Singleton: How should it be used

Question

Edit: From another question I provided an answer that has links to a lot of questions/answers about singletons: More info about singletons here:

So I have read the thread Singletons: good design or a crutch?
And the argument still rages.

I see Singletons as a Design Pattern (good and bad).

The problem with Singleton is not the Pattern but rather the users (sorry everybody). Everybody and their father thinks they can implement one correctly (and from the many interviews I have done, most people can't). Also because everybody thinks they can implement a correct Singleton they abuse the Pattern and use it in situations that are not appropriate (replacing global variables with Singletons!).

So the main questions that need to be answered are:

• When should you use a Singleton
• How do you implement a Singleton correctly

My hope for this article is that we can collect together in a single place (rather than having to google and search multiple sites) an authoritative source of when (and then how) to use a Singleton correctly. Also appropriate would be a list of Anti-Usages and common bad implementations explaining why they fail to work and for good implementations their weaknesses.

---

So get the ball rolling:
I will hold my hand up and say this is what I use but probably has problems.
I like "Scott Myers" handling of the subject in his books "Effective C++"

Good Situations to use Singletons (not many):

• Logging frameworks
• Thread recycling pools

/*
 * C++ Singleton
 * Limitation: Single Threaded Design
 * See: http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf
 *      For problems associated with locking in multi threaded applications
 *
 * Limitation:
 * If you use this Singleton (A) within a destructor of another Singleton (B)
 * This Singleton (A) must be fully constructed before the constructor of (B)
 * is called.
 */
class MySingleton
{
    private:
        // Private Constructor
        MySingleton();
        // Stop the compiler generating methods of copy the object
        MySingleton(MySingleton const& copy);            // Not Implemented
        MySingleton& operator=(MySingleton const& copy); // Not Implemented

    public:
        static MySingleton& getInstance()
        {
            // The only instance
            // Guaranteed to be lazy initialized
            // Guaranteed that it will be destroyed correctly
            static MySingleton instance;
            return instance;
        }
};

OK. Lets get some criticism and other implementations together.
:-)

Answer 1

Answer:

Use a Singleton if:

• You need to have one and only one object of a type in system

Do not use a Singleton if:

• You want to save memory
• You want to try something new
• You want to show off how much you know
• Because everyone else is doing it (See cargo cult programmer in wikipedia)
• In user interface widgets
• It is supposed to be a cache
• In strings
• In Sessions
• I can go all day long

How to create the best singleton:

• The smaller, the better. I am a minimalist
• Make sure it is thread safe
• Make sure it is never null
• Make sure it is created only once
• Lazy or system initialization? Up to your requirements
• Sometimes the OS or the JVM creates singletons for you (e.g. in Java every class definition is a singleton)
• Provide a destructor or somehow figure out how to dispose resources
• Use little memory

Answer 2

Singletons give you the ability to combine two bad traits in one class. That's wrong in pretty much every way.

A singleton gives you:

1. Global access to an object, and
2. A guarantee that no more than one object of this type can ever be created

Number one is straightforward. Globals are generally bad. We should never make objects globally accessible unless we really need it.

Number two may sound like it makes sense, but let's think about it. When was the last time you **accidentally* created a new object instead of referencing an existing one? Since this is tagged C++, let's use an example from that language. Do you often accidentally write

std::ostream os;
os << "hello world\n";

When you intended to write

std::cout << "hello world\n";

Of course not. We don't need protection against this error, because that kind of error just doesn't happen. If it does, the correct response is to go home and sleep for 12-20 hours and hope you feel better.

If only one object is needed, simply create one instance. If one object should be globally accessible, make it a global. But that doesn't mean it should be impossible to create other instances of it.

The "only one instance is possible" constraint doesn't really protect us against likely bugs. But it does make our code very hard to refactor and maintain. Because quite often we find out later that we did need more than one instance. We do have more than one database, we do have more than one configuration object, we do want several loggers. Our unit tests may want to be able to create and recreate these objects every test, to take a common example.

So a singleton should be used if and only if, we need both the traits it offers: If we need global access (which is rare, because globals are generally discouraged) and we need to prevent anyone from ever creating more than one instance of a class (which sounds to me like a design issue). The only reason I can see for this is if creating two instances would corrupt our application state - probably because the class contains a number of static members or similar silliness. In which case the obvious answer is to fix that class. It shouldn't depend on being the only instance.

If you need global access to an object, make it a global, like std::cout. But don't constrain the number of instances that can be created.

If you absolutely, positively need to constrain the number of instances of a class to just one, and there is no way that creating a second instance can ever be handled safely, then enforce that. But don't make it globally accessible as well.

If you do need both traits, then 1) make it a singleton, and 2) let me know what you need that for, because I'm having a hard time imagining such a case.

