UML

Specialization “is-a”(C++)

• One class is a “version” of another class
• Typically subclassing or subtyping relationship
• If B is-a A, then we should be able to substitute something of type B, wherever we expect something of type A.

How is "is-a" implemented?

In C++, achieve specialization via public Inheritance.

class Book {
	string title, author;
	int length;
	protected:
		int getLength() const {
			return length;
		}
	public:
		Book(string title, string author, int length): title{title}, author{author}, length{length} {}
}
		virtual bool isHeavy() const {
			return length > 200;
		}
}

class Text: public Book {
	string topic;
	public:
		Text(string title, string author, int legnth, string topic): Book{title, author, length}, topic {topic} {}
	bool isHeavy() const override {
		return getLength() > 500;
	}
}

Why Book{title, author, length}? Recall the 4 steps of object construction (from Constructor)

1. Space is allocated
2. Call the superclass constructor
3. Initialize fields via a member initialization list (MIL)
4. Run the constructor body

We cannot set title, author, length because they are private to book. It will use Books default constructor, won’t compile if default constructor does not exist.

Related

• has-a
• owns-a