Diffrences between Singleton and global variable [duplicate]

3

I know that singleton allow only one instance of an object. Each method declared in the singleton will only operate on this object. I was wondering why to not simply declare a global object which will achieve the same goal?

I am certainly forgetting something. If singletons exist there must be specific uses or help to realize specific mechanisms.

For instance:

class Singleton
{
public:
    static Singleton& Instance()
    {
        static Singleton sg;
        return sg;
    }

    void function();
};

would be the same as:

class NotSingleton
{
public:
    NotSingleon();
    ~NotSingleton()
    void function();
};
NotSingleton nsg;

However, nothing prevent me to use more than one instance of NotSingleton

---

• Don't remember SO user but someone told me that it safer in a multi-threaded context. - Papipone
• Also, if use global var, when the program starts, this object has to be initialized. If you use Singleton, then you can create it until the time that you are gonna use it for the first time. In the extreme situation, you don't have to use it until the program ends, so you don't have to create the Singleton at all. So using Singleton can let you create it only when you need it. Second, in the multi-thread program, if you just access the global var directly, it may cause problem which is hard to debug. - Jiahao

Class Singleton{
public static Singleton object==null;



  public void singleton(){
    if(object==null)
       object = new Singleton();
    return object;
    }

    }