CSC/ECE 517 Fall 2012/ch2b 2w69 as

The Open/Closed principle

Introduction

In object-oriented programming the open/closed principle states,

Software entities (Classes, Modules, Functions, etc.) should be open for extension, but closed for modification

It sounds like a contradiction in terms, but it's not. All it means is that you should structure an application so that you can add new functionality with minimal modification to existing code

All systems change during their life cycles. One should keep this in mind when designing systems that are expected to last longer than the first version.”

Motivation

All software systems are subject to change. Thus designing a system which is stable is a very crucial task. When a single change to a program results in a cascade of changes to dependent modules, that program exhibits the undesirable attributes that we have come to associate with “bad” design. The program becomes fragile, rigid, unpredictable and unreusable. The open-closed principle attacks this in a very straightforward way. Open-closed principle states that the code should be designed in such a way that it should not change. Whenever requirements change, the existing code should be extended by adding new code and leave the old working code intact.

Most of the times it easier to write all new code rather thank making changes to existing code. Modifying old code adds the risk of breaking existing functionality. With new code you generally only have to test the new functionality. When you modify old code you have to both test your changes and then perform a set of regression tests to make sure you did not break any of the existing code.

See Also

• Single-Responsibility Principle

References

[1] Open/closed principle

[2] "The Open-Closed Principle", C++ Report, January 1996"

[3] "Patterns in Practice: The Open Closed Principle"