What is a singleton in C

Question

What is a Singleton and when should I use it

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A Singleton  and this isn t tied to C   it s an OO design pattern  is when you want to allow only ONE instance of a class to be created throughout your application  Useages would typically include global resources  although I will say from personal experience  they re very often the source of great pain

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another way to implement singleton in c   i personally prefer this way because you can access the instance of the singeton class as a property instead of a method   public class Singleton               private static Singleton instance           private Singleton                public static Singleton Instance                       get                               if  instance    null                      instance   new Singleton                    return instance                                     instance methods         but well  as far as i know both ways are considered  right  so it s just a thing of personal flavor

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I use it for lookup data  Load once from DB    public sealed class APILookup               private static readonly APILookup  instance   new APILookup            private Dictionary lt string  int gt   lookup           private APILookup                         try                                lookup   Utility GetLookup                              catch                        static APILookup                                            public static APILookup Instance                       get                               return  instance                                  public Dictionary lt string  int gt  GetLookup                         return  lookup

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using System  using System Collections Generic  class MainApp       static void Main                 LoadBalancer oldbalancer   null          for  int i   0  i  lt  15  i                          LoadBalancer balancerNew   LoadBalancer GetLoadBalancer                 if  oldbalancer    balancerNew  amp  amp  oldbalancer    null                                Console WriteLine   0  SameInstance  1    oldbalancer Server  balancerNew Server                             oldbalancer   balancerNew                    Console ReadKey             class LoadBalancer       private static LoadBalancer  instance      private List lt string gt   servers   new List lt string gt         private Random  random   new Random         private static object syncLock   new object         private LoadBalancer                  servers Add  ServerI             servers Add  ServerII             servers Add  ServerIII             servers Add  ServerIV             servers Add  ServerV               public static LoadBalancer GetLoadBalancer                 if   instance    null                        lock  syncLock                                if   instance    null                                         instance   new LoadBalancer                                                       return  instance             public string Server               get                       int r    random Next  servers Count               return  servers r  ToString                        I took code from dofactory com  nothing so fancy but I find this far good than examples with Foo and Bar additionally book from Judith Bishop on C  3 0 Design Patterns has example about active application in mac dock   If you look at code we are actually building new objects on for loop  so that creates new object but reuses instance as a result of which the oldbalancer and newbalancer has same instance  How  its due to static keyword used on function GetLoadBalancer    despite of having different server value which is random list  static on GetLoadBalancer   belongs to the type itself rather than to a specific object   Additionally there is double check locking here  if   instance    null                                lock  syncLock                                        if   instance    null    since from MSDN          The lock keyword ensures that one thread does not enter a critical section of code while another thread is in the critical section  If another thread tries to enter a locked code  it will wait  block  until the object is released       so every-time mutual-exclusion lock is issued  even if it don t need to which is unnecessary so we have null check   Hopefully it helps in clearing more   And please comment if I my understanding is directing wrong ways

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You asked for C    Trivial example    public class Singleton       private Singleton                    Prevent outside instantiation            private static readonly Singleton  singleton   new Singleton         public static Singleton GetSingleton                 return  singleton

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Whilst the there can only ever be one instance of a singleton  it is not the same as a static class   A static class can only contain static methods and can never be instantiated  whereas the instance of a singleton may be used in the same way as any other object

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E X You can use Singleton for global information that needs to be injected   In my case  I was keeping the Logged user detail username  permissions etc   in Global Static Class  And when I tried to implement the Unit Test  there was no way I could inject dependency into Controller classes  Thus I have changed my Static Class to Singleton pattern   public class SysManager       private static readonly SysManager instance   new SysManager         static SysManager           private SysManager          public static SysManager Instance               get  return  instance             http   csharpindepth com Articles General Singleton aspx cctor

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A singleton is a class which only allows one instance of itself to be created - and gives simple  easy access to said instance  The singleton premise is a pattern across software development   There is a C  implementation  Implementing the Singleton Pattern in C   covering most of what you need to know - including some good advice regarding thread safety   To be honest  It s very rare that you need to implement a singleton - in my opinion it should be one of those things you should be aware of  even if it s not used too often

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I know it s very late to answer the question but with Auto-Property you can do something like that   public static Singleton Instance   get      new Singleton     Where Singleton is you class and can be via  in this case the readonly property Instance

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What is a singleton   It is a class which only allows one instance of itself to be created  and usually gives simple access to that instance   When should you use   It depends on the situation   Note   please do not use on db connection  for a detailed answer please refer to the answer of  Chad Grant   Here is a simple example of a Singleton   public sealed class Singleton       private static readonly Singleton instance   new Singleton            Explicit static constructor to tell C  compiler        not to mark type as beforefieldinit     static Singleton                    private Singleton                    public static Singleton Instance               get                       return instance                      You can also use Lazy lt T gt  to create your Singleton   See here for a more detailed example using Lazy lt T gt

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Thread Safe Singleton without using locks and no lazy instantiation   This implementation has a static constructor  so it executes only once per Application Domain   public sealed class Singleton        static Singleton          private Singleton          public static Singleton Instance   get      new Singleton

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Here s what singleton is  http   en wikipedia org wiki Singleton pattern  I don t know C   but it s actually the same thing in all languages  only implementation differs   You should generally avoid singleton when it s possible  but in some situations it s very convenient   Sorry for my English

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It s a design pattern and it s not specific to c   More about it all over the internet and SO  like on this wikipedia article      In software engineering  the singleton   pattern is a design pattern that is   used to restrict instantiation of a   class to one object  This is useful   when exactly one object is needed to   coordinate actions across the system    The concept is sometimes generalized   to systems that operate more   efficiently when only one object   exists  or that restrict the   instantiation to a certain number of   objects  say  five   Some consider it   an anti-pattern  judging that it is   overused  introduces unnecessary   limitations in situations where a sole   instance of a class is not actually   required  and introduces global state   into an application    You should use it if you want a class that can only be instanciated once

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What it is  A class for which there is just one  persistent instance across the lifetime of an application   See Singleton Pattern   When you should use it  As little as possible   Only when you are absolutely certain that you need it   I m reluctant to say  never   but there is usually a better alternative  such as Dependency Injection or simply a static class

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