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• calling a function from a set of overloads depending on the dynamic type of an object

  - by Jasper

  I feel like the answer to this question is really simple, but I really am having trouble finding it. So here goes: Suppose you have the following classes: class Base; class Child : public Base; class Displayer { public: Displayer(Base* element); Displayer(Child* element); } Additionally, I have a Base* object which might point to either an instance of the class Base or an instance of the class Child. Now I want to create a Displayer based on the element pointed to by object, however, I want to pick the right version of the constructor. As I currently have it, this would accomplish just that (I am being a bit fuzzy with my C++ here, but I think this the clearest way) object->createDisplayer(); virtual void Base::createDisplayer() { new Displayer(this); } virtual void Child::createDisplayer() { new Displayer(this); } This works, however, there is a problem with this: Base and Child are part of the application system, while Displayer is part of the GUI system. I want to build the GUI system independently of the Application system, so that it is easy to replace the GUI. This means that Base and Child should not know about Displayer. However, I do not know how I can achieve this without letting the Application classes know about the GUI. Am I missing something very obvious or am I trying something that is not possible?

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• How to implement a C# interface in F#?

  - by Anton Andreev

  I would like to implement the following C# interface in F#: using System; using System.Collections.Generic; using System.Linq; using System.Text; using Mono.Addins; [TypeExtensionPoint] public interface ISparqlCommand { string Name { get; } object Run(Dictionary<string, string> NamespacesDictionary, org.openrdf.repository.Repository repository, params object[] argsRest); } This is what I have tried, but it gives me: "Incomplete structured construct at or before this point in expression" #light module Module1 open System open System.Collections.Generic; type MyClass() = interface ISparqlCommand with member this.Name = "Finding the path between two tops in the Graph" member this.Run(NamespacesDictionary, repository, argsRest) = new System.Object What am I doing wrong? Maybe indentation is wrong?

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• How to manage feeds with subclassed object in Django 1.2?

  - by Matteo

  Hi, I'm trying to generate a feed rss from a model like this one, selecting all the Entry objects: from django.db import models from django.contrib.sites.models import Site from django.contrib.auth.models import User from imagekit.models import ImageModel import datetime class Entry(ImageModel): date_pub = models.DateTimeField(default=datetime.datetime.now) author = models.ForeignKey(User) via = models.URLField(blank=True) comments_allowed = models.BooleanField(default=True) icon = models.ImageField(upload_to='icon/',blank=True) class IKOptions: spec_module = 'journal.icon_specs' cache_dir = 'icon/resized' image_field = 'icon' class Post(Entry): title = models.CharField(max_length=200) description = models.TextField() slug = models.SlugField(unique=True) def __unicode__(self): return self.title class Photo(Entry): alt = models.CharField(max_length=200) description = models.TextField(blank=True) original = models.ImageField(upload_to='photo/') class IKOptions: spec_module = 'journal.photo_specs' cache_dir = 'photo/resized' image_field = 'original' def __unicode__(self): return self.alt class Quote(Entry): blockquote = models.TextField() cite = models.TextField(blank=True) def __unicode__(self): return self.blockquote When I use the render_to_response in my views I simply call: def get_journal_entries(request): entries = Entry.objects.all().order_by('-date_pub') return render_to_response('journal/entries.html', {'entries':entries}) And then I use a conditional template to render the right snippets of html: {% extends "base.html" %} {% block main %} <hr> {% for entry in entries %} {% if entry.post %}[...]{% endif %}[...] But I cannot do the same with the Feed Framework in django 1.2... Any suggestion, please?

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• Why doesn't C++ allow you to request a pointer to the most derived class?

  - by Matthew Lowe

  (This question should probably be answered with a reference to Stroustrup.) It seems extremely useful to be able to request a pointer to the most derived class, as in the following: class Base { ... }; class DerivedA { ... }; class DerivedB { ... }; class Processor { public: void Do(Base* b) {...} void Do(DerivedA* d) {...} void Do(DerivedB* d) {...} }; list<Base*> things; Processor p; for(list<Base*>::iterator i=things.begin(), e=things.end(); i!=e; ++i) { p.Do(CAST_TO_MOST_DERIVED_CLASS(*i)); } But this mechanism isn't provided in c++. Why?

