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Applicability
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There must be
exactly one instance of a class and it must be accessible
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to multiple
clients
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The sole
instance should be extensible by subclassing, and clients
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should be able
to use subclass without modifying code
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Consequences
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Controlled
access to sole instance
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Reduced name
space (over global variable)
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Extendable
implementation
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Permits a
variable number of instances (easy to change if dont want
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singleton)
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More flexible
than static member functions allows subclassing and
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easy to change
to multiple number of instances
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