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• Is OOP becoming easier or harder?

  - by tunmise fasipe

  When the concepts of Object Oriented Programming were introduced to programmers years back it looks interesting and programming was cleaner. OOP was like this Stock stock = new Stock(); stock.addItem(item); stock.removeItem(item); That was easier to understand with self-descriptive name. But now OOP, with pattern like Data Transfer Objects (or Value Objects), Repository, Dependency Injection etc, has become more complex. To achieve the above you may have to create several classes (e.g. abstract, factory, DAO etc) and Implement several interfaces Note: I am not against best practices that makes Collaboration, Testing and Integration easier

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• Figuring out the Call chain

  - by BDotA

  Let's say I have an assemblyA that has a method which creates an instance of assemblyB and calls its MethodFoo(). Now assemblyB also creates an instance of assemblyC and calls MethodFoo(). So no matter if I start with assemblyB in the code flow or with assemlyA, at the end we are calling that MethodFoo of AssemblyC(). My question is when I am in the MethodFoo() how can I know who has called me? Has it been a call originally from assemblyA or was it from assemlyB? Is there any design pattern or a good OO way of solving this?

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• Anemic Domain Model, Business Logic and DataMapper (PHP)

  - by sunwukung

  I've implemented a rudimentary ORM layer based on DataMapper (I don't want to use a full blown ORM like Propel/Doctrine - for anything beyond simple fetch/save ops I prefer to access the data directly layer using a SQL abstraction layer). Following the DataMapper pattern, I've endeavoured to keep all persistence operations in the Mapper - including the location of related entities. My Entities have access to their Mapper, although I try not to call Mapper logic from the Entity interface (although this would be simple enough). The result is: // get a mapper and produce an entity $ProductMapper = $di->get('product_mapper'); $Product = $ProductMapper->find('[email protected]','email'); //.. mutaute some values.. save $ProductMapper->save($Product) // uses __get to trigger relation acquisition $Manufacturer = $Product->manufacturer; I've read some articles regarding the concept of an Anemic Domain model, i.e. a Model that does not contain any "business logic". When demonstrating the sort of business logic ideally suited to a Domain Model, however, acquiring related data items is a common example. Therefore I wanted to ask this question: Is persistence logic appropriate in Domain Model objects?

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• Client-server application design issue

  - by user2547823

  I have a collection of clients on server's side. And there are some objects that need to work with that collection - adding and removing clients, sending message to them, updating connection settings and so on. They should perform these actions simultaneously, so mutex or another synchronization primitive is required. I want to share one instance of collection between these objects, but all of them require access to private fields of collection. I hope that code sample makes it more clear[C++]: class Collection { std::vector< Client* > clients; Mutex mLock; ... } class ClientNotifier { void sendMessage() { mLock.lock(); // loop over clients and send message to each of them } } class ConnectionSettingsUpdater { void changeSettings( const std::string& name ) { mLock.lock(); // if client with this name is inside collection, change its settings } } As you can see, all these classes require direct access to Collection's private fields. Can you give me an advice about how to implement such behaviour correctly, i.e. keeping Collection's interface simple without it knowing about its users?

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• Why are MVC & TDD not employed more in game architecture?

  - by secoif

  I will preface this by saying I haven't looked a huge amount of game source, nor built much in the way of games. But coming from trying to employ 'enterprise' coding practices in web apps, looking at game source code seriously hurts my head: "What is this view logic doing in with business logic? this needs refactoring... so does this, refactor, refactorrr" This worries me as I'm about to start a game project, and I'm not sure whether trying to mvc/tdd the dev process is going to hinder us or help us, as I don't see many game examples that use this or much push for better architectural practices it in the community. The following is an extract from a great article on prototyping games, though to me it seemed exactly the attitude many game devs seem to use when writing production game code: Mistake #4: Building a system, not a game ...if you ever find yourself working on something that isn’t directly moving your forward, stop right there. As programmers, we have a tendency to try to generalize our code, and make it elegant and be able to handle every situation. We find that an itch terribly hard not scratch, but we need to learn how. It took me many years to realize that it’s not about the code, it’s about the game you ship in the end. Don’t write an elegant game component system, skip the editor completely and hardwire the state in code, avoid the data-driven, self-parsing, XML craziness, and just code the damned thing. ... Just get stuff on the screen as quickly as you can. And don’t ever, ever, use the argument “if we take some extra time and do this the right way, we can reuse it in the game”. EVER. is it because games are (mostly) visually oriented so it makes sense that the code will be weighted heavily in the view, thus any benefits from moving stuff out to models/controllers, is fairly minimal, so why bother? I've heard the argument that MVC introduces a performance overhead, but this seems to me to be a premature optimisation, and that there'd more important performance issues to tackle before you worry about MVC overheads (eg render pipeline, AI algorithms, datastructure traversal, etc). Same thing regarding TDD. It's not often I see games employing test cases, but perhaps this is due to the design issues above (mixed view/business) and the fact that it's difficult to test visual components, or components that rely on probablistic results (eg operate within physics simulations). Perhaps I'm just looking at the wrong source code, but why do we not see more of these 'enterprise' practices employed in game design? Are games really so different in their requirements, or is a people/culture issue (ie game devs come from a different background and thus have different coding habits)?

