Differences between Singleton and global variable [duplicate]

Question

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

Answer 1

Singleton is used when we do not want to create more than one object. Singleton class ensures that not more than one object is created. However, having a global object doesn't ensure this.

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

This class will not create more than one object. This is the purpose of the Singleton class.