Beware singletons

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(New page: The Singleton design pattern has several apparent advantages. The programmer can be sure that only one instance will exist. That instance is static. This saves the mess of having to pa...)
 
 
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The [[Singleton]] design pattern has several apparent advantages. The programmer can be sure that only one instance will exist. That instance is static. This saves the mess of having to pass around many references to the object. Also, [[lazy initialization]] is an inherent part of the pattern.
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:''Do not turn objects of a class into derived classes of the class. Be very suspicious of any derived class for which there is only one instance.'' --Riel's Heuristic 5.15, [[Arthur Riel 1996]]
  
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However, these advantages can tempt the programmer to make unwise design decisions.
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For the purposes of this maxim, "singleton" refers not specifically to the [[Singleton]] design pattern, but to any class of which there is only one instance. The risks of using the Singleton pattern are discussed on the [[Singleton]] page.
  
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* The assumption that only one instance will ever be required can be shortsighted. For example, one may have a settings object as a singleton. But if one later wishes to extend the software to run multiple objects each with their own settings, much painstaking refactoring will be required.
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[[Category:Riel's heuristics]]
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* If you're using the static nature of the pattern to avoid passing around many references, it's a sign you might be overusing that class, and not following the maxim of [[Tell, don't ask]].
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Latest revision as of 00:13, 21 July 2009

Do not turn objects of a class into derived classes of the class. Be very suspicious of any derived class for which there is only one instance. --Riel's Heuristic 5.15, Arthur Riel 1996

For the purposes of this maxim, "singleton" refers not specifically to the Singleton design pattern, but to any class of which there is only one instance. The risks of using the Singleton pattern are discussed on the Singleton page.

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