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• Is "convention over configuration" not violating basic programming principles?

  - by Geerten

  I was looking at the WPF MVVM framework Caliburn.Micro and read that a lot of standard things are based on naming conventions. For example, automatic binding of properties in the View to properties in the ViewModel. Although this seems to be convenient (removes some boilerplate code), my first instinct reaction is that it isn't completely obvious to a new programmer that will read this code. In other words, the functionality of the application is not completely explained by its own code, but also by the documentation of the framework. EDIT: So this approach is called convention over configuration. Since I could not find any questions concerning this, I altered my question: My question is: Is convention over configuration a correct way of simplifying things, or is it violating some programming principles (and if so, which ones)?

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• Should this code/logic be included in Business Objects class or a separate class?

  - by aspdotnetuser

  I have created a small application which has a three tier architecture and I have business object classes to represent entities such as User, Orders, UserType etc. In these classes I have methods that are executed when the Constuctor method of, for example, User is called. These methods perform calculations and generate details that setup data for attributes that are part of each User object. Here is the structure for the project in Visual Studio: Here is some code from the business object class User.cs: Public Class User { public string Name { get; set; } public int RandomNumber { get; set; } etc public User { Name = GetName(); RandomNumber = GetRandomNumber(); } public string GetName() { .... return name; } public int GetRandomNumber() { ... return randomNumber; } } Should this logic be included in the Business Object classes or should it be included in a Utilities class of some kind? Or in the business rules?

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• Flags with deferred use

  - by Trenton Maki

  Let's say I have a system. In this system I have a number of operations I can do but all of these operations have to happen as a batch at a certain time, while calls to activate and deactivate these operations can come in at any time. To implement this, I could use flags like doOperation1 and doOperation2 but this seems like it would become difficult to maintain. Is there a design pattern, or something similar, that addresses this situation?

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• Many ui panels needs interaction with same object

  - by user877329

  I am developing a tool for simulating systems like the Gray-Scott model (That is systems where spatial distribution depends on time). The actual model is loaded from a DLL or shared object and the simulation is performed by a Simulation object. There are at least two situations when the simulation needs to be destroyed: The user loads a new model The user changes the size of the domain To make sure nothing goes wrong, the current Model, Simulation, and rendering Thread are all managed by an ApplicationState object. But the two cases above are initiated from two different UI objects. Is it then ok to distribute a reference to the ApplicationState object to all panels that need to access at least one method on the ApplicationState object? Another solution would be to use aggregation so that the panel from which the user chooses model knows the simulation parameter panel. Also, the ApplicationState class seems somewhat clumsy, so I would like to have something else

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• Using packages (gems, eggs, etc.) to create decoupled architectures

  - by Juan Carlos Coto

  The main issue Seeing the good support most modern programming platforms have for package management (think gem, npm, pip, etc), does it make sense to design an application or system be composed of internally developed packages, so as to promote and create a loosely coupled architecture? Example An example of this would be to create packages for database access, as well as for authentication and other components of the system. These, of course, use external packages as well. Then, your system imports and uses these packages - instead of including their code within its own code base. Considerations To me, it seems that this would promote code decoupling and help maintainability, almost in a Web-based-vs.-desktop-application kind of way (updates are applied almost automatically, single code base for single functionality, etc.). Does this seem like a rational and sane design concept? Is this actually used as a standard way of structuring applications today? Thanks very much!

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• Implementing Command Pattern in Web Application

  - by KingOfHypocrites

  I'm looking to implement the command pattern in a web application (asp.net c#)... Since the commands come in text format from the client, what is the best way to translate the string to a command object? Should I use reflection? Currently I just assume the command that comes in matches the file name of a user control. This is a bit of a hack. Rather than have a select case statement that says if string = "Dashboard" then call Dashboard.Execute(), is there a pattern for working with commands that originate as strings?

