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This paper presents one view of what ‘‘object oriented’’ ought to mean in the context of a general purposeprogramming language. §2 Distinguishes ‘‘object-oriented programming’’ and ‘‘data abstraction’’ from each other and from other styles of programming and presents the mechanisms that are essential for supporting the vari-ous styles of programming. §3 Presents features needed to make data abstraction effective.§4 Discusses facilities needed to support object-oriented programming.§5 Presents some limits imposed on data abstraction and object-oriented programming by traditional hardware architectures and operating systems. Examples will be presented in C++. The reason for this is partly to introduce C++ and partly because C++ isone of the few languages that supports both data abstraction and object-oriented programming in addition totraditional programming techniques. Issues of concurrency and of hardware support for specific higher-level language constructs are ignored in this paper.

Programming Paradigms Object-oriented programming is a technique for programming – a paradigm for writing ‘‘good’’ pro- grams for a set of problems. If the term ‘‘object-oriented programming language’’ means anything it mustmean a programming language that provides mechanisms that support the object-oriented style of program-ming well. There is an important distinction here. A language is said to support a style of programming if it pro- vides facilities that makes it convenient (reasonably easy, safe, and efficient) to use that style. A languagedoes not support a technique if it takes exceptional effort or exceptional skill to write such programs; itmerely enables the technique to be used. For example, you can write structured programs in Fortran, writetype-secure programs in C, and use data abstraction in Modula-2, but it is unnecessarily hard to do becausethese languages do not support those techniques. Support for a paradigm comes not only in the obvious form of language facilities that allow direct use of the paradigm, but also in the more subtle form of compile-time and/or run-time checks against uninten-tional deviation from the paradigm. Type checking is the most obvious example of this; ambiguity detec-tion and run-time checks can be used to extend linguistic support for paradigms. Extra-linguistic facilitiessuch as standard libraries and programming environments can also provide significant support for para-digms.