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• What would be a better implementation of shared variable among subclass

  - by Churk

  So currently I have a spring unit testing application. And it requires me to get a session cookie from a foreign authentication source. Problem what that is, this authentication process is fairly expensive and time consuming, and I am trying to create a structure where I am authenticate once, by any subclass, and any subsequent subclass is created, it will reuse this session cookie without hitting the authentication process again. My problem right now is, the static cookie is null each time another subclass is created. And I been reading that using static as a global variable is a bad idea, but I couldn't think of another way to do this because of Spring framework setting things during run time and how I would set the cookie so that all other classes can use it. Another piece of information. The variable is being use, but is change able during run time. It is not a single user being signed in and used across the board. But more like a Sub1 would call login, and we have a cookie. Then multiple test will be using that login until SubX will come in and say, I am using different credential, so I need to login as something else. And repeats. Here is a outline of my code: public class Parent implements InitializingBean { protected static String BASE_URL; public static Cookie cookie; ... All default InitializingBean methods ... afterPropertiesSet() { cookie = // login process returns a cookie } } public class Sub1 extends Parent { @resource public String baseURL; @PostConstruct public void init() { // set parents with my baseURL; BASE_URL = baseURL; } public void doSomething() { // Do something with cookie, because it should have been set by parent class } } public class Sub2 extends Parent { @resource public String baseURL; @PostConstruct public void init() { // set parents with my baseURL; BASE_URL = baseURL; } public void doSomethingElse() { // Do something with cookie, because it should have been set by parent class } }

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• "Overriding" instance variables in subtype: Possible risks?

  - by sebastiangeiger

  Say I had a class SuperClass and two subclasses SubClassA and SubClassB that inherit from SuperClass. abstract class SuperClass{ ... List someList; ... } class SubClassA extends SuperClass{ ... List<String> someList; ... } class SubClassB extends SuperClass{ ... List<Integer> someList; ... } That way it is convenient because I can get someList.size() in Superclass and have Typesafety in the Subclasses. The problem is that it does not "feel" right, can you think of potential hazards this apporach has that I am not aware of?

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• [C++] Use abstract within base expecting it to be a derived class?

  - by Jonathan

  take this simple code: class A{ public: virtual void foo() = 0; void x(){ foo(); } }; class B: public A{ foo(){ ... } }; main(){ B b; b.x(); } What I want is to build an abstract class that will have a function that will call a function expecting it to be implemented in the derived class Thanks, Jonathan

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• Does this keyword exist? When an overriding method needs to call the parent

  - by Carlos

  Often, in C# documentation, you come across a member where the description says something along the lines of "be sure to call the base method if you override this." Is there a way to ensure at compile time that one has actually called the base function?

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• What's an elegant solution to get the property values from two classes (that have the same property

  - by SlipToFall

  Essentially I have to deal with a poorly implemented web service. They have two classes that don't derive from a parent class, but have the same properties (Ughh...). So it looks like this in my web service proxy class file: public partial class Product1 { public int Quantity; public int Price; } public partial class Product2 { public int Quantity; public int Price; } So what's the best way to grab the values from known properties without duplicating the code and casting to their respective classes? I know I probably could use reflection, but that can get ugly. If there is an easier less crazier way to do it (maybe in the new c# features?) please let me know.

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• what would be the output?

  - by Abhishek Jain

  Please explain me below situation What would be the output? interface A{} class B implements A{} class C extends B{} Class D extends C{} class E extends D{ public static void main(String args[]){ C c = new C(); B b = c; A a = (E)c; A a = (B)c; C c = (C)(B)c; } }

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• Dynamic Object Not Creating for Privately Inherited Class.

  - by mahesh

  Hi, What is the reason for the following code that does not let me to create object. class base { public: void foo() { cout << "base::foo()"; } }; class derived : private base { public: void foo() { cout << "deived::foo()"; } }; void main() { base *d = new derived(); d->foo(); } It Gives me error : " 'type cast' : conversion from 'derived *' to 'base *' exists, but is inaccessible" Thanks in advance :)

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• What is the best way to "override" enums?

  - by Tyler

  I have a number of classes which extend an abstract class. The abstract parent class defines an enum with a set of values. Some of the subclasses inherit the parent class's enum values, but some of the subclasses need the enum values to be different. Is there any way to somehow override the enum for these particular subclasses, and if not, what is a good way to achieve what I'm describing? class ParentClass { private MyEnum m_EnumVal; public virtual MyEnum EnumVal { get { return m_EnumVal; } set { m_EnumVal = value; } } public enum MyEnum { a, b, c }; } class ChildClass : ParentClass { private MyEnum m_EnumVal; public virtual MyEnum EnumVal { get { return m_EnumVal; } set { m_EnumVal = value; } } public enum MyEnum { d, e, f }; }

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• How can one enforce calling a base class function after derived class constructor?

