Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification.
Source: Robert C. Martin. Agile Principle Patterns and Practices [PPP] from Meyer97.
Design your classes so that you can change their behavior without changing their code.
Source: symfonycasts
OCP advises us to refactor the system so that further changes to dependent modules will not cause more modifications.
Modules that conform to OCP have two primary attributes:
- They are open for extension - This means that the behavior of the module can be extended.
- They are closed for modification - Extending the behavior of a module does not result in changes to the source
You should be able to extend a classes behavior, without modifying it. OR You should be able to change what a class does, without changing its code.
Real example: an open source library (in /vendor folder).
How is it possible that the behaviors of a module can be modified without changing its source code? Without changing the module, how can we change what a module does? The answer is abstraction.
The primary mechanisms behind the Open/Closed Principle are abstraction and polymorphism. One of the key mechanisms that supports abstraction and polymorphism is inheritance. It is by using inheritance that we can create
derived classesthat implement abstract methods in base classes.
