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  • Database Context and Singleton injection with IoC

    - by zaitsman
    All of the below relates to a ASP.NET c# app. I have a Singleton Settings MemoryCache that reads values from database on first access and caches these, then invalidates them using SQL Service Broker message and re-reads as required. For the purposes of standard controllers, i create my Db Context in a request scope. However, this obviously means that i can't use the same context in the Settings Cache class, since that is a singleton and we have a scope collision. At the moment, i ended up with two db contexts - the Controllers get it via IoC container, whereas a Singleton just creates it's own. However, i am not satisfied with this approach (mostly due to the way i feel about two contexts, the cache doesn't set anything on the db hence concurrency is not an issue as much). What is a better way to do it?

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  • Singleton & Multi-threading

    - by ronan
    Friends I have the following class that class Singleton { private: static Singleton *p_inst; Singleton(); public: static Singleton * instance() { if (!p_inst) { p_inst = new Singleton(); } return p_inst; } }; Please do elaborate on precautions taken while implementing Singleton in multi-threaded environment .. Many thanks

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  • Is it possible to apply inheritance to a Singleton class?

    - by Bragaadeesh
    Hi, Today I faced one question in interview. Is it possible to apply inheritance concept on Singleton Classes. I said since the constructor is private, we cannot extend that Singleton class (can someone please validate this). Next thing he asked me is to apply inheritance on that Singleton class. So, I made the Singleton's constructor as protected thinking that child's constructor also has be protected. But I was wrong the child can have a modifier either equal to or higher than that. So, I asked him to give a real world example on such a case. He was not able to give me one and said that I cant ask questions and wanted me to tell whether this scenario is possible or not. I went kind of blank. My question here is, Is this possible? Even if its possible, what is the use of it? What real world scenario would demand such a use. Thanks

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  • Can I use an abstract class instead of a private __construct() when creating a singleton in PHP?

    - by Pheter
    When creating a Singleton in PHP, I ensure that it cannot be instantiated by doing the following: class Singleton { private function __construct() {} private function __clone() {} public static function getInstance() {} } However, I realised that defining a class as 'abstract' means that it cannot be instantiated. So is there anything wrong with doing the following instead: abstract class Singleton { public static function getInstance() {} } The second scenario allows me to write fewer lines of code which would be nice. (Not that it actually makes much of a difference.)

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  • Constructor and Destructor of a singleton object called twice

    - by Bikram990
    I'm facing a problem in singleton object in c++. Here is the explanation: Problem info: I have a 4 shared libraries (say libA.so, libB.so, libC.so, libD.so) and 2 executable binary files each using one another shared library( say libE.so) which deals with files. The purpose of libE.so is to write data into a file and if the executable restarts or size of file exceeds a certain limit it is zipped and a new file is created with time stamp in name. It is using singleton object. It exports a handler class for getting and using singleton. Compressing only happens in the above said two cases. The user/loader executable can specify the starting name of file only no other control is provided by handler class. libA.so, libB.so, libC.so and libD.so have almost same behavior. They all have a class and declare and object of an handler which gets the instance of the singleton in libE.so and uses it for further purpose. All these libraries are linked to two executable binary files. If only one of the two executable runs then its fine, But if both executable runs one after other then the file of the first started executable gets compressed. Debug info: The constructor and destructor of the singleton object is called twice.(for each executable) The object of singleton is a static object and never deleted. The executable is not able to exit/return gives: glibc detected * (exe1 or exe2): double free or corruption (!prev): some_addr * Running with binaries valgrind gives that the above error is due to the destructor of the singleton object. Thanks

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  • Alternatives to the Singleton Design Pattern

    The Singleton Design Pattern is one of the simplest and most widely known design patterns in use in software development. However, despite its simplicity, it is very easy to get wrong and the consequences of its use even when properly implemented can outweigh its benefits. It turns out there are other ways to achieve the goals of the Singleton pattern which will often prove to be simpler, safer, and more maintainable.

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  • Hide or Show singleton?

