The Open-Closed Principle

This is the first of my Engineering Notebook columns for The C++ Report. The articles that will appear in this column will focus on the use of C++ and OOD, and will address issues of software engineering. I will strive for articles that are pragmatic and directly useful to the software engineer in the trenches. In these articles I will make use of Booch's notation for documenting object oriented designs. The sidebar provides a brief lexicon of Booch's notation.

There are many heuristics associated with object oriented design. For example, "all member variables should be private", or "global variables should be avoided", or "using run time type identification (RTTI) is dangerous". What is the source of these heuristics? What makes them true? Are they always true? This column investigates the design principle that underlies these heuristics—the open-closed principle.

As Ivar Jacobson said: "All systems change during their life cycles. This must be borne in mind when developing systems expected to last longer than the first version." How can we create designs that are stable in the face of change and that will last longer than the first version? Bertrand Meyer gave us guidance as long ago as 1988 when he coined the now famous open-closed principle. To paraphrase him:

SOFTWARE ENTITIES (CLASSES, MODULES, FUNCTIONS, ETC.) SHOULD BE OPEN FOR EXTENSION, BUT CLOSED FOR MODIFICATION.