Singleton

[Rajesh1978]

Hi,
I can achieve the same functionality as of singleton by creating a static object.
So what is the advantage of using the singleton class or design pattern.
Plz correct me if I am wrong.
Thanks

[Richard.J]

a static object is one possibility of implementing a Singleton. Another possibility would be to create a new object in the free store on the first access to the Singleton.

[ixSci]

I can achieve the same functionality as of singleton by creating a static object.

No, you can't. Singlteon means you have only one object of a such type with all the advantages of it whereas a static object is the static object only. It is not a replacement for the singleton. You should read more about the singleton http://sourcemaking.com/design_patterns/singleton

[Rajesh1978]

Why,what extra advantage is there in Singleton

[ixSci]

Rajesh1978, Did you read the text on the link i've posted above?

[Speedo]

The point of the singleton pattern is supposedly to enforce only one instance of a particular class, but in practice it seems it's mostly used to wrap global state in a more attractive package. I'm generally not a fan of it.

[Lindley]

It's essentially the same advantage over global state that full class objects have over data-only structures: It comes complete with methods to refine and manipulate that state, all wrapped up in a tidy package.

Could you achieve the same thing without a singleton, just using free functions? Absolutely. But that might not give you as useful of an abstraction about what's happening.

[kempofighter]

I think that I have seen what the OP is talking about. I have seen a pattern where all data members of the class are static. Even though you can create multiple instances, every instances reads and writes to the same class attributes, which are all static. In this case, the constructors are still public as are the member functions. Member functions are also non-static. Of course you still have to implement mutexes to protect the data in the case of multiple threads creating other instances. In some cases I have seen this done where all of the member functions are also static. In that case no instance is ever created.

The main problem I have with this is that it isn't a singleton by definition since there are multiple instances but the design realizes similar goals. It actually works and I have had a hard time criticizing it. The main problem is that static member functions can't be virtual so it is impossible to extend the pattern using inheritance. On the other hand, in most cases that I have seen inheritance wasn't really needed so the designer didn't care. I'm not sure if this is what the OP is talking about. In the past I've seen these and disliked them but didn't have a real good argument for insisting that they be designed into real singleton's at the time.

I've come to dislike singleton honestly. I think it is an overused pattern and for such a simple concept there seem to be a lot of complex issues associated with guaranteeing that only one instance can ever exist. There are plenty of other options for providing access to objects without using the singleton pattern and in a lot of cases there really isn't any danger of multiple objects being created unecessarily if factories and delegators are coded correctly.

[kempofighter]

a static object is one possibility of implementing a Singleton. Another possibility would be to create a new object in the free store on the first access to the Singleton.

To be honest, I am not sure what is meant by a static object. If the class declaration contains a public constructor then it isn't a singleton. The purpose of the singleton pattern is to "enforce" one and only instance of a type while providing a convenient way of accessing that one and only instance. If you have a type with a public constructor and you just create a static instance, there is nothing being enforced that would allow you to call that pattern a real implementation of singleton.

[Speedo]

It's essentially the same advantage over global state that full class objects have over data-only structures: It comes complete with methods to refine and manipulate that state, all wrapped up in a tidy package.

I don't disagree with that aspect of it. What get me is, you poll programmers about global state, and most will say it's a bad thing. Poll them again about singletons, and most tend to say that they're good. What's the difference?

There are certainly ways to get the encapsulation aspect and the single-instance aspect without having the global state aspect.

[laserlight]

I think that I have seen what the OP is talking about. I have seen a pattern where all data members of the class are static. Even though you can create multiple instances, every instances reads and writes to the same class attributes, which are all static.

Heheh, I decided to check my copy of the GoF book, and yeah, that pattern is listed as one of the things that the singleton pattern can replace (and conversely, the singleton pattern is one of those things that that pattern can replace).

If you have a type with a public constructor and you just create a static instance, there is nothing being enforced that would allow you to call that pattern a real implementation of singleton.

Yes, though I'd say that you can still refer to that lone object as a singleton, literally speaking.

What get me is, you poll programmers about global state, and most will say it's a bad thing. Poll them again about singletons, and most tend to say that they're good. What's the difference?

I would say that "most tend to say that they're good" is an exaggeration, but my take is that the difference is because in those specific cases where one really needs to enforce the singleton property and a single point of access makes sense, the singleton pattern is tolerable (but woe to you if your requirements change too much!), whereas ordinary global variables give you global state with no mitigating factors (other than allowing you to be careless about design).

[Lindley]

The primary problem with global data, as I see it, is that it's uncontrolled and makes thread-safety hard.

Singletons solve the second problem well enough. As for the first, that just requires careful design. Using a singleton helps to enforce the fact that you aren't just arbitrarily changing data; you're altering state which is designed and expected to be queryable in the first place.

[toraj58]

a class with all static methods is not singleton it is a nothington that lacks polymorphism and a built-in mechanism for construction and destruction and hence you can make a hierarchy of classes when using nothington. in another point of view when using nothingnot there is no object orientation at all and in fact your class is a collection of some functions that is not good as perspective of design. this issue that constructor and destructor are called automatically when using singleton is a very important issue rather than using nothington.

[Speedo]

I would say that "most tend to say that they're good" is an exaggeration, but my take is that the difference is because in those specific cases where one really needs to enforce the singleton property and a single point of access makes sense, the singleton pattern is tolerable (but woe to you if your requirements change too much!), whereas ordinary global variables give you global state with no mitigating factors (other than allowing you to be careless about design).

Of course that's true in theory, but in practice it seems that singletons are a favorite point of abuse.

The primary problem with global data, as I see it, is that it's uncontrolled and makes thread-safety hard.

It also hides hides dependancies, which can make testing and reusing code a royal PITA.

[Zaccheus@Work]

What get me is, you poll programmers about global state, and most will say it's a bad thing. Poll them again about singletons, and most tend to say that they're good. What's the difference?

One difference is that 'global data' usually was lumped together in one big interdependent mess, while singletons can be used on their own. If a class uses a number of singletons, those singletons can be reused along with that class.