    - by Sinker
    Singleton is a common pattern implemented in both native libraries of .NET and Java. You will see it as such: C#: MyClass.Instance Java: MyClass.getInstance() The question is: when writing APIs, is it better to expose the singleton through a property or getter, or should I hide it as much as possible? Here are the alternatives for illustrative purposes: Exposed(C#): private static MyClass instance; public static MyClass Instance { get { if (instance == null) instance = new MyClass(); return instance; } } public void PerformOperation() { ... } Hidden (C#): private static MyClass instance; public static void PerformOperation() { if (instance == null) { instance = new MyClass(); } ... } EDIT: There seems to be a number of detractors of the Singleton design. Great! Please tell me why and what is the better alternative. Here is my scenario: My whole application utilises one logger (log4net/log4j). Whenever, the program has something to log, it utilises the Logger class (e.g. Logger.Instance.Warn(...) or Logger.Instance.Error(...) etc. Should I use Logger.Warn(...) or Logger.Warn(...) instead? If you have an alternative to singletons that addresses my concern, then please write an answer for it. Thank you :)

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  • Singleton Properties

    - by coffeeaddict
    Ok, if I create a singleton class and expose the singleton object through a public static property...I understand that. But my singleton class has other properties in it. Should those be static? Should those also be private? I just want to be able to access all properties of my singleton class by doing this: MySingletonClass.SingletonProperty.SomeProperty2 Where SingletonProperty returns me the single singleton instance. I guess my question is, how do you expose the other properties in the singleton class..make them private and then access them through your public singleton static property?

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  • How to bind a control to a singleton in Cocoa?

    - by SphereCat1
    I have a singleton in my FTP app designed to store all of the types of servers that the app can handle, such as FTP or Amazon S3. These types are plugins which are located in the app bundle. Their path is located by applicationWillFinishLoading: and sent to the addServerType: method inside the singleton to be loaded and stored in an NSMutableDictionary. My question is this: How do I bind an NSDictionaryController to the dictionary inside the singleton instance? Can it be done in IB, or do I have to do it in code? I need to be able to display the dictionary's keys in an NSPopupButton so the user can select a server type. Thanks in advance! SphereCat1

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  • Do I still have to implement a singleton class by hand in .net, even when using .Net4.0?

    - by Hamish Grubijan
    Once the singleton pattern is understood, writing subsequent singleton classes in C# is a brainless exercise. I would hope that the framework would help you by providing an interface or a base class to do that. Here is how I envision it: public sealed class Schablone : ISingleton<Schablone> { // Stuff forced by the interface goes here // Extra logic goes here } Does what I am looking for exist? Is there some syntactic sugar for constructing a singleton class - whether with an interface, a class attribute, etc.? Can one write a useful and bullet-proof ISingleton themselves? Care to try? Thanks!

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  • Is this a valid, lazy, thread-safe Singleton implementation for C#?

    - by Matthew
    I implemented a Singleton pattern like this: public sealed class MyClass { ... public static MyClass Instance { get { return SingletonHolder.instance; } } ... static class SingletonHolder { public static MyClass instance = new MyClass (); } } From Googling around for C# Singleton implementations, it doesn't seem like this is a common way to do things in C#. I found one similar implementation, but the SingletonHolder class wasn't static, and included an explicit (empty) static constructor. Is this a valid, lazy, thread-safe way to implement the Singleton pattern? Or is there something I'm missing?

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  • Is there any reason for an object pool to not be treated as a singleton?

    - by Chris Charabaruk
    I don't necessarily mean implemented using the singleton pattern, but rather, only having and using one instance of a pool. I don't like the idea of having just one pool (or one per pooled type). However, I can't really come up with any concrete situations where there's an advantage to multiple pools for mutable types, at least not any where a single pool can function just as well. What advantages are there to having multiple pools over a singleton pool?

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  • How TiVo is messing up customer support.