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• What is a useful pattern to maintaining an object state in a one to many relationship?

  - by ahenderson

  I am looking for a design for my application, here are the players(classes) involved. struct Transform { // Uses a matrix to transform the position. // Also acts acts as the state of a Dialog. Position transform(Position p); //other methods. }; struct Dialog { // There are multiple dialog for the user to transform the output. Transform& t; void ChangeTranformation(){t.rotate(360);} } struct Algorithm { //gives us a position based on an implementation. For example this can return points on a circle or line. Transform& t; Position m_p; Dialog& d; Position GetCurrentPosition(){ return t.transform(m_p);} //other methods. } Properties I need: Each algorithms has one dialog and each dialog can have many algorithms associated with it. When the user selects an algorithm a dialog associated with that algorithm is displayed. If the user selects a different algorithm then re-selects back the state is restored in the dialog. Basically I want a good design pattern to maintain the state of the dialog given that many algorithms use it and they can be switched back and forth. Does anyone have any suggestions? Here is a use case: Dialog1 has a single edit box to control the radius. Algorithm1 generates points on a unit circle. Algorithm2 is the same as Algorithm1. The user has selected Algorithm1 and entered 2 into the edit box. This will generate points on a circle of radius 2. The user then selects Algorithm2 and enters 10 into the edit box of Dialog1. This will generate points on a circle of radius 10. Finally Algorithm1 is selected again. The edit box of Dialog1 should show 2 and points on a circle of radius 2 should be generated.

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• Learning how to design knowledge and data flow [closed]

  - by max

  In designing software, I spend a lot of time deciding how the knowledge (algorithms / business logic) and data should be allocated between different entities; that is, which object should know what. I am asking for advice about books, articles, presentations, classes, or other resources that would help me learn how to do it better. I code primarily in Python, but my question is not really language-specific; even if some of the insights I learn don't work in Python, that's fine. I'll give a couple examples to clarify what I mean. Example 1 I want to perform some computation. As a user, I will need to provide parameters to do the computation. I can have all those parameters sent to the "main" object, which then uses them to create other objects as needed. Or I can create one "main" object, as well as several additional objects; the additional objects would then be sent to the "main" object as parameters. What factors should I consider to make this choice? Example 2 Let's say I have a few objects of type A that can perform a certain computation. The main computation often involves using an object of type B that performs some interim computation. I can either "teach" A instances what exact parameters to pass to B instances (i.e., make B "dumb"); or I can "teach" B instances to figure out what needs to be done when looking at an A instance (i.e., make B "smart"). What should I think about when I'm making this choice?

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• How can I design my classes for a calendar based on database events?

  - by Gianluca78

  I'm developing a web calendar in php (using Symfony2) inspired by iCal for a project of mine. At this moment, I have two classes: a class "Calendar" and a class "CalendarCell". Here you are the two classes properties and method declarations. class Calendar { private $month; private $monthName; private $year; private $calendarCellList = array(); private $translator; public function __construct($month, $year, $translator) {} public function getCalendarCells() {} public function getMonth() {} public function getMonthName() {} public function getNextMonth() {} public function getNextYear() {} public function getPreviousMonth() {} public function getPreviousYear() {} public function getYear() {} private function calculateDaysPreviousMonth() {} private function calculateNumericDayOfTheFirstDayOfTheWeek() {} private function isCurrentDay(\DateTime $dateTime) {} private function isDifferentMonth(\DateTime $dateTime) {} } class CalendarCell { private $day; private $month; private $dayNameAbbreviation; private $numericDayOfTheWeek; private $isCurrentDay; private $isDifferentMonth; private $translator; public function __construct(array $parameters) {} public function getDay() {} public function getMonth() {} public function getDayNameAbbreviation() {} public function isCurrentDay() {} public function isDifferentMonth() {} } Each calendar day can includes many events stored in a database. My question is: which is the best way to manage these events in my classes? I think to add a eventList property in CalendarCell and populate it with an array of CalendarEvent objects fetched by the database. This kind of solution doesn't allow other coders to reuse the classes without db (because I should inject at least a repository services also) just to create and visualize a calendar... so maybe it could be better to extend CalendarCell (for instance in CalendarCellEvent) and add the database features? I feel like I'm missing some crucial design pattern! Any suggestion will be very appreciated!

