SOLID Design Principles

Open-Closed Principle (OCP)

The Open-Closed Principle states that classes should be open to extension, but closed to modification.

Changes to a program should occur primarily by writing new code, not changing code that already exists.

Example

• What if we wanted our Hero to use a different type of weapon? Bow or MagicWand?
• Requires us changing all the places we referenced the Sword class - lots of modifications, not much extension.

Fix is to use Strategy Pattern:

The Open-Closed Principle is closely related to the concept of Inheritance and Virtual Functions.

For example:

int countHeavy(const vector<Book*>& v) {
	int count = 0;
	for (auto p: v) {
		if (p->isHeavy()) ++count;
	}
	return count;
}

This function is open to extension: we can add more functionality by defining new types of Books, and closed to modification: countHeavy never needs to change now that it has been written.

Compare this to WhatIsIt (see dynamic_cast)

• not open to extension: cannot get new behavior without modifying the source code
• not closed to modification either

OCP is an Ideal

Open/closed principle is an ideal - writing a program will require modifications at some point.

Plan for things that are most likely to change, account for those.

Next

• Liskov Substitution Principle