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In a comment Buy ketoconazole cream without prescription, a while ago, David wanted to know why are Singletons the root of all evil. Cheap ketoconazole cream from canada, Well, that's just because they are, cheapest generic ketoconazole cream online. Cheap ketoconazole cream in usa, Ok, it is not that easy, order ketoconazole cream overnight delivery. Buy ketoconazole cream generic, Nor that black-and-white, either, buying generic ketoconazole cream. Find ketoconazole cream, Let's see if I can articulate some of the reasons why Singletons are bad for you and for your code.

From all the varieties of pattern abuse out there, singletonitis is the most irritating one because of two simple reasons:

Singleton is one of the patterns with the worst side effects.

Singleton abuse is one of the most widespread varieties of pattern abuse.

In 99% of Singleton occurrences, the Singleton pattern was unnecessarily introduced with one of the following excuses:

Some object needs to access a resource that isn't available in that abstraction layer, buy ketoconazole cream without prescription. The right solution to this problem is to redesign, tablet ketoconazole cream. Order discount ketoconazole cream, Solving bad design by throwing a pattern at it just makes things worse.

As a glorified global variable. Global variables can be avoided most of the time, ketoconazole cream pills, Ketoconazole cream internet, and they should be avoided wherever possible. See previous point on solving bad design by layering patterns on top.

Premature optimization ("we shouldn't instantiate this more than once, purchase ketoconazole cream overnight delivery, Buy ketoconazole cream lowest price, or it will bring down performance; let's singletonize it!"). Thousands of wise words have been written on the evils of premature optimization, ketoconazole cream in australia, Buy ketoconazole cream online without prescription, so I won't go into that here.

So it all boils down to the following: in most cases, programmers introduce Singletons either as a misguided attempt to improve a bad design, order discount ketoconazole cream online, Ketoconazole cream order, or as a means for premature optimization.

Singletons not only have the noxious effect of obscuring how objects collaborate, buy ketoconazole cream in canada, Cheapest ketoconazole cream price, but they also hamper dependency injection, which in turn makes testing and refactoring extremely difficult, ketoconazole cream online stores. Order ketoconazole cream, Thus, Singletons lead you into a spiral of unmaintainability, cheap ketoconazole cream. Buy ketoconazole cream no prescription required, To think that they were unnecessary in the first place. Compare ketoconazole cream prices online. Cheap ketoconazole cream pharmacy. Order ketoconazole cream without prescription. Ketoconazole cream us. Lowest price for ketoconazole cream. Cheapest generic ketoconazole cream. Cheap ketoconazole cream tablet. Cheap ketoconazole cream no prescription. Cheapest ketoconazole cream online. Cheap ketoconazole cream from uk. Cheap ketoconazole cream without prescription. Cheap ketoconazole cream in canada. Order cheap ketoconazole cream. Generic ketoconazole cream. Canadian ketoconazole cream. Buy ketoconazole cream from us. Ketoconazole cream from canada.

February 5, 2008 Filed under: programming

http://fer-martin.com Fer Martin

Ana,

I love when you come back techie I think this article can bring a hell lot of conversation about it; some technical, but some philosophical as well.

In my opinion, Singleton is not the root of evil, that is an exemplification of the misusage of any technology after all.

Singleton can be a great tool to avoid having multiple copies of an object that it must be unique in the whole application.

For example, in the last application I wrote, we had to create a Cache Manager. This class could not be layered on top, and I needed a “singleton” access to the Cache from all objects. They should not care were a single instance of the Cache exists or not, they just want to use it, but it should be unique.

I agree that something that usually falls into the “dark side of bad habits” should be advertised as “evil” (specially for children!), but as you said… not everything is white or black

Cheers!!
Fer
http://fer-martin.com Fer Martin

Ana,

I love when you come back techie I think this article can bring a hell lot of conversation about it; some technical, but some philosophical as well.

In my opinion, Singleton is not the root of evil, that is an exemplification of the misusage of any technology after all.

Singleton can be a great tool to avoid having multiple copies of an object that it must be unique in the whole application.

For example, in the last application I wrote, we had to create a Cache Manager. This class could not be layered on top, and I needed a “singleton” access to the Cache from all objects. They should not care were a single instance of the Cache exists or not, they just want to use it, but it should be unique.

I agree that something that usually falls into the “dark side of bad habits” should be advertised as “evil” (specially for children!), but as you said… not everything is white or black

Cheers!!
Fer
Mario Tanev

Fer,

If your objects wanted to use a global cache, why not have the cache be an argument to the instantiations of all those objects? Is it because someone could still choose to instantiate another version of the cache and pass it to one of the objects? Why are the objects so dependent on having a single version of the cache, is there some special interaction where one object writes and another one reads from the cache? Why couldn’t the Cache Manager be layered on top?

I was not convinced in the need for singletons in that instance.
Mario Tanev

Fer,

If your objects wanted to use a global cache, why not have the cache be an argument to the instantiations of all those objects? Is it because someone could still choose to instantiate another version of the cache and pass it to one of the objects? Why are the objects so dependent on having a single version of the cache, is there some special interaction where one object writes and another one reads from the cache? Why couldn’t the Cache Manager be layered on top?

I was not convinced in the need for singletons in that instance.
ana

Fer, I have to agree with Mario here: I see no reasons in your example why the cache manager had to be implemented as a singleton. An instance and dependency injection probably would have sufficed. Instead, you constrained yourself to the use of a singleton, probably unnecessarily.
ana

Fer, I have to agree with Mario here: I see no reasons in your example why the cache manager had to be implemented as a singleton. An instance and dependency injection probably would have sufficed. Instead, you constrained yourself to the use of a singleton, probably unnecessarily.

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