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• Passing Services to MainViewModel - SHOULD I use a dependency injection container ?

  - by msfanboy

  Hello, I have this code: public partial class App : Application { protected override void OnStartup(StartupEventArgs e) { base.OnStartup(e); var mainVM = new MainViewModel ( new Service1(), ... new Service10(), ); var window = new MainWindow(); window.DataContext = mainVM; window.Show(); } } I pass all my Services instances to the MainViewModel. Within the MainViewModel I spread those services to other ViewModels via constructor parameter passing. Should I use any DI framework for the services in the App class? If yes whats the benefit of resolving the services instead of just creating the instance manually... ?

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• Is std::move really needed on initialization list of constructor for heavy members passed by value?

  - by PiotrNycz

  Recently I read an example from cppreference.../vector/emplace_back: struct President { std::string name; std::string country; int year; President(std::string p_name, std::string p_country, int p_year) : name(std::move(p_name)), country(std::move(p_country)), year(p_year) { std::cout << "I am being constructed.\n"; } My question: is this std::move really needed? My point is that compiler sees that this p_name is not used in the body of constructor, so, maybe, there is some rule to use move semantics for it by default? That would be really annoying to add std::move on initialization list to every heavy member (like std::string, std::vector). Imagine hundreds of KLOC project written in C++03 - shall we add everywhere this std::move? This question: move-constructor-and-initialization-list answer says: As a golden rule, whenever you take something by rvalue reference, you need to use it inside std::move, and whenever you take something by universal reference (i.e. deduced templated type with &&), you need to use it inside std::forward But I am not sure: passing by value is rather not universal reference?

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• Factory for arrays of objects in python

  - by Vorac

  Ok, the title might be a little misleading. I have a Window class that draws widgets inside itself in the constructor. The widgets are all of the same type. So I pass a list of dictionaries, which contain the parameters for each widget. This works quite nicely, but I am worried that the interface to callers is obfuscated. That is, in order to use Window, one has to study the class, construct a correct list of dictionaries, and then call the constructor with only one parameter - widgets_params. Is this good or bad design? What alternatives does the python syntax provide?

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• Constructor vs setter validations

  - by Jimmy

  I have the following class : public class Project { private int id; private String name; public Project(int id, String name, Date creationDate, int fps, List<String> frames) { if(name == null ){ throw new NullPointerException("Name can't be null"); } if(id == 0 ){ throw new IllegalArgumentException("id can't be zero"); } this.name = name; this.id = id; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } } I have three questions: Do I use the class setters instead of setting the fields directly. One of the reason that I set it directly, is that in the code the setters are not final and they could be overridden. If the right way is to set it directly and I want to make sure that the name filed is not null always. Should I provide two checks, one in the constructor and one in the setter. I read in effective java that I should use NullPointerException for null parameters. Should I use IllegalArgumentException for other checks, like id in the example.

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• Initializing entities vs having a constructor parameter

  - by Vee

  I'm working on a turn-based tile-based puzzle game, and to create new entities, I use this code: Field.CreateEntity(10, 5, Factory.Player()); This creates a new Player at [10; 5]. I'm using a factory-like class to create entities via composition. This is what the CreateEntity method looks like: public void CreateEntity(int mX, int mY, Entity mEntity) { mEntity.Field = this; TileManager.AddEntity(mEntity, true); GetTile(mX, mY).AddEntity(mEntity); mEntity.Initialize(); InvokeOnEntityCreated(mEntity); } Since many of the components (and also logic) of the entities require to know what the tile they're in is, or what the field they belong to is, I need to have mEntity.Initialize(); to know when the entity knows its own field and tile. The Initialize(); method contains a call to an event handler, so that I can do stuff like this in the factory class: result.OnInitialize += () => result.AddTags(TDLibConstants.GroundWalkableTag, TDLibConstants.TrapdoorTag); result.OnInitialize += () => result.AddComponents(new RenderComponent(), new ElementComponent(), new DirectionComponent()); This works so far, but it is not elegant and it's very open to bugs. I'm also using the same idea with components: they have a parameterless constructor, and when you call the AddComponent(mComponent); method in an entity, it is the entity's job to set the component's entity to itself. The alternative would be having a Field, int, int parameters in the factory class, to do stuff like: new Entity(Field, 10, 5); But I also don't like the fact that I have to create new entities like this. I would prefer creating entities via the Field object itself. How can I make entity/component creation more elegant and less prone to bugs?

