polymorphism

问题 So I am making a registration page in ASP.NET MVC and theres a list of fields which changes depending on a number of conditions (taken from a database). What would be considered the best approach for displaying these fields on a view without code changes each time the DB is updated. The current system I am thinking about is making a Dictionary with "Name, DataType" then just iterating through the list and putting a new input type of each name (using a switch for data type to get the correct

问题 if: interface I{} class A implements I{} class B extends A{} class C extends B{} A a = new A(); B b = new B(); Why a = (B)(I)b; is correct but b = (B)(I)a; is false? I find casting to be very confusing, what is the best way to understand if I can down cast or up cast an object? 回答1: Your class hierarchy looks like this: Object x can be casted to class Y , if runtime type of x is subclass of Y . Or, in other words, if there is a path from runtime type of x to Y . By "runtime type" i mean the

问题 While I've been working in c++ for a while, I haven't had need to use polymorphic features until now, and I'm very intrigued by them. If I have a base class ClassA and another ClassB derives from it, I understand that I can have virtual member function in ClassA that, when implemented in ClassB , will be called in a ClassB instance even if that instance is pointed at using a ClassA pointer. Without this virtual keyword, I presume the base class implementation would prevail when using a base

问题 I have an interface IEnumerable for template classes of List<>, Array<> and Dictionary<>. I was hoping to use typedef to get their templated types T. I was hoping to do the following. class IEnumerable { public: virtual typedef int TemplateType; } And then override in inherited member, but you cant make a virtual typedef. So is there any other way that i could get the type of an unknown template class (IEnumerable is not template)? 回答1: Well, here is what is discussed in the comments in case

问题 What I have known till now is that a subclass if overriding a superclass method should throw the same exception or a subclass of the exception. For example: This is correct class SuperClass { public int doIt(String str, Integer... data)throws ArrayIndexOutOfBoundsException{ String signature = "(String, Integer[])"; System.out.println(str + " " + signature); return 1; } } public final class SubClass extends SuperClass { public int doIt(String str, Integer... data) throws

问题 This question is one of the big doubts that looms around my head and is also hard to describe it in terms of words . Some times it seems obvious and sometimes a tough one to crack.So the question goes like this:: class Base{ public: int a_number; Base(){} virtual void function1() {} virtual void function2() {} void function3() {} }; class Derived:public Base{ public: Derived():Base() {} void function1() {cout &lt&lt "Derived from Base" &lt&lt endl; virtual void function4() {cout &lt&lt "Only

问题 Using enum Axes to confine Coordinate and Quaternion : #[derive(Clone)] pub enum Axes { Coordinate {x: f64, y: f64, z: f64, reserve: Vec<f64>,}, Quaternion {x: f64, y: f64, z: f64}, } impl Axes { pub fn shift(&mut self, Sample: &Axes) -> () { let Dup: Axes = self.clone(); match Dup { Axes::Coordinate {x, y, z, reserve} => { match &Sample { Axes::Coordinate {x, y, z, reserve} => { *self = Axes::Coordinate {x: *x, y: *y, z: *z, reserve: reserve.to_vec()}; } _ => panic!(), } } Axes::Quaternion

问题 I have this code: class Class { public: virtual void first() {}; virtual void second() {}; }; Class* object = new Class(); object->first(); object->second(); delete object; that I compile with Visual C++ 10 with /O2 and have this disassembly: 282: Class* object = new Class(); 00403953 push 4 00403955 call dword ptr [__imp_operator new (4050BCh)] 0040395B add esp,4 0040395E test eax,eax 00403960 je wmain+1Ch (40396Ch) 00403962 mov dword ptr [eax],offset Class::`vftable' (4056A4h) 00403968 mov

问题 I have a C# WCF service using a webHttpBinding endpoint that will receive and return data in JSON format. The data to send/receive needs to use a polymorphic type so that data of different types can be exchanged in the same "data packet". I have the following data model: [DataContract] public class DataPacket { [DataMember] public List<DataEvent> DataEvents { get; set; } } [DataContract] [KnownType(typeof(IntEvent))] [KnownType(typeof(BoolEvent))] public class DataEvent { [DataMember] public

问题 There is this field in a table: room_id INT NOT NULL CONSTRAINT room_id_ref_room REFERENCES room I have three 2 tables for two kinds of rooms: standard_room and family_room How to do something like this: room_id INT NOT NULL CONSTRAINT room_id_ref_room REFERENCES standard_room or family_room I mean, room_id should reference either standard_room or family_room . Is it possible to do so? 回答1: Here is the pattern I've been using. CREATE TABLE room ( room_id serial primary key, room_type VARCHAR