<const Base*> vec; vector<shared_ptr<const Base> > vec_smart; }; //------------------------------------------------------ void Store::Add(const Base& b){ vec.push_back(&b); //got sliced } When using vector of pointers to Base, it got sliced. I suppose i have to use