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Can the Singleton pattern [Gamma95] be turned into a reusable component? To help answer this question, we have reviewed existing implementations and tried to improve them. This article explains the difficulties of having a single-instance class in Eiffel and proposes language extensions, namely once creation procedures, which would be satisfactory in most(More)
Goals of expressiveness and flexibility in typed object-oriented programming suggest a " covariant " type policy, where routine redefinitions can change the types of both arguments and results in the same direction as the inheritance hierarchy. Unfortunately, a careless covariant policy, when combined with polymorphism, genericity and dynamic binding —(More)
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