Answer 3

The problem with singletons is not their implementation. It is that they conflate two different concepts, neither of which is obviously desirable.

1) Singletons provide a global access mechanism to an object. Although they might be marginally more threadsafe or marginally more reliable in languages without a well-defined initialization order, this usage is still the moral equivalent of a global variable. It's a global variable dressed up in some awkward syntax (foo::get_instance() instead of g_foo, say), but it serves the exact same purpose (a single object accessible across the entire program) and has the exact same drawbacks.

2) Singletons prevent multiple instantiations of a class. It's rare, IME, that this kind of feature should be baked into a class. It's normally a much more contextual thing; a lot of the things that are regarded as one-and-only-one are really just happens-to-be-only-one. IMO a more appropriate solution is to just create only one instance--until you realize that you need more than one instance.

Answer 4

One thing with patterns: don't generalize. They have all cases when they're useful, and when they fail.

Singleton can be nasty when you have to test the code. You're generally stuck with one instance of the class, and can choose between opening up a door in constructor or some method to reset the state and so on.

Other problem is that the Singleton in fact is nothing more than a global variable in disguise. When you have too much global shared state over your program, things tend to go back, we all know it.

It may make dependency tracking harder. When everything depends on your Singleton, it's harder to change it, split to two, etc. You're generally stuck with it. This also hampers flexibility. Investigate some Dependency Injection framework to try to alleviate this issue.

Answer 5

Singletons basically let you have complex global state in languages which otherwise make it difficult or impossible to have complex global variables.

Java in particular uses singletons as a replacement for global variables, since everything must be contained within a class. The closest it comes to global variables are public static variables, which may be used as if they were global with import static

C++ does have global variables, but the order in which constructors of global class variables are invoked is undefined. As such, a singleton lets you defer the creation of a global variable until the first time that variable is needed.

Languages such as Python and Ruby use singletons very little because you can use global variables within a module instead.

So when is it good/bad to use a singleton? Pretty much exactly when it would be good/bad to use a global variable.

Answer 6

• How do you implement a Singleton correctly

There's one issue I've never seen mentioned, something I ran into at a previous job. We had C++ singletons that were shared between DLLs, and the usual mechanics of ensuring a single instance of a class just don't work. The problem is that each DLL gets its own set of static variables, along with the EXE. If your get_instance function is inline or part of a static library, each DLL will wind up with its own copy of the "singleton".

The solution is to make sure the singleton code is only defined in one DLL or EXE, or create a singleton manager with those properties to parcel out instances.

Answer 7

Modern C++ Design by Alexandrescu has a thread-safe, inheritable generic singleton.

For my 2p-worth, I think it's important to have defined lifetimes for your singletons (when it's absolutely necessary to use them). I normally don't let the static get() function instantiate anything, and leave set-up and destruction to some dedicated section of the main application. This helps highlight dependencies between singletons - but, as stressed above, it's best to just avoid them if possible.

Answer 8

As others have noted, major downsides to singletons include the inability to extend them, and losing the power to instantiate more than one instance, e.g. for testing purposes.

Some useful aspects of singletons:

1. lazy or upfront instantiation
2. handy for an object which requires setup and/or state

However, you don't have to use a singleton to get these benefits. You can write a normal object that does the work, and then have people access it via a factory (a separate object). The factory can worry about only instantiating one, and reusing it, etc., if need be. Also, if you program to an interface rather than a concrete class, the factory can use strategies, i.e. you can switch in and out various implementations of the interface.

Finally, a factory lends itself to dependency injection technologies like Spring etc.

Answer 9

The first example isn't thread safe - if two threads call getInstance at the same time, that static is going to be a PITA. Some form of mutex would help.

Answer 10

Singletons are handy when you've got a lot code being run when you initialize and object. For example, when you using iBatis when you setup a persistence object it has to read all the configs, parse the maps, make sure its all correct, etc.. before getting to your code.

If you did this every time, performance would be much degraded. Using it in a singleton, you take that hit once and then all subsequent calls don't have to do it.

Answer 11

Because a singleton only allows one instance to be created it effectively controls instance replication. for example you'd not need multiple instances of a lookup - a morse lookup map for example, thus wrapping it in a singleton class is apt. And just because you have a single instance of the class does not mean you are also limited on the number of references to that instance. You can queue calls(to avoid threading issues) to the instance and effect changes necessary. Yes, the general form of a singleton is a globally public one, you can certainly modify the design to create a more access restricted singleton. I haven't tired this before but I sure know it is possible. And to all those who commented saying the singleton pattern is utterly evil you should know this: yes it is evil if you do not use it properly or within it confines of effective functionality and predictable behavior: do not GENERALIZE.

Answer 12

Most people use singletons when they are trying to make themselves feel good about using a global variable. There are legitimate uses, but most of the time when people use them, the fact that there can only be one instance is just a trivial fact compared to the fact that it's globally accessible.

