The Open-Closed Principle

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

How?

• Abstraction is the Key.
  Give client an interface instead of an implementation.
• Strategic Closure
  It should be clear that no significant program can be 100% closed. The designer must choose the kinds of changes against which to close his design.
• Using a “Data Driven” Approach to Achieve Closure.
  The change part could be considered “data”, not program. So, “data” could be put outside the program.

Heuristics and Conventions based on the open-closed principle

• Make all Member Variables Private.
• No Global Variables – Ever.
• RTTI is Dangerous. （run time type identification）