CSC 143 O 1 CSC 143 Inheritance and Object Oriented Design

CSC 143 O 2 Public Inheritance means "isa"  If class D publicly inherits from class B, every object of type D is also an object of type B (not vice-versa) class Person{…}; class Student: public Person {…}; void dance(const Person& p); // anyone can dance void study(const Student& s); // only students study Person p; Student s; // p is a Person, s a Student dance(p); // fine, p is a Person dance(s); // fine, s is a Student, therefore isa Person study(s); // OK study(p); // error! p is not a Student

CSC 143 O 3 IsA Design (1)  Trouble: penguins can fly!  C++ is stricter than the English language. In C++, "Birds can fly" means "All birds can fly"  Solution? class Bird{ public: virtual void fly() // birds can fly … }; class Penguin: public Bird{ // penguins are birds … };

CSC 143 O 4 IsA Design (2)  Refine the hierarchy NonFlyingBird Penguin Bird (no fly function) FlyingBird (fly function declared here)

CSC 143 O 5 IsA Design (3)  Could also do: Penguins can fly, but it is an error for them to try to do so. How? class Bird {… //no fly function}; class FlyingBird : public Bird{ virtual void fly(); … }; class NonFlyingBird{ … //no fly function}; class Penguin: public NonFlyingBird{ … // no fly function};

CSC 143 O 6 Interface  Implementation  Interface = function declaration  Implementation = code for the function  Member function interfaces are always inherited. If a function applies to a class, it must also apply to its subclasses.  If you want the subclasses to inherit  the interface only, make the member function pure virtual (virtual … = 0)  the interface and a default implementation, make the member function virtual  Non virtual means inherit the interface as well as a mandatory implementation

CSC 143 O 7 Example: A Shape Class class Shape { public: virtual void draw() const = 0; // Abstract class virtual void error(const string& msg); int ObjectID() const; … }; class Rectangle: public Shape{…}; class Ellipse: public Shape{…};  Rectangle and Ellipse must provide a draw function  Every class must support an error function (but you don't have to write your own if you don't want to)  Every object has an ID. That ID is determined by the definition of Shape::objectID. Don't change it!

CSC 143 O 8 Virtual Destructor  Make sure that base classes have a virtual destructor  If D is derived from B, what happens in the following? B* pB = new D; … delete pB;  If you try to delete a derived class object through a base pointer and the base class doesn't have a virtual destructor, the result is undefined.