Answer 13

The real downfall of Singletons is that they break inheritance. You can't derive a new class to give you extended functionality unless you have access to the code where the Singleton is referenced. So, beyond the fact the the Singleton will make your code tightly coupled (fixable by a Strategy Pattern ... aka Dependency Injection) it will also prevent you from closing off sections of the code from revision (shared libraries).

So even the examples of loggers or thread pools are invalid and should be replaced by Strategies.

Answer 14

Below is the better approach for implementing a thread safe singleton pattern with deallocating the memory in destructor itself. But I think the destructor should be an optional because singleton instance will be automatically destroyed when the program terminates:

#include<iostream>
#include<mutex>

using namespace std;
std::mutex mtx;

class MySingleton{
private:
    static MySingleton * singletonInstance;
    MySingleton();
    ~MySingleton();
public:
    static MySingleton* GetInstance();
    MySingleton(const MySingleton&) = delete;
    const MySingleton& operator=(const MySingleton&) = delete;
    MySingleton(MySingleton&& other) noexcept = delete;
    MySingleton& operator=(MySingleton&& other) noexcept = delete;
};

MySingleton* MySingleton::singletonInstance = nullptr;
MySingleton::MySingleton(){ };
MySingleton::~MySingleton(){
    delete singletonInstance;
};

MySingleton* MySingleton::GetInstance(){
    if (singletonInstance == NULL){
        std::lock_guard<std::mutex> lock(mtx);
        if (singletonInstance == NULL)
            singletonInstance = new MySingleton();
    }
    return singletonInstance;
}

Regarding the situations where we need to use singleton classes can be- If we want to maintain the state of the instance throughout the execution of the program If we are involved in writing into execution log of an application where only one instance of the file need to be used....and so on. It will be appreciable if anybody can suggest optimisation in my above code.

Answer 15

But when I need something like a Singleton, I often end up using a Schwarz Counter to instantiate it.

Answer 16

I find them useful when I have a class that encapsulates a lot of memory. For example in a recent game I've been working on I have an influence map class that contains a collection of very large arrays of contiguous memory. I want that all allocated at startup, all freed at shutdown and I definitely want only one copy of it. I also have to access it from many places. I find the singleton pattern to be very useful in this case.

I'm sure there are other solutions but I find this one very useful and easy to implement.

Answer 17

I use Singletons as an interview test.

When I ask a developer to name some design patterns, if all they can name is Singleton, they're not hired.

Answer 18

I still don't get why a singleton has to be global.

I was going to produce a singleton where I hid a database inside the class as a private constant static variable and make class functions that utilize the database without ever exposing the database to the user.

I don't see why this functionality would be bad.

Answer 19

If you are the one who created the singleton and who uses it, dont make it as singleton (it doesn't have sense because you can control the singularity of the object without making it singleton) but it makes sense when you a developer of a library and you want to supply only one object to your users (in this case you are the who created the singleton, but you aren't the user).

Singletons are objects so use them as objects, many people accesses to singletons directly through calling the method which returns it, but this is harmful because you are making your code knows that object is singleton, I prefer to use singletons as objects, I pass them through the constructor and I use them as ordinary objects, by that way, your code doesn't know if these objects are singletons or not and that makes the dependencies more clear and it helps a little for refactoring ...

Answer 20

The Meyers singleton pattern works well enough most of the time, and on the occasions it does it doesn't necessarily pay to look for anything better. As long as the constructor will never throw and there are no dependencies between singletons.

A singleton is an implementation for a globally-accessible object (GAO from now on) although not all GAOs are singletons.

Loggers themselves should not be singletons but the means to log should ideally be globally-accessible, to decouple where the log message is being generated from where or how it gets logged.

Lazy-loading / lazy evaluation is a different concept and singleton usually implements that too. It comes with a lot of its own issues, in particular thread-safety and issues if it fails with exceptions such that what seemed like a good idea at the time turns out to be not so great after all. (A bit like COW implementation in strings).

With that in mind, GOAs can be initialised like this:

namespace {

T1 * pt1 = NULL;
T2 * pt2 = NULL;
T3 * pt3 = NULL;
T4 * pt4 = NULL;

}

int main( int argc, char* argv[])
{
   T1 t1(args1);
   T2 t2(args2);
   T3 t3(args3);
   T4 t4(args4);

   pt1 = &t1;
   pt2 = &t2;
   pt3 = &t3;
   pt4 = &t4;

   dostuff();

}

T1& getT1()
{
   return *pt1;
}

T2& getT2()
{
   return *pt2;
}

T3& getT3()
{
  return *pt3;
}

T4& getT4()
{
  return *pt4;
}

It does not need to be done as crudely as that, and clearly in a loaded library that contains objects you probably want some other mechanism to manage their lifetime. (Put them in an object that you get when you load the library).

