An implementation of the singleton pattern must:

1. ensure that only one instance of the singleton class ever exists
2. class creates its own singleton pattern instance
3. provide global access to that instance.

Typically, this is done by:

1. declaring all constructors of the class to be private
2. providing a static method that returns a reference to the instance.

Java

//Lazy loading(not recommend)
public class Singleton {  
    private static Singleton instance;  
    private Singleton (){}  
    public static synchronized Singleton getInstance() {  
    if (instance == null) {  
        instance = new Singleton();  
    }  
    return instance;  
    }  
}
//Non-lazy loading(recommend)
public final class Singleton {
    // create instance
    private static final Singleton INSTANCE = new Singleton();
    // set constructor as null, so that no instance will be created 
    private Singleton() {}
    // return this singletion instance 
    public static Singleton getInstance() {
        return INSTANCE;
    }
}
//enum(recommend)
public enum Singleton {  
    INSTANCE;  
    public void dummy() {  
    }  
}
// how to use
 SingletonEnum singleton = SingletonEnum.INSTANCE;
 singleton.dummy();

Python

// Lazy loading
class Singleton(object):
    def __new__(cls, *args, **kw):
        if not hasattr(cls, '_instance'):
            orig = super(Singleton, cls)
            cls._instance = orig.__new__(cls, *args, **kw)
        return cls._instance

class MyClass(Singleton):
    a = 1

//meta class
class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super(Singleton, cls).__call__(*args, **kwargs)
        return cls._instances[cls]

class MyClass(metaclass=Singleton):
     pass

//decorator
from functools import wraps

def singleton(cls):
    instances = {}
    @wraps(cls)
    def getinstance(*args, **kw):
        if cls not in instances:
            instances[cls] = cls(*args, **kw)
        return instances[cls]
    return getinstance

@singleton
class MyClass(object):
    a = 1

C++

#include <iostream>

class Singleton
{
    private:
        /* Here will be the instance stored. */
        static Singleton* instance;

        /* Private constructor to prevent instancing. */
        Singleton();

    public:
        /* Static access method. */
        static Singleton* getInstance();
};

/* Null, because instance will be initialized on demand. */
Singleton* Singleton::instance = 0;

Singleton* Singleton::getInstance()
{
    if (instance == 0)
    {
        instance = new Singleton();
    }

    return instance;
}

Singleton::Singleton()
{}

int main()
{
    //new Singleton(); // Won't work
    Singleton* s = Singleton::getInstance(); // Ok
    Singleton* r = Singleton::getInstance();

    /* The addresses will be the same. */
    std::cout << s << std::endl;
    std::cout << r << std::endl;
}

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