polymorphism

问题 I'm pretty new to Rails, and i'm trying to do a polymorphic HABTM relationship. The problem is that I have three models that I want to relate. The first one is the Event model and then are two kind of attendees: Users and Contacts. What I want to do is to be able to relate as an attendee both users and contacts. So, what i have right now in my code is: Event Model has_and_belongs_to_many :attendees, :polymorphic => true User Model has_and_belongs_to_many :events, :as => :attendees Contact

问题 I have the following code in which I have a parent class and its child. I am trying to determine how the code benefits from using polymorphism. class FlyingMachines { public void fly() { System.out.println("No implementation"); } } class Jet extends FlyingMachines { public void fly() { System.out.println("Start, Taxi, Fly"); } public void bombardment() { System.out.println("Throw Missile"); } } public class PolymorphicTest { public static void main(String[] args) { FlyingMachines flm = new

问题 I'm working on a multithreaded application, and I want to debug it using GDB. Problem is, one of my threads keeps dying with the message: pure virtual method called terminate called without an active exception Abort I know the cause of that message, but I have no idea where in my thread it occurs. A backtrace would really be helpful. When I run my app in GDB, it pauses every time a thread is suspended or resumed. I want my app to continue running normally until one of the threads dies with

问题 Case 1 static void call(Integer i) { System.out.println("hi" + i); } static void call(int i) { System.out.println("hello" + i); } public static void main(String... args) { call(10); } Output of Case 1 : hello10 Case 2 static void call(Integer... i) { System.out.println("hi" + i); } static void call(int... i) { System.out.println("hello" + i); } public static void main(String... args) { call(10); } Shows compilation error reference to call ambiguous . But, I was unable to understand. Why ? But

问题 Closed . This question is opinion-based. It is not currently accepting answers. Want to improve this question? Update the question so it can be answered with facts and citations by editing this post. Closed last year . Is Method Overloading considered part of polymorphism? 回答1: "Polymorphism" is just a word and doesn't have a globally agreed-upon, precise definition. You will not be enlightened by either a "yes" or a "no" answer to your question because the difference will be in the chosen

问题 I have a tricky situation. Its simplified form is something like this class Instruction { public: virtual void execute() { } }; class Add: public Instruction { private: int a; int b; int c; public: Add(int x, int y, int z) {a=x;b=y;c=z;} void execute() { a = b + c; } }; And then in one class I do something like... void some_method() { vector<Instruction> v; Instruction* i = new Add(1,2,3) v.push_back(*i); } And in yet another class... void some_other_method() { Instruction ins = v.back(); ins

问题 I am actually working on my first VBA project. (come from C++ ) I would like to improve an existing VBA project used by a Microsoft Excel workbook by implementing classes and polymorphism. My problem is: 1 - I read a lot of articles/forums which explain that VBA is not an Object Oriented Programming ( OOP ) language and do not support Polymorphism. Some of them propose a workaround using the keyword Implements . 2 - I also found some webpages like this one which explain how to perform OOP and

问题 I am attempting to work through a tutorial from Programmer Bruce that is supposed to allow the deserialization of polymorphic JSON. The complete list can be found here Programmer Bruce tutorials (Great stuff btw) I have worked through the first five with no problems but I have hit a snag on the last one (Example 6), which of course is the one I really need to get working. I am getting the following error at compile time The method readValue(JsonParser, Class) in the type ObjectMapper is not

问题 Take the following: class A {} class B : A {} class C { C() { var b = new B(); Foo(b); Foo2(ref b); // <= compile-time error: // "The 'ref' argument doesn't match the parameter type" } void Foo(A a) {} void Foo2(ref A a) {} } Why does the above compile-time error occur? This happens with both ref and out arguments. 回答1: ============= UPDATE: I used this answer as the basis for this blog entry: Why do ref and out parameters not allow type variation? See the blog page for more commentary on

问题 How can I access state variable of class KeyBoardPlayer with an object of class KalaPlayer ? /** * An abstract class representing a player in Kala. Extend this class * to make your own players (e.g. human players entering moves at the keyboard * or computer players with programmed strategies for making moves). */ public abstract class KalaPlayer { /** * Method by which a player selects a move. * @param gs The current game state * @return A side pit number in the range 1-6 * @throws