    - by James Fleming
    Ok,  So I've gotten a TiVo and overall, I'm happy, but there have been issues and I suspect I've a defective unit. - Now the nice folks after many service calls were happy to swap it out, and to ensure continuity of service, they sent me a new unit (after a $109 deposit).  That was yesterday. Today, when we go to watch a little TV, and wait for our replacement unit to arrive we find our TiVo service has been suspended. WTF? They have an exchange program, but your unit your waiting to exchange is as dead as a doornail until the replacement arrives. How hard is it to keep the old unit active for an extra week? Here is the exchange w/Tivo below... You are currently number 1 in the queue. We apologize for the delay. We will assign you to an agent as soon as one is available.The average amount of time a customer has to wait is 00:13.  Kaylene (Listening)  Kaylene: Thank you for contacting TiVo! My name is Kaylene. So that I may better assist you, are you an existing customer?  james Fleming: yes I am, but I'm now having second thoughts about being one    Kaylene: Thank you for verifying your information. How may I assist you today James?  james Fleming: I've been having issues w/a tivo box & I'm getting a replacement sent out to me (after paying an additional deposit) and now my current unit is no longer activated  Kaylene: I can help you today!  Kaylene: When we process an exchange we do transfer over the service to the replacement box so it is active and ready to go when you receive it.  james Fleming: which is to say you also make my current box worthless until such time I receive a new box?!?!?  Kaylene: I apologize that your original box was deactivated so we could activate your replacement box.  james Fleming: Why on Earth would I bother to pay in advance for a new box if you were going to kill my existing box.  Kaylene: What features are you needing to use on your current box?  james Fleming: I need to be able to access my netflix subscription (if I'm lucky enough to have it work without rebooting)  Kaylene: Can I have you verify the TiVo Service Number of your TiVo box please?  james Fleming: 7460011906979b4  Kaylene: We have your current box temporary service but not all features are available with temporary service as it is not paid for service.  Kaylene: If you like I can transfer your service back to your current box for now. Then once you receive the new box you will have to call in and have the service transferred back to the new box.  james Fleming: Not paid for? Let's see> one tivo box + 3 year service plan + monthly service + $109 deposit on a second box = what?  Kaylene: Would you like me to transfer your service back to your current box?  james Fleming: Yes - that would be helpful  Kaylene: All you will need to do is contact us again once you receive the new box so we can transfer it back.  Kaylene: I have put your service back on TiVo box 7460011906979b4.  james Fleming: What would also be helpful is your firm informing me to how you'd be cutting service in the interim.  james Fleming: Again - I opted to pay to have a second box delivered BEFORE returning the box I have - thus trying to have a continuity of service..  Kaylene: This is not something we normally do so it is important when you contact us to transfer the service back to the new box when you receive it that you reference this case number: 110622-006089.  Kaylene: I apologize about the inconvenience. You may need  force a few connections for the box to recognize the service again.  james Fleming: If it's not something you normally do than WHY would you have a $109 fee and a term for the service.  james Fleming: I am not mad at you, but your company is not impressing me and I'm blogging about this experience  Kaylene: Again I apologize about the inconvenience but you should be good to go now. Is there anything else I can help you with today?  james Fleming: so I need to go through the re-actviate process or is that somethign you do  Kaylene: When you receive the new TiVo box you need to contact us so we can transfer the service to the new box for you.  james Fleming: sure  Kaylene: Is there anything else I can help you with today James?  james Fleming: Nope - please email this transcript to me  Kaylene: I apologize but we do not have the ability to e-mail you a copy of this transcript. You can view it online at  http://www.tivo.com when you sign into your account or you can copy and paste it now to save it.  Kaylene: Thank you for contacting TiVo today. Your reference number for our conversation is 110622-006089. You can save this for your records, and if necessary, provide this to a later agent to pull up what we discussed. There will be a brief satisfaction survey emailed to you. We would appreciate any feedback on your TiVo Chat Support experience today.  Kaylene: Thank you for using TiVo Chat and have a great day James! Good-bye.  Kaylene has disconnected.

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  • Could a singleton type replace static methods and classes?