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• Correct way to inject dependencies in Business logic service?

  - by Sri Harsha Velicheti

  Currently the structure of my application is as below Web App -- WCF Service (just a facade) -- Business Logic Services -- Repository - Entity Framework Datacontext Now each of my Business logic service is dependent on more than 5 repositories ( I have interfaces defined for all the repos) and I am doing a Constructor injection right now(poor mans DI instead of using a proper IOC as it was determined that it would be a overkill for our project). Repositories have references to EF datacontexts. Now some of the methods in the Business logic service require only one of the 5 repositories, so If I need to call that method I would end up instantiating a Service which will instatiate all 5 repositories which is a waste. An example: public class SomeService : ISomeService { public(IFirstRepository repo1, ISecondRepository repo2, IThirdRepository repo3) {} // My DoSomething method depends only on repo1 and doesn't use repo2 and repo3 public DoSomething() { //uses repo1 to do some stuff, doesn't use repo2 and repo3 } public DoSomething2() { //uses repo2 and repo3 to do something, doesn't require repo1 } public DoSomething3() { //uses repo3 to do something, doesn't require repo1 and repo2 } } Now if my I have to use DoSomething method on SomeService I end up creating both IFirstRepository,ISecondRepository and IThirdRepository but using only IFirstRepository, now this is bugging me, I can seem to accept that I am un-necessarily creating repositories and not using them. Is this a correct design? Are there any better alternatives? Should I be looking at Lazy instantiation Lazy<T> ?

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• Implenting ActiveRecord with inheritance?

  - by King

  I recently converted an old application that was using XML files as the data store to use SQL instead. To avoid a lot of changes I basically created ActiveRecord style classes that inherited from the original business objects. For example SomeClassRecord :SomeClass //ID Property //Save method I then used this new class in place of the other one, because of polymorphism I didn't need to change any methods that took SomeClass as a parameter. Would this be considered 'Bad'? What would be a better alternative?

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• Motivation for service layer (instead of just copying dlls)?

  - by BornToCode

  I'm creating an application which has 2 different UIs so I'm making it with a service layer which I understood is appropriate for such case. However I found myself just creating web methods for every single method I have in the BL layer, so the services basically built from methods that looks like this: return customers_bl.Get_Customer_Prices(customer_id); I understood that a main point of the service layer is to prevent duplication of code so I asked myself - well, why not just import the BL.dll (and the DAL.dll) to the other UI, and whenever making a change re-copy the dll files, it might not be so 'neat', but is the all purpose of the service layer to prevent this? {I know something is wrong in my approach, I'm probably missing the importance of service layer, I'd like to get more motivation to create another layer, especially because as it is I found that many of my BL functions ALREADY looks like: return customers_dal.Get_Customer_Prices(cust_id) which led me to ask: was it really necessary to create the BL just because on several functions I actually have LOGIC inside the BL?} so I'm looking for more motivation to creating ONE MORE layer, I'm sure it's not just to make it more convenient that I won't have to re-copy the dlls on changes? Am I grasping it wrong? Any simple guidelines on how to design service layer (corresponding to all the BL layer functions or not? any simple example?) any enlightenment on the subject?

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• Is it okay to have many Abstract classes in your application?

  - by JoseK

  We initially wanted to implement a Strategy pattern with varying implementations of the methods in a commmon interface. These will get picked up at runtime based on user inputs. As it's turned out, we're having Abstract classes implementing 3 - 5 common methods and only one method left for a varying implementation i.e. the Strategy. Update: By many abstract classes I mean there are 6 different high level functionalities i.e. 6 packages , and each has it's Interface + AbstractImpl + (series of Actual Impl). Is this a bad design in any way? Any negative views in terms of later extensibility - I'm preparing for a code/design review with seniors.

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• Motivation for a service layer (instead of just copying dlls)?

  - by BornToCode

  I'm creating an application which has 2 different UIs so I'm making it with a service layer which I understood is appropriate for such scenario. However I found myself just creating web methods for every single method I have in the BL layer, so the services basically built from methods that looks like this: return customers_bl.Get_Customer_Prices(customer_id); I understood that a main point of the service layer is to prevent duplication of code so I asked myself - why not just import the BL.DLL (and the dal.dll) to the other UI, and whenever making a change re-copy the dlls, it might not be so 'neat', but still less hassle than one more layer? {I know something is wrong in my approach, I'm probably missing the importance of service layer, I'd like to get more motivation to create another layer, especially because as it is I found that many of my BL functions ALREADY looks like: return customers_dal.Get_Customer_Prices(cust_id) which led me to ask: was it really necessary to create the BL just because on several functions I actually have LOGIC inside the BL?} so I'm looking for more motivation to creating ONE MORE layer, I'm sure it's not just to make it more convenient that I won't have to re-copy the dlls on changes? Am I grasping it wrong? Any simple guidelines on how to design service layer (corresponding to all the BL layer functions or not? any simple example?) any enlightenment on the subject?

