Chapter33 - 33 O-O programming and Ada In the...

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33 O-O programming and Ada I n the nineteen-seventies, advances in programming methodology brought about a new generation of languages combining the control structures of Algol 60 and the data structuring constructs of Algol W and Pascal with better system structuring facilities and support for information hiding. Although their precise traits differ, these languages share a common spirit and may be collectively called the encapsulation languages . (They are also known in the literature as “object-based”, a terminology that will be discussed in the next chapter.) Although a complete list of encapsulation languages would be long, only a few have developed a sizable user community. Five deserve particular attention: Modula-2 , a successor to Pascal designed at the Swiss Federal Institute of Technology by Niklaus Wirth, creator of Algol W, Pascal itself and (later) Oberon; CLU , developed at MIT under the direction of Barbara Liskov, which comes closest to realizing object-oriented concepts but lacks inheritance; Mesa , a Xerox effort with particular emphasis on describing inter- module relationships of large systems; Alphard , by Mary Shaw, William Wulf and Ralph London of Carnegie-Mellon University, which included an assertion mechanism; and Ada . We will limit our study of how to approach O-O techniques in encapsulation languages to Ada, which, besides having attracted the most attention, is also the most complete (and complex) of these languages, embodying in some form most of the features found in the others. Modula-2, for example, does not offer genericity or overloading. 33.1 A BIT OF CONTEXT Ada was a response to a crisis perceived in the mid-seventies by the software policy- makers of the US Department of Defense (DoD). They noted in particular that the various branches of the military were using more than 450 programming languages, many of them technically obsolete, gravely hampering contractor management, programmer training, technical progress, software quality and cost control. Bearing in mind the successful precedent of COBOL (the result, in the late fifties, of a DoD call for a COmmon Business-Oriented Language), they put out successive versions of a Request For Proposals for a modern software engineering language capable of supporting embedded real-time applications. A first winnowing out of the several dozen initial responses led to four candidate designs, sealed and color-coded for fairness. The field was narrowed down to two, finally leading in 1979 to the selection of the Green language designed by Jean D. Ichbiah and his group at CII-Honeywell Bull in France
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O-O PROGRAMMING AND ADA § 33.1 1080 (today’s Bull). Following a few years’ experience with the first industrial implementations, the language was revised and made into an ANSI standard in 1983. Ada (as Green was renamed) began a new era in language design. Never before had
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This note was uploaded on 10/02/2009 for the course CS 4376 taught by Professor Christeansan during the Spring '09 term at Dallas Colleges.

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Chapter33 - 33 O-O programming and Ada In the...

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