In software design, laziness is a virtue: it's better to reuse than to redo. Design patterns are a good illustration. Patterns, a major advance in software architecture, provide a common vocabulary and a widely known catalog of design solutions addressing frequently encountered situations. But they do not support reuse, which assumes components: off-the-shelf modules ready to be integrated into an application through the sole knowledge of a program interface (API). Is it possible to go beyond patterns by componentizing them-turning them into components? We have built a component library which answers this question positively for a large subset of the bestknown design patterns. Here we summarize these results and analyze the componentization process through the example of an important pattern, Visitor, showing how to take advantage of object-oriented language mechanisms to replace the design work of using a pattern by mere "ready-to-wear" reuse through an API. The reusable solution is not only easier to use but more general than the pattern, removing its known limitations; performance analysis on a large industrial application shows that the approach is realistic and scales up gracefully.
Reliabiliiy is a key concern in the current state of sofmare industry. Producing software without defects is still very difJicult. Therefore it is crucial to benefit from high-level language mechanisms such as assertions. Nevertheless contracts are mostly a specificity of the Eiffel language. This observation convinced ISE to build a new product, the Contract Wizard, which aims at providing .NET developers with the abiliiy to add contracts to a .NET assembly independently from the .NET language it was initially written in.This paper first gives an overview of the existing implementation of Design by Contract in various programming languages. It then explains how the new ISE Contract Wizard works by providing a guided tour of the product.
Can Design Patterns be turned into reusable components? To help answer this question, we have performed a systematic study of the standard design patterns. One of the most interesting is Abstract Factory, for which we were indeed able to build a reusable component fulfilling the same needs as the original pattern. This article presents the component's design and its lessons for the general issue of pattern componentization. Patterns and componentsIn hardly more than a decade, Design Patterns have established themselves as a major resource for effective software design. "Each pattern describes a problem which occurs over and over again in our environment and then describes the core of the solution to this problem" [17]. Thanks to the published catalogs of such solutions, starting with [17] itself, software designers can benefit from the wisdom and experience of their predecessors. The idea that we should avoid reinventing the wheel in software construction predates patterns by a long time; reuse is a running theme in standard discussions of software engineering. The idea of reuse is to provide software components covering standard needs and make them available through a standard API (Abstract Program Interface 1 ) to any program that experiences the corresponding needs.For all the benefits of design patterns, it is hard to ignore that the idea as usually expressed runs contrary to decades of advances in reuse, which have profoundly improved the state of software development. Pattern advocates themselves often sound skeptical about reuse. The preceding citation was actually truncated: the full sentence states that a pattern describes the core of the solution to the problem "in such a way that you can use this solution a million times over, without ever doing it the same way twice". The pattern literature frequently confirms this view that patterns are not reusable components; for example [20]: "Patterns are not, by definition, fully formalized descriptions. They can't appear as a deliverable".Why not? It is hard to accept that patterns, however useful, should force us to step back to pre-reuse times, when sorting an array required finding a solution outline in an algorithms textbook and then adapting it to your needs. Today we use library routines for such tasks. Patterns should provide similarly reusable solution at a higher level of granularity. Inexplicably, the pattern lit-1 The original expansion of "API", "Application Program Interface", apparently going back to old IBM software, is no longer meaningful. The acronym itself remains well-understood and relevant. We expand it as "Abstract Program Interface", which captures the idea precisely.
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 cases, or frozen classes. HOW TO GET A SINGLETON IN EIFFEL? 76 J OURNAL OF OBJECT TECHNOLOGY V OL. 3, NO. 4 Note that once routines are executed once in the whole system not once per class.
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