inheritance1 Inheritance Software re-use with derived classes

inheritance2 Inheritance Object-oriented languages allow new classes to acquire properties from an existing class - this is known as inheritance Original class is called the base or parent class, or the superclass The new, slightly different class is the derived or child class, or the subclass

inheritance3 Derived classes A derived class contains all members and functions of the base class The subclass inherits these members, along with their access characteristics The declaration of the child class determines how these access characteristics are manifested in the child class

inheritance4 Derived class declaration In the declaration of the child class, the parent class is listed as either public or private; for example: class Set : public Bag –With this declaration, public members of Bag are inherited as public members of Set –private members are inherited, but can be accessed only through inherited functions

inheritance5 Derived class declaration A derived class can also be declared with a private base class: class Set : private Bag –public members of Bag are inherited as private members of Set –private members of Bag are accessible only through Bag member functions

inheritance6 Protected class members If a base class member is declared as protected, rather than public or private, that member is directly accessible by any class derived from the base Protected members act just like private members in terms of access by non-member functions, except for members of derived classes

inheritance7 Example of inheritance in action We will define two classes, Point and Circle Circle inherits Point members and adds members of its own The base class (Point) data members are declared protected, rather than private, enabling direct access by the derived class (Circle)

inheritance8 Base Class Declaration class Point { public: Point (double a=0.0, double b=0.0); void setPoint(double a, double b); double getX()const {return x;} double getY()const {return y;} friend ostream& operator << (ostream &o, const Point &p); protected: double x; double y; };

inheritance9 Base class function definitions Point::Point (double a, double b)void Point::setPoint(double a, double b){x = a;y = b;} ostream & operator << (ostream &o, const Point &p) { o << “[” << p.x << “, ” << p.y << “]”; return o; }

inheritance10 Child class definition class Circle : public Point { public: Circle (double r =0.0, double x=0, double y=0); void setRadius(double r) {radius = r;} double getRadius () const {return radius;} double area () const {return radius*radius* ;} friend ostream& operator << (ostream &o, const Circle &c); protected: double radius; };

inheritance11 Circle constructor Circle::Circle(double r, double x, double y) : Point(x,y) { radius = r; } The line preceding the function’s begin bracket :Point (x,y) indicates a call to the constructor of the parent class This is not actually a separate line of code, but part of the function’s heading

inheritance12 Circle friend function ostream& operator << (ostream &o, const Circle &c) { o << “Center = [” << c.x << “, ” << c.y << “]” << “ Radius = ” << c.radius; return o; } Members x and y of parent class Point are accessible to child class Circle because of protected status Members x and y are members of the Circle object; in other words, there is a Point in every Circle

inheritance13 The “is-a” relationship The Point/Circle example is atypical of the way inheritance is usually used Inheritance usually involves creating a generic base class, then deriving several child classes from it The relationship of the child to the parent is usually described using the phrase “is a” -- we say a derived class object is a parent class object

inheritance14 The “is-a” relationship For example, your textbook describes a generic Clock class, then creates the derived class CuckooClock It is natural to say “a CuckooClock is a Clock” - meaning a cuckoo clock is a kind of clock It wouldn’t be too much of a stretch to derive other Clock types - alarm clock, 24- hour clock