C++ Singleton design pattern

You could avoid memory allocation. There are many variants, all having problems in case of multithreading environment.

I prefer this kind of implementation (actually, it is not correctly said I prefer, because I avoid singletons as much as possible):

class Singleton
{
private:
   Singleton();

public:
   static Singleton& instance()
   {
      static Singleton INSTANCE;
      return INSTANCE;
   }
};

It has no dynamic memory allocation.


Being a Singleton, you usually do not want it to be destructed.

It will get torn down and deallocated when the program terminates, which is the normal, desired behavior for a singleton. If you want to be able to explicitly clean it, it's fairly easy to add a static method to the class that allows you to restore it to a clean state, and have it reallocate next time it's used, but that's outside of the scope of a "classic" singleton.


In 2008 I provided a C++98 implementation of the Singleton design pattern that is lazy-evaluated, guaranteed-destruction, not-technically-thread-safe:
Can any one provide me a sample of Singleton in c++?

Here is an updated C++11 implementation of the Singleton design pattern that is lazy-evaluated, correctly-destroyed, and thread-safe.

class S
{
    public:
        static S& getInstance()
        {
            static S    instance; // Guaranteed to be destroyed.
                                  // Instantiated on first use.
            return instance;
        }
    private:
        S() {}                    // Constructor? (the {} brackets) are needed here.

        // C++ 03
        // ========
        // Don't forget to declare these two. You want to make sure they
        // are inaccessible(especially from outside), otherwise, you may accidentally get copies of
        // your singleton appearing.
        S(S const&);              // Don't Implement
        void operator=(S const&); // Don't implement

        // C++ 11
        // =======
        // We can use the better technique of deleting the methods
        // we don't want.
    public:
        S(S const&)               = delete;
        void operator=(S const&)  = delete;

        // Note: Scott Meyers mentions in his Effective Modern
        //       C++ book, that deleted functions should generally
        //       be public as it results in better error messages
        //       due to the compilers behavior to check accessibility
        //       before deleted status
};

See this article about when to use a singleton: (not often)
Singleton: How should it be used

See this two article about initialization order and how to cope:
Static variables initialisation order
Finding C++ static initialization order problems

See this article describing lifetimes:
What is the lifetime of a static variable in a C++ function?

See this article that discusses some threading implications to singletons:
Singleton instance declared as static variable of GetInstance method, is it thread-safe?

See this article that explains why double checked locking will not work on C++:
What are all the common undefined behaviours that a C++ programmer should know about?
Dr Dobbs: C++ and The Perils of Double-Checked Locking: Part I