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• Use constructor or setter method?

  - by user633600

  I am working on a UI code where I have an Action class, something like this - public class MyAction extends Action { public MyAction() { setText("My Action Text"); setToolTip("My Action Tool tip"); setImage("Some Image"); } } When this Action class was created it was pretty much assumed that the Action class wont be customizable (in a sense- its text, tooltip or image will be not be changed anywhere in the code). Of late, now we are in need of changing the action text at some location in code. So I suggested my co-worker to remove the hardcoded action text from the constructor and accept it as an argument, so that everybody is forced to pass the action text. Something like this code below - public class MyAction extends Action { public MyAction(String actionText) { setText(actionText); setTooltip("My Action tool tip); setImage("My Image"); } } He however thinks that since setText() method belongs to base class. It can be flexibly used to pass the action text wherever action instance is created. That way, there is no need to change the existing MyAction class. So his code would look something like this. MyAction action = new MyAction(); //this creates action instance with the hardcoded text action.setText("User required new action text"); //overwrite the exisitng text. I am not sure if that is a correct way to deal with problem. I think in above mentioned case user is anyway going to change the text, so why not force him while constructing the action. The only benefit I see with the original code is that user can create Action class without much thinking about setting text.

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• WCF Runtime Error while using Constructor

  - by Pranesh Nair

  Hi all, I am new to WCF i am using constructor in my WCF service.svc.cs file....It throws this error when i use the constructor The service type provided could not be loaded as a service because it does not have a default (parameter-less) constructor. To fix the problem, add a default constructor to the type, or pass an instance of the type to the host. When i remove the constructor its working fine....But its compulsory that i have to use constructor... This is my code namespace UserAuthentication { [ServiceBehavior(InstanceContextMode=System.ServiceModel.InstanceContextMode.Single)] public class UserAuthentication : UserRepository,IUserAuthentication { private ISqlMapper _mapper; private IRoleRepository _roleRepository; public UserAuthentication(ISqlMapper mapper): base(mapper) { _mapper = mapper; _roleRepository = new RoleRepository(_mapper); } public string EduvisionLogin(EduvisionUser aUser, int SchoolID) { UserRepository sampleCode= new UserRepository(_mapper); sampleCode.Login(aUser); return "Login Success"; } } } can anyone provide ideas or suggestions or sample code hw to resolve this issue...

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• Generic Abstract Singleton with Custom Constructor in C#

  - by Heka

  I want to write a generic singleton with an external constructor. In other words the constructor can be modified. I have 2 designs in my mind but I don't know whether they are practical or not. First one is to enforce derived class' constructor to be non-public but I do not know if there is a way of it? Second one is to use a delegate and call it inside the constructor? It isn't necessarily to be a constructor. The reason I chose custom constructor is doing some custom initializations. Any suggestions would be appreciated :)

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• How is a constructor executed?

  - by simion

  I am doing some reviison from the lecture slides and it says a constructor is executed in the following way; If the constructor starts with this, recursively execute the indicated constructor, then go to step 4. Invoke the explicitly or implicitly indicated superclass constructor (unless this class is java.lang.Object) Initialise the fields of the object in the order in which they were declared in this class Execute the rest of the body of this constructor. What i dont undertsand is that, a constructor can never "start" with this, because even if it forms no class heirarchy/relationship then super() is inserted by default. How would this fit in with the description above? Thanks

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• Is a Java static block equivalent to a C3 static constructor?

  - by Mackenzie

  What is the real difference between a C# static constructor and a Java static block? They both must be parameterless. They are both called only once, when the related class is first used. Am I missing something, or are they the same thing, just with different names?

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• What is the meaning of ": base" in the constructor definition ?