  - by Mike Elkins

  I'm looking for a clean C++ idiom for the following situation: class SomeLibraryClass { public: SomeLibraryClass() { /* start initialization */ } void addFoo() { /* we are a collection of foos */ } void funcToCallAfterAllAddFoos() { /* Making sure this is called is the issue */ } }; class SomeUserClass : public SomeLibraryClass { public: SomeUserClass() { addFoo(); addFoo(); addFoo(); // SomeUserClass has three foos. } }; class SomeUserDerrivedClass : public SomeUserClass { public: SomeUserDerrivedClass() { addFoo(); // This one has four foos. } }; So, what I really want is for SomeLibraryClass to enforce the calling of funcToCallAfterAllAddFoos at the end of the construction process. The user can't put it at the end of SomeUserClass::SomeUserClass(), that would mess up SomeUserDerrivedClass. If he puts it at the end of SomeUserDerrivedClass, then it never gets called for SomeUserClass. To further clarify what I need, imagine that /* start initialization */ acquires a lock, and funcToCallAfterAllAddFoos() releases a lock. The compiler knows when all the initializations for an object are done, but can I get at that information by some nice trick?

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• How to access the backing field of a base class using fluent nhibernate?

  - by Akk

  How do i set the Access Strategy in the mapping class to point to the base _photos field? public class Content { private IList<Photo> _photos; public Content() { _photos = new List<Photo>(); } public virtual IEnumerable<Photo> Photos { get { return _photos; } } public virtual void AddPhoto() {...} } public class Article : Content { public string Body {get; set;} } I am currently using the following to try and locate the backing field but an exception is thrown as it cannot be found. public class ArticleMap : ClassMap<Article> { HasManyToMany(x => x.Photos) .Access.CamelCaseField(Prefix.Underscore) //_photos //... } i tried moving the backing field _photos directly into the class and the access works. So how can i access the backing field of the base class?

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• are there requirements for Struts setters beyond variable name matching?

  - by slk

  I have a model-driven Struts Web action: public class ModelDrivenAction<T extends Object> implements ModelDriven<T>, Preparable { protected Long id; protected T model; @Override public void prepare() {} public void setId(Long id) { this.id = id; } @Override public T getModel() { return model; } public void setModel(T model) { this.model = model; } } I have another action which is not currently model-driven: public class OtherAction implements Preparable { private ModelObj modelObj; private Long modelId; @Override public void prepare() { modelObj = repoService.retrieveModelById(modelId); } public void setModelId(Long modelId) { this.modelId = modelId; } } I wish to make it so, and would like to avoid having to track down all the instances in JavaScript where the action is passed a "modelId" parameter instead of "id" if at all possible. I thought this might work, so either modelId or id could be passed in: public class OtherAction extends ModelDrivenAction<ModelObj> { @Override public void prepare() { model = repoService.retrieveModelById(id); } public void setModelId(Long modelId) { this.id = modelId; } } However, server/path/to/other!method?modelId=123 is failing to set id. I thought so long as a setter matched a parameter name the Struts interceptor would call it on action invocation. Am I missing something here?

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• C# private (hidden) base class

  - by Loadmaster

  It is possible to make a C# base class accessible only within the library assembly it's compiled into, while making other subclasses that inherit from it public? For example: using System.IO; class BaseOutput: Stream // Hidden base class { protected BaseOutput(Stream o) { ... } ...lots of common methods... } public class MyOutput: BaseOutput // Public subclass { public BaseOutput(Stream o): base(o) { ... } public override int Write(int b) { ... } } Here I'd like the BaseOutput class to be inaccessible to clients of my library, but allow the subclass MyOutput to be completely public. I know that C# does not allow base classes to have more restrictive access than subclasses, but is there some other legal way of achieving the same effect?

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• C++ abstract class template + type-specific subclass = trouble with linker