    - by MKO
    In C# Static methods has long served a purpose allowing us to call them without instantiating classes. Only in later year have we became more aware of the problems of using static methods and classes. They can’t use interfaces They can’t use inheritance They are hard to test because you can’t make mocks and stubs Is there a better way ? Obviously we need to be able to access library methods without instantiated classes all the time otherwise our code would become pretty cluttered One possibly solution is to use a new keyword for an old concept: the singleton. Singleton’s are global instances of a class, since they are instances we can use them as we would normal classes. In order to make their use nice and practical we'd need some syntactic sugar however Say that the Math class would be of type singleton instead of an actual class. The actual class containing all the default methods for the Math singleton is DefaultMath, which implements the interface IMath. The singleton would be declared as singleton Math : IMath { public Math { this = new DefaultMath(); } } If we wanted to substitute our own class for all math operations we could make a new class MyMath that inherits DefaultMath, or we could just inherit from the interface IMath and create a whole new Class. To make our class the active Math class, you'd do a simple assignment Math = new MyMath(); and voilá! the next time we call Math.Floor it will call your method. Note that for a normal singleton we'd have to write something like Math.Instance.Floor but the compiler eliminates the need for the Instance property Another idea would be to be able to define a singletons as Lazy so they get instantiated only when they're first called, like lazy singleton Math : IMath What do you think, would it have been a better solution that static methods and classes? Is there any problems with this approach?

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  • Achieving C# "readonly" behavior in C++

    - by Tommy Fisk
    Hi guys, this is my first question on stack overflow, so be gentle. Let me first explain the exact behavior I would like to see. If you are familiar with C# then you know that declaring a variable as "readonly" allows a programmer to assign some value to that variable exactly once. Further attempts to modify the variable will result in an error. What I am after: I want to make sure that any and all single-ton classes I define can be predictably instantiated exactly once in my program (more details at the bottom). My approach to realizing my goal is to use extern to declare a global reference to the single-ton (which I will later instantiate at a time I choose. What I have sort of looks like this, namespace Global { extern Singleton& mainInstance; // not defined yet, but it will be later! } int main() { // now that the program has started, go ahead and create the singleton object Singleton& Global::mainInstance = Singleton::GetInstance(); // invalid use of qualified name Global::mainInstance = Singleton::GetInstance(); // doesn't work either :( } class Singleton { /* Some details ommited */ public: Singleton& GetInstance() { static Singleton instance; // exists once for the whole program return instance; } } However this does not really work, and I don't know where to go from here. Some details about what I'm up against: I'm concerned about threading as I am working on code that will deal with game logic while communicating with several third-party processes and other processes I will create. Eventually I would have to implement some kind of synchronization so multiple threads could access the information in the Singleton class without worry. Because I don't know what kinds of optimizations I might like to do, or exactly what threading entails (never done a real project using it), I was thinking that being able to predictably control when Singletons were instantiated would be a Good Thing. Imagine if Process A creates Process B, where B contains several Singletons distributed against multiple files and/or libraries. It could be a real nightmare if I can not reliably ensure the order these singleton objects are instantiated (because they could depend on each other, and calling methods on a NULL object is generally a Bad Thing). If I were in C# I would just use the readonly keyword, but is there any way I can implement this (compiler supported) behavior in C++? Is this even a good idea? Thanks for any feedback.

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  • Apple Mail authentication failure to Apache James while Thunderbird connects

    - by dacracot
    I have an Apache James 2.3.2 email server running on RHEL 5. I have been connecting to it successfully for months using Thunderbird (currently version 12.0.1). I am attempting to connect to the same account using Apple's Mail 6.5. On the first dialog, to add an account to Apple's Mail, it asks for full name, email address, and password. It then asks for an incoming mail server. I put account type equal to POP, the incoming mail server equal to the host in my email address, and my username and password. It comes back with the error: "Logging in to the POP server "" failed. Make sure the user name and password you entered are correct, then click Continue. If the information isn't correct, you cannot receive messages." While the dialogs are different in Thunderbird, I believe that I am giving it exactly the same parameters, and succeeding with authentication.

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  • using a Singleton to pass credentials in a multi-tenant application a code smell?