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• What are the caveats of the event system built on Messenger rather than on classic .NET events?

  - by voroninp

  MVVM Light and PRISM offer messenger to implement event system. the approximate interface looks like the following one: interface Messanger { void Subscribe<TMessageParam>(Action<TMessageParam> action); void Unsubscribe<TMessageParam>(Action<TMessageParam> action); void Unsubscribe<TMessageParam>(objec actionOwner); void Notify<TMessageParam>(TMessageParam param); } Now this model seems beneficial comparing to classic .net events. It works well with Dependency Injection. Actions are stored as weak references so memory leaks are avioded and unsubscribe is not a must. The only annoyance is the need to declare new TMessageParam for each specific message. But everything comes at a cost. And what I'm really worried about is that I see no shortcomings of this approach. Has anoyne the experience of some troubles with this design pattern?

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• How to choose between using a Domain Event, or letting the application layer orchestrate everything

  - by Mr Happy

  I'm setting my first steps into domain driven design, bought the blue book and all, and I find myself seeing three ways to implement a certain solution. For the record: I'm not using CQRS or Event Sourcing. Let's say a user request comes into the application service layer. The business logic for that request is (for whatever reason) separated into a method on an entity, and a method on a domain service. How should I go about calling those methods? The options I have gathered so far are: Let the application service call both methods Use method injection/double dispatch to inject the domain service into the entity, letting the entity do it's thing and then let it call the method of the domain service (or the other way around, letting the domain service call the method on the entity) Raise a domain event in the entity method, a handler of which calls the domain service. (The kind of domain events I'm talking about are: http://www.udidahan.com/2009/06/14/domain-events-salvation/) I think these are all viable, but I'm unable to choose between them. I've been thinking about this a long time and I've come to a point where I no longer see the semantic differences between the three. Do you know of some guidelines when to use what?

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• Help with MVC design pattern?

  - by user3681240

  I am trying to build a java program for user login but I am not sure if my MVC design is accurate. I have the following classes: LoginControl - servlet LoginBean - data holder java class with private variables getters and setters LoginDAO - concrete java class where I am running my SQL queries and doing rest of the logical work. Connection class - java class just to connect to the database view - jsp to display the results html - used for form Is this how you design a java program based on MVC design pattern? Please provide some suggestions?

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• What are the best practices to use NHiberante sessions in asp.net (mvc/web api) ?

  - by mrt181

  I have the following setup in my project: public class WebApiApplication : System.Web.HttpApplication { public static ISessionFactory SessionFactory { get; private set; } public WebApiApplication() { this.BeginRequest += delegate { var session = SessionFactory.OpenSession(); CurrentSessionContext.Bind(session); }; this.EndRequest += delegate { var session = SessionFactory.GetCurrentSession(); if (session == null) { return; } session = CurrentSessionContext.Unbind(SessionFactory); session.Dispose(); }; } protected void Application_Start() { AreaRegistration.RegisterAllAreas(); FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters); RouteConfig.RegisterRoutes(RouteTable.Routes); BundleConfig.RegisterBundles(BundleTable.Bundles); var assembly = Assembly.GetCallingAssembly(); SessionFactory = new NHibernateHelper(assembly, Server.MapPath("/")).SessionFactory; } } public class PositionsController : ApiController { private readonly ISession session; public PositionsController() { this.session = WebApiApplication.SessionFactory.GetCurrentSession(); } public IEnumerable<Position> Get() { var result = this.session.Query<Position>().Cacheable().ToList(); if (!result.Any()) { throw new HttpResponseException(new HttpResponseMessage(HttpStatusCode.NotFound)); } return result; } public HttpResponseMessage Post(PositionDataTransfer dto) { //TODO: Map dto to model IEnumerable<Position> positions = null; using (var transaction = this.session.BeginTransaction()) { this.session.SaveOrUpdate(positions); try { transaction.Commit(); } catch (StaleObjectStateException) { if (transaction != null && transaction.IsActive) { transaction.Rollback(); } } } var response = this.Request.CreateResponse(HttpStatusCode.Created, dto); response.Headers.Location = new Uri(this.Request.RequestUri.AbsoluteUri + "/" + dto.Name); return response; } public void Put(int id, string value) { //TODO: Implement PUT throw new NotImplementedException(); } public void Delete(int id) { //TODO: Implement DELETE throw new NotImplementedException(); } } I am not sure if this is the recommended way to insert the session into the controller. I was thinking about using DI but i am not sure how to inject the session that is opened and binded in the BeginRequest delegate into the Controllers constructor to get this public PositionsController(ISession session) { this.session = session; } Question: What is the recommended way to use NHiberante sessions in asp.net mvc/web api ?

