How can one make two instances of this Singleton Class?

Question

This code should be enough for restricting only one instance of singleton class. How can we make two instances of this object?

class Singleton
{
private:
    Singleton()
    {}
    ~Singleton()
    {}
    static Singleton * ptr;
public:
    static Singleton * CreateObject()
    {
        if (!ptr)
            ptr = new Singleton;
        return ptr;
    }
    static void freeObject()
    {
        if (ptr)
        {
            delete ptr;
            ptr = 0;
        }
    }
};
Singleton * Singleton::ptr = 0;

Sounds to me like you need to rethink your design (assuming this isn't an academic exercise). Perhaps share with us why you'd need 2 instances of something that is designed to only need 1? — Steve, Nov 14 '17 at 09:09
the whole purpose (which in itself is questionable) of a singleton is to allow only a single instance. Why do you want to have two of them? Can you change the class? — 463035818_is_not_an_ai, Nov 14 '17 at 09:11
@YSC The question doesn't make more sense even if that were the case — Passer By, Nov 14 '17 at 13:03

score 1 · Answer 1 · answered Nov 14 '17 at 09:31

The singleton pattern is about not having two instances of a class.

The easiest way to get rid of that constraint... is to remove the singleton pattern:

class Singleton
{
};

Singleton one = new Singleton();
Singleton two = new Singleton();

As a side-effect, you just made your code cleaner, too, as Singleton is considered an anti-pattern by many.

Federico · Answer 2 · 2017-11-14T10:26:16.363

Well, another possibility not yet exposed is the cuncurrency. That singleton pattern is not secure enough. If you have two (or more) different threads calling simultaneously the CreateObject() method they could easily get different instances of the object.

You should add a mutex to secure that pattern. Or move to another pattern like this one:

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

YSC · Answer 3 · 2017-11-14T10:34:01.817

-4

This code should be enough for restricting only one instance of singleton class.

It is not.

How can we make two instances of this object?

With the singleton implementation you gave in your question, it is fairly easy to create a second instance:

Singleton* s1 = Singleton::CreateObject();
Singleton* s2 = new Singleton(*s1);

And enjoy memory leaks.

To fix your singleton, you should respect the rule of five (if you define at least one constructor, define all five of them). Or you could use something else than a singleton.

edited Nov 14 '17 at 10:34

answered Nov 14 '17 at 09:34

YSC

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If you doubt me: please see [demo on coliru](http://coliru.stacked-crooked.com/a/c8e8c4efe8671a5c). – YSC Nov 14 '17 at 10:35
2

I didn't downvote. I think the answer strictly correct, but ill-advised. – Passer By Nov 14 '17 at 13:05

How can one make two instances of this Singleton Class?

3 Answers3