As for when I use singletons? I used them for 2 things - A singleton table that indicates what libraries have been loaded with dlopen - A message handler that loggers can subscribe to and that you can send messages to. Required specifically for signal handlers.

Answer 21

Anti-Usage:

One major problem with excessive singleton usage is that the pattern prevents easy extension and swapping of alternate implementations. The class-name is hard coded wherever the singleton is used.

Answer 22

I think this is the most robust version for C#:

using System;
using System.Collections;
using System.Threading;

namespace DoFactory.GangOfFour.Singleton.RealWorld
{

  // MainApp test application

  class MainApp
  {
    static void Main()
    {
      LoadBalancer b1 = LoadBalancer.GetLoadBalancer();
      LoadBalancer b2 = LoadBalancer.GetLoadBalancer();
      LoadBalancer b3 = LoadBalancer.GetLoadBalancer();
      LoadBalancer b4 = LoadBalancer.GetLoadBalancer();

      // Same instance?
      if (b1 == b2 && b2 == b3 && b3 == b4)
      {
        Console.WriteLine("Same instance\n");
      }

      // All are the same instance -- use b1 arbitrarily
      // Load balance 15 server requests
      for (int i = 0; i < 15; i++)
      {
        Console.WriteLine(b1.Server);
      }

      // Wait for user
      Console.Read();    
    }
  }

  // "Singleton"

  class LoadBalancer
  {
    private static LoadBalancer instance;
    private ArrayList servers = new ArrayList();

    private Random random = new Random();

    // Lock synchronization object
    private static object syncLock = new object();

    // Constructor (protected)
    protected LoadBalancer()
    {
      // List of available servers
      servers.Add("ServerI");
      servers.Add("ServerII");
      servers.Add("ServerIII");
      servers.Add("ServerIV");
      servers.Add("ServerV");
    }

    public static LoadBalancer GetLoadBalancer()
    {
      // Support multithreaded applications through
      // 'Double checked locking' pattern which (once
      // the instance exists) avoids locking each
      // time the method is invoked
      if (instance == null)
      {
        lock (syncLock)
        {
          if (instance == null)
          {
            instance = new LoadBalancer();
          }
        }
      }

      return instance;
    }

    // Simple, but effective random load balancer

    public string Server
    {
      get
      {
        int r = random.Next(servers.Count);
        return servers[r].ToString();
      }
    }
  }
}

Here is the .NET-optimised version:

using System;
using System.Collections;

namespace DoFactory.GangOfFour.Singleton.NETOptimized
{

  // MainApp test application

  class MainApp
  {

    static void Main()
    {
      LoadBalancer b1 = LoadBalancer.GetLoadBalancer();
      LoadBalancer b2 = LoadBalancer.GetLoadBalancer();
      LoadBalancer b3 = LoadBalancer.GetLoadBalancer();
      LoadBalancer b4 = LoadBalancer.GetLoadBalancer();

      // Confirm these are the same instance
      if (b1 == b2 && b2 == b3 && b3 == b4)
      {
        Console.WriteLine("Same instance\n");
      }

      // All are the same instance -- use b1 arbitrarily
      // Load balance 15 requests for a server
      for (int i = 0; i < 15; i++)
      {
        Console.WriteLine(b1.Server);
      }

      // Wait for user
      Console.Read();    
    }
  }

  // Singleton

  sealed class LoadBalancer
  {
    // Static members are lazily initialized.
    // .NET guarantees thread safety for static initialization
    private static readonly LoadBalancer instance =
      new LoadBalancer();

    private ArrayList servers = new ArrayList();
    private Random random = new Random();

    // Note: constructor is private.
    private LoadBalancer()
    {
      // List of available servers
      servers.Add("ServerI");
      servers.Add("ServerII");
      servers.Add("ServerIII");
      servers.Add("ServerIV");
      servers.Add("ServerV");
    }

    public static LoadBalancer GetLoadBalancer()
    {
      return instance;
    }

    // Simple, but effective load balancer
    public string Server
    {
      get
      {
        int r = random.Next(servers.Count);
        return servers[r].ToString();
      }
    }
  }
}

You can find this pattern at dotfactory.com.

Answer 23

In desktop apps (I know, only us dinosaurs write these anymore!) they are essential for getting relatively unchanging global application settings - the user language, path to help files, user preferences etc which would otherwise have to propogate into every class and every dialog.

Edit - of course these should be read-only !

Answer 24

Another implementation

class Singleton
{
public:
    static Singleton& Instance()
    {
        // lazy initialize
        if (instance_ == NULL) instance_ = new Singleton();

        return *instance_;
    }

private:
    Singleton() {};

    static Singleton *instance_;
};