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• Where we should put validation for domain model

  - by adisembiring

  I still looking best practice for domain model validation. Is that good to put the validation in constructor of domain model ? my domain model validation example as follows: public class Order { private readonly List<OrderLine> _lineItems; public virtual Customer Customer { get; private set; } public virtual DateTime OrderDate { get; private set; } public virtual decimal OrderTotal { get; private set; } public Order (Customer customer) { if (customer == null) throw new ArgumentException("Customer name must be defined"); Customer = customer; OrderDate = DateTime.Now; _lineItems = new List<LineItem>(); } public void AddOderLine //.... public IEnumerable<OrderLine> AddOderLine { get {return _lineItems;} } } public class OrderLine { public virtual Order Order { get; set; } public virtual Product Product { get; set; } public virtual int Quantity { get; set; } public virtual decimal UnitPrice { get; set; } public OrderLine(Order order, int quantity, Product product) { if (order == null) throw new ArgumentException("Order name must be defined"); if (quantity <= 0) throw new ArgumentException("Quantity must be greater than zero"); if (product == null) throw new ArgumentException("Product name must be defined"); Order = order; Quantity = quantity; Product = product; } } Thanks for all of your suggestion.

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• Is this a pattern? Should it be?

  - by Arkadiy

  The following is more of a statement than a question - it describes something that may be a pattern. The question is: is this a known pattern? Or, if it's not, should it be? I've had a situation where I had to iterate over two dissimilar multi-layer data structures and copy information from one to the other. Depending on particular use case, I had around eight different kinds of layers, combined in about eight different combinations: A-B-C B-C A-C D-E A-D-E and so on After a few unsuccessful attempts to factor out the repetition of per-layer iteration code, I realized that the key difficulty in this refactoring was the fact that the bottom level needed access to data gathered at higher levels. To explicitly accommodate this requirement, I introduced IterationContext class with a number of get() and set() methods for accumulating the necessary information. In the end, I had the following class structure: class Iterator { virtual void iterateOver(const Structure &dataStructure1, IterationContext &ctx) const = 0; }; class RecursingIterator : public Iterator { RecursingIterator(const Iterator &below); }; class IterateOverA : public RecursingIterator { virtual void iterateOver(const Structure &dataStructure1, IterationContext &ctx) const { // Iterate over members in dataStructure1 // locate corresponding item in dataStructure2 (passed via context) // and set it in the context // invoke the sub-iterator }; class IterateOverB : public RecursingIterator { virtual void iterateOver(const Structure &dataStructure1, IterationContext &ctx) const { // iterate over members dataStructure2 (form context) // set dataStructure2's item in the context // locate corresponding item in dataStructure2 (passed via context) // invoke the sub-iterator }; void main() { class FinalCopy : public Iterator { virtual void iterateOver(const Structure &dataStructure1, IterationContext &ctx) const { // copy data from structure 1 to structure 2 in the context, // using some data from higher levels as needed } } IterationContext ctx(dateStructure2); IterateOverA(IterateOverB(FinalCopy())).iterate(dataStructure1, ctx); } It so happens that dataStructure1 is a uniform data structure, similar to XML DOM in that respect, while dataStructure2 is a legacy data structure made of various structs and arrays. This allows me to pass dataStructure1 outside of the context for convenience. In general, either side of the iteration or both sides may be passed via context, as convenient. The key situation points are: complicated code that needs to be invoked in "layers", with multiple combinations of layer types possible at the bottom layer, the information from top layers needs to be visible. The key implementation points are: use of context class to access the data from all levels of iteration complicated iteration code encapsulated in implementation of pure virtual function two interfaces - one aware of underlying iterator, one not aware of it. use of const & to simplify the usage syntax.

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