  - by DotNetBeginner

  What is the meaning of ": base" in the costructor of following class(MyClass) ? Please explain the concept behind constructor definition given below for class MyClass. public class MyClass: WorkerThread { public MyClass(object data): base(data) { // some code } } public abstract class WorkerThread { private object ThreadData; private Thread thisThread; public WorkerThread(object data) { this.ThreadData = data; } public WorkerThread() { ThreadData = null; } }

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• For structs, do I have to call the constructor explicitly in C#?

  - by Gulshan

  The question is about the structs. When I declare a struct type variable/object (don't know which one suits better) or an array or list of structs, do I have to call the constructor explicitly like objects or just declaring will suffice like variables?

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• Is it true that the assigned final object field may still be null inside a constructor?

  - by Tom Brito

  Is it true that the assigned final object field may still be null inside a constructor? class MyClass { private final Object obj = new Object(); public MyClass() { System.out.println(obj); // may print null? } } if yes, isn't this a bug?

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• In Scala, how does one write a class with a constructor, not all of whose arguments are class member

  - by user318816

  I want to write class whose constructor takes two parameters, but the arguments are not actually members of the class. e.g. class P(V1:Int, V2:Int) { val set = Set(V1, V2) } Having constructed the 'set', I don't actually care about V1 and V2. Is there a way of expressing this in Scala ?

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• Passing parameter to base class constructor or using instance variable?

  - by deamon

  All classes derived from a certain base class have to define an attribute called "path". In the sense of duck typing I could rely upon definition in the subclasses: class Base: pass # no "path" variable here def Sub(Base): def __init__(self): self.path = "something/" Another possiblity would be to use the base class constructor: class Base: def __init__(self, path): self.path = path def Sub(Base): def __init__(self): super().__init__("something/") What would you prefer and why? Is there a better way?

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• Can I refer to an object in its constructor?

  - by Roman

  Can I do the following? public Manager(String userName) { game = new Game(userName); game.addManager(this); } The problem is that I refer to an object (this) in its constructor (before it was actually created).

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• Is it a good or bad practice to call instance methods from a java constructor?

  - by Steve

  There are several different ways I can initialize complex objects (with injected dependencies and required set-up of injected members), are all seem reasonable, but have various advantages and disadvantages. I'll give a concrete example: final class MyClass { private final Dependency dependency; @Inject public MyClass(Dependency dependency) { this.dependency = dependency; dependency.addHandler(new Handler() { @Override void handle(int foo) { MyClass.this.doSomething(foo); } }); doSomething(0); } private void doSomething(int foo) { dependency.doSomethingElse(foo+1); } } As you can see, the constructor does 3 things, including calling an instance method. I've been told that calling instance methods from a constructor is unsafe because it circumvents the compiler's checks for uninitialized members. I.e. I could have called doSomething(0) before setting this.dependency, which would have compiled but not worked. What is the best way to refactor this? Make doSomething static and pass in the dependency explicitly? In my actual case I have three instance methods and three member fields that all depend on one another, so this seems like a lot of extra boilerplate to make all three of these static. Move the addHandler and doSomething into an @Inject public void init() method. While use with Guice will be transparent, it requires any manual construction to be sure to call init() or else the object won't be fully-functional if someone forgets. Also, this exposes more of the API, both of which seem like bad ideas. Wrap a nested class to keep the dependency to make sure it behaves properly without exposing additional API:class DependencyManager { private final Dependency dependency; public DependecyManager(Dependency dependency) { ... } public doSomething(int foo) { ... } } @Inject public MyClass(Dependency dependency) { DependencyManager manager = new DependencyManager(dependency); manager.doSomething(0); } This pulls instance methods out of all constructors, but generates an extra layer of classes, and when I already had inner and anonymous classes (e.g. that handler) it can become confusing - when I tried this I was told to move the DependencyManager to a separate file, which is also distasteful because it's now multiple files to do a single thing. So what is the preferred way to deal with this sort of situation?

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• Does binding temporary to a reference require a copy constructor in C++?

  - by vitaut

  Consider the following code: class A { A(const A&); public: A() {} }; int main() { const A &a = A(); } This code compiles fine with GCC, but fails to compile with Visual C++ with the following error: test.cc(8) : error C2248: 'A::A' : cannot access private member declared in class 'A' test.cc(2) : see declaration of 'A::A' test.cc(1) : see declaration of 'A' So is it necessary to have a copy constructor accessible when binding a temporary to a reference?