    - by Hans Gruber
    Currently working on a multi-tenant application that employs Shared DB/Shared Schema approach. IOW, we enforce tenant data segregation by defining a TenantID column on all tables. By convention, all SQL reads/writes must include a Where TenantID = '?' clause. Not an ideal solution, but hindsight is 20/20. Anyway, since virtually every page/workflow in our app must display tenant specific data, I made the (poor) decision at the project's outset to employ a Singleton to encapsulate the current user credentials (i.e. TenantID and UserID). My thinking at the time was that I didn't want to add a TenantID parameter to each and every method signature in my Data layer. Here's what the basic pseudo-code looks like: public class UserIdentity { public UserIdentity(int tenantID, int userID) { TenantID = tenantID; UserID = userID; } public int TenantID { get; private set; } public int UserID { get; private set; } } public class AuthenticationModule : IHttpModule { public void Init(HttpApplication context) { context.AuthenticateRequest += new EventHandler(context_AuthenticateRequest); } private void context_AuthenticateRequest(object sender, EventArgs e) { var userIdentity = _authenticationService.AuthenticateUser(sender); if (userIdentity == null) { //authentication failed, so redirect to login page, etc } else { //put the userIdentity into the HttpContext object so that //its only valid for the lifetime of a single request HttpContext.Current.Items["UserIdentity"] = userIdentity; } } } public static class CurrentUser { public static UserIdentity Instance { get { return HttpContext.Current.Items["UserIdentity"]; } } } public class WidgetRepository: IWidgetRepository{ public IEnumerable<Widget> ListWidgets(){ var tenantId = CurrentUser.Instance.TenantID; //call sproc with tenantId parameter } } As you can see, there are several code smells here. This is a singleton, so it's already not unit test friendly. On top of that you have a very tight-coupling between CurrentUser and the HttpContext object. By extension, this also means that I have a reference to System.Web in my Data layer (shudder). I want to pay down some technical debt this sprint by getting rid of this singleton for the reasons mentioned above. I have a few thoughts on what an better implementation might be, but if anyone has any guidance or lessons learned they could share, I would be much obliged.

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  • C++ Singleton design pattern.

    - by Artem Barger
    Recently I've bumped into realization/implementation of Singleton design pattern for C++. It has looked in the following way (I have adopted it from real life example): // a lot of methods is omitted here class Singleton { public: static Singleton* getInstance( ); ~Singleton( ); private: Singleton( ); static Singleton* instance; }; From this declaration I can deduce that instance field is initiated on the heap, that means there is a memory allocation. That is completely unclear for me is when does exactly memory is going to be deallocated? Or there is a bug and memory leak? It seems like there is a problem in implementation. PS. And main question how to implement it in the right way?

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  • super(type,subclass) in simple singleton implementation

    - by Tianchen Wu
    when I was implementing naive singleton in python, I came up with a problem with super key word. As usual the behavior of super is always tricky and buggy, hope someone can shed light on it. Thanks :) The problem is that: class Singleton(object): def __new__(cls,*args,**kw): if not hasattr(cls,'_instance'): #create a instance of type cls, origin=super(Singleton,Singleton).__new__(cls,*args,**kw) cls._instance=origin return cls._instance class B(Singleton): def __init__(self,b): self.b=b It actually works, but I am wondering Will it be better if I change line 5 to the below, like in most of the books? origin=super(Singleton,cls).__new__(cls,*args,**ks) what's the difference to make?

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  • Singleton

    Imagine that you need some global logging system in your application.You need to be able log your messages to some file at any point of your application, but also you need to numerate your messages.How can you accomplish this? - SINGLETON

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  • Singelton restricted to instance of dll

    - by codeySmurf
    If I create a singleton class in the context of a dll, the singleton class is instantiated once and used by all instances of the dll. I am using a dll as a plug-in for an application. Now the following thing came to my mind: If I use a singleton Class, it will be shared across multiple instances of the plug-in. However, this makes it difficult to manage the lifetime of the singleton class efficiently. The only way I could think of would be to use a reference count and to make the singleton delete its self when the reference count is 0. Does anyone have any better ideas on that? Is there any good way to restrict the singleton object to one instance of the dll? Language is c++

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  • How to access 'private functions' in a singleton from another object inside it.

    - by Cedric Dugas
    I am currently trying to create a test suite for my javascript apps. My problem is that, it seems I cannot get access to init() from my utils object, as you can see below: I have my app that follow a singleton pattern: var appModal = function () { var utils = Object.create(moduleUtils); function init(caller, options ) { } }(); My test suite is in moduleUtils, this is a object literal converted to a prototype moduleUtils.debug = { addSlideTest : function(){ /* this function cannot fire init() from appModal */ }}

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