  - by user333279

  Hi there, The project in question is about different endpoints communicating with each other. An endpoint sends events (beyond the scope of the current problem) and can process incoming events. Each event is represented in a generic object as follows: #pragma interface ... // some includes template<typename T> class Event { public: Event(int senderId, Type type, T payload); // Type is an enum Event(int senderId, Type type, int priority, T payload); virtual ~Event(); virtual int getSenderId(); virtual int getPriority(); virtual T getPayload(); void setPriority(const int priority); protected: const int senderId; const Type type; const T payload; int priority; }; It has its implementing class with #pragma implementation tag. An endpoint is defined as follows: #pragma interface #include "Event.h" template<typename T> class AbstractEndPoint { public: AbstractEndPoint(int id); virtual ~AbstractEndPoint(); virtual int getId(); virtual void processEvent(Event<T> event) = 0; protected: const int id; }; It has its implementing class too, but only the constructor, destructor and getId() are defined. The idea is to create concrete endpoints for each different payload type. Therefore I have different payload objects and specific event classes for each type, e.g. Event<TelegramFormatA>, Event<TelegramFormatB> and ConcreteEndPoint for TelegramFormatA, ConcreteEndPoint for TelegramFormatB respectively. The latter classes are defined as class ConcreteEndPoint : AbstractEndPoint<TelegramFormatA> { ... } I'm using g++ 4.4.3 and ld 2.19. Everything compiles nicely, but the linker complaints about undefined references to type-specific event classes, like Event<TelegramFormatA>::Event(....) . I tried explicit instantiation using template class AbstractEndPoint<TelegramFormatA>; but couldn't get past the aforementioned linker errors. Any ideas would be appreciated.

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• Why can't I create an abstract constructor on an abstract C# class?

  - by Anthony D

  I am creating an abstract class. I want each of my derived classes to be forced to implement a specific signature of constructor. As such, I did what I would have done has I wanted to force them to implement a method, I made an abstract one. public abstract class A { abstract A(int a, int b); } However I get a message saying the abstract modifier is invalid on this item. My goal was to force some code like this. public class B : A { public B(int a, int b) : base(a, b) { //Some other awesome code. } } This is all C# .NET code. Can anyone help me out? Update 1 I wanted to add some things. What I ended up with was this. private A() { } protected A(int a, int b) { //Code } That does what some folks are saying, default is private, and the class needs to implement a constructor. However that doesn't FORCE a constructor with the signature A(int a, int b). public abstract class A { protected abstract A(int a, int b) { } } Update 2 I should be clear, to work around this I made my default constructor private, and my other constructor protected. I am not really looking for a way to make my code work. I took care of that. I am looking to understand why C# does not let you do this.

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• How do I change the parent class in ANTLR-3?

  - by etheros

  By default, a parser generated using ANTLR-3 extends from org.antlr.runtime.Parser. How do I have it extend my custom class instead?

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• Should downcasting be avoided while using a class hierarchy in C++?

  - by neuviemeporte

  Let's say I'm writing an application which works with projects, and exposes different functionality depending on the type of the project. I have a hierarchy of classes for the different types of projects: class AbstractProject { }; class ProjectA : public AbstractProject { }; class ProjectB : public AbstractProject { }; class ProjectC : public AbstractProject { }; Now, I was planning to have an AbstractProject *_currentProject pointer as a member in the application's main class, pop up a dialog box on startup and based on the selection, do: _currentProject = new ProjectB(); // e.g. Later, I'll have to downcast the pointer to the specific type to utilize the functionality specific to different Project-s. Somehow this makes me feel uneasy. Is there a Better Way of doing this?

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• Reading a Text file in xcode

  - by Nicolaj Zefting

  First off, I'm a complete beginner. This might be a stupid question, but here it goes: I'm currently working on an App than contains Latin texts that the users can view and read. I'm using Xcode 4 with the storybord function. Theway the app is built: user selects author - then the book - then app shows the text. I am kind of confused because i need to have various text files, depending on the users choice.

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• Nhibernate Common columns in base class

  - by sukh

  I want to design following scenario Base class (Id, Name, order, Value) 3 Derived classes derive1, derive2, derive3 inheriting properties from base There is no table for base class. And 1 table for each derived class. 3 tables have same columns. How can I create mapping file ignoring base class? Do I need to create 1 mapping file for each derived class? can I achieve this using only 1 mapping file?

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• How To Get Classes To Expose The Same Shared/Static Method

  - by Jason

  I have a base class with several derived classes. I want all of my derived classes to have the same Public Shared (static) method with their own implementation. How do I do this? Is it even possible?

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• Are private members inherited in C#?

  - by Petr

  Just seen one tutorial saying that: Class Dog { private string Name; } Class SuperDog:Dog { private string Mood; } Then there was an UML displaying that SuperDog will inherit Name as well. I have tried but to me it seems that only public members are inherited. At least I could not access Name unless it was declared as public.

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• Base form controls not visible in child form

  - by Kildareflare

  Hello I'm using C#.Net and have a base form that is inherited by several forms. Until yesterday, when the child (derived) form was opened in the designer the base forms controls would be displayed and shown as locked. Now, however the form is simply blank. None of the base forms controls are visible in the designer. Everything compiles, builds and runs OK. Has anyone else seen this? I've tried placing a call to the base forms InitializeComponent method in the derived forms OnLoad method but to no avail.

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