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• How can I define a constructor in an open generic type?

  - by Cort

  I am trying to create on open generic type that has a constructor to be used by derived types, but I either don't know how to do it or it is not possible -- not sure which. public struct DataType<T> : IDataType { private T myValue; private TypeState myState; private DataType<T>(T initialValue, TypeState state) { myValue = initialValue; myState = state; } } Any help much appreciated! Cort

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• Can I pass constructor parameters to Unity's Resolve() method?

  - by NotDan

  I am using Microsoft's Unity for dependency injection and I want to do something like this: IDataContext context = _unityContainer.Resolve<IDataContext>(); var repositoryA = _unityContainer.Resolve<IRepositoryA>(context); //Same instance of context var repositoryB = _unityContainer.Resolve<IRepositoryB>(context); //Same instance of context IDataContext context2 = _unityContainer.Resolve<IDataContext>(); //New instance var repositoryA2 = _unityContainer.Resolve<IRepositoryA>(context2); RepositoryA and RepositoryB both have a constructor that takes an IDataContext parameter, and I want Unity to initialize the repository with the context that I pass it. Also note that IDataContext is not registered with Unity (I dont want 3 instances of IDataContext).

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• Using IoC and Dependency Injection, how do I wrap an existing implementation with a new layer of imp

  - by Dividnedium

  I'm trying to figure out how this would be done in practice, so as not to violate the Open Closed principle. Say I have a class called HttpFileDownloader that has one function that takes a url and downloads a file returning the html as a string. This class implements an IFileDownloader interface which just has the one function. So all over my code I have references to the IFileDownloader interface and I have my IoC container returning an instance of HttpFileDownloader whenever an IFileDownloader is Resolved. Then after some use, it becomes clear that occasionally the server is too busy at the time and an exception is thrown. I decide that to get around this, I'm going to auto-retry 3 times if I get an exception, and wait 5 seconds in between each retry. So I create HttpFileDownloaderRetrier which has one function that uses HttpFileDownloader in a for loop with max 3 loops, and a 5 second wait between each loop. So that I can test the "retry" and "wait" abilities of the HttpFileDownloadRetrier I have the HttpFileDownloader dependency injected by having the HttpFileDownloaderRetrier constructor take an IFileDownloader. So now I want all Resolving of IFileDownloader to return the HttpFileDownloaderRetrier. But if I do that, then HttpFileDownloadRetrier's IFileDownloader dependency will get an instance of itself and not of HttpFileDownloader. So I can see that I could create a new interface for HttpFileDownloader called IFileDownloaderNoRetry, and change HttpFileDownloader to implement that. But that means I'm changing HttpFileDownloader, which violates Open Closed. Or I could implement a new interface for HttpFileDownloaderRetrier called IFileDownloaderRetrier, and then change all my other code to refer to that instead of IFileDownloader. But again, I'm now violating Open Closed in all my other code. So what am I missing here? How do I wrap an existing implementation (downloading) with a new layer of implementation (retrying and waiting) without changing existing code? Here's some code if it helps: public interface IFileDownloader { string Download(string url); } public class HttpFileDownloader : IFileDownloader { public string Download(string url) { //Cut for brevity - downloads file here returns as string return html; } } public class HttpFileDownloaderRetrier : IFileDownloader { IFileDownloader fileDownloader; public HttpFileDownloaderRetrier(IFileDownloader fileDownloader) { this.fileDownloader = fileDownloader; } public string Download(string url) { Exception lastException = null; //try 3 shots of pulling a bad URL. And wait 5 seconds after each failed attempt. for (int i = 0; i < 3; i++) { try { fileDownloader.Download(url); } catch (Exception ex) { lastException = ex; } Utilities.WaitForXSeconds(5); } throw lastException; } }

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• What does the Spring framework do? Should I use it? Why or why not?

  - by sangfroid

  So, I'm starting a brand-new project in Java, and am considering using Spring. Why am I considering Spring? Because lots of people tell me I should use Spring! Seriously, any time I've tried to get people to explain what exactly Spring is or what it does, they can never give me a straight answer. I've checked the intros on the SpringSource site, and they're either really complicated or really tutorial-focused, and none of them give me a good idea of why I should be using it, or how it will make my life easier. Sometimes people throw around the term "dependency injection", which just confuses me even more, because I think I have a different understanding of what that term means. Anyway, here's a little about my background and my app : Been developing in Java for a while, doing back-end web development. Yes, I do a ton of unit testing. To facilitate this, I typically make (at least) two versions of a method : one that uses instance variables, and one that only uses variables that are passed in to the method. The one that uses instance variables calls the other one, supplying the instance variables. When it comes time to unit test, I use Mockito to mock up the objects and then make calls to the method that doesn't use instance variables. This is what I've always understood "dependency injection" to be. My app is pretty simple, from a CS perspective. Small project, 1-2 developers to start with. Mostly CRUD-type operations with a a bunch of search thrown in. Basically a bunch of RESTful web services, plus a web front-end and then eventually some mobile clients. I'm thinking of doing the front-end in straight HTML/CSS/JS/JQuery, so no real plans to use JSP. Using Hibernate as an ORM, and Jersey to implement the webservices. I've already started coding, and am really eager to get a demo out there that I can shop around and see if anyone wants to invest. So obviously time is of the essence. I understand Spring has quite the learning curve, plus it looks like it necessitates a whole bunch of XML configuration, which I typically try to avoid like the plague. But if it can make my life easier and (especially) if make it can make development and testing faster, I'm willing to bite the bullet and learn Spring. So please. Educate me. Should I use Spring? Why or why not?

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• Move constructor and assignment operator: why no default for derived classes?

  - by doublep

  Why there is default move constructor or assignment operator not created for derived classes? To demonstrate what I mean; having this setup code: #include <utility> struct A { A () { } A (A&&) { throw 0; } A& operator= (A&&) { throw 0; } }; struct B : A { }; either of the following lines throws: A x (std::move (A ()); A x; x = A (); but neither of the following does: B x (std::move (B ()); B x; x = B (); In case it matters, I tested with GCC 4.4.

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• Why must "stride" in the System.Drawing.Bitmap constructor be a multiple of 4?

  - by Gorchestopher H

  I am writing an application that requires me to take a proprietary bitmap format (an MVTec Halcon HImage) and convert it into a System.Drawing.Bitmap in C#. The only proprietary functions given to me to help me do this involve me writing to file, except for the use of a "get pointer" function. This function is great, it gives me a pointer to the pixel data, the width, the height, and the type of the image. My issue is that when I create my System.Drawing.Bitmap using the constructor: new System.Drawing.Bitmap(width, height, stride, format, scan) I need to specify a "stride" that is a multiple of 4. This may be a problem as I am unsure what size bitmap my function will be hit with. Supposing I end up with a bitmap that is 111x111 pixels, I have no way to run this function other than adding a bogus column to my image or subtracting 3 columns. Is there a way I can sneak around this limitation?

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• Can an interface define the signature of a c#-constructor

  - by happyclicker

  I have a .net-app that provides a mechanism to extend the app with plugins. Each plugin must implement a plugin-interface and must provide furthermore a constructor that receives one parameter (a resource context). During the instantiation of the plugin-class I look via reflection, if the needed constructor exists and if yes, I instantiate the class (via Reflection). If the constructor does not exists, I throw an exception that says that the plugin not could be created, because the desired constructor is not available. My question is, if there is a way to declare the signature of a constructor in the plugin-interface so that everyone that implements the plugin-interface must also provide a constructor with the desired signature. This would ease the creation of plugins. I don’t think that such a possibility exists because I think such a feature falls not in the main purpose for what interfaces were designed for but perhaps someone knows a statement that does this, something like: public interface IPlugin { ctor(IResourceContext resourceContext); int AnotherPluginFunction(); } I want to add that I don't want to change the constructor to be parameterless and then set the resource-context through a property, because this will make the creation of plugins much more complicated. The persons that write plugins are not persons with deep programming experience. The plugins are used to calculate statistical data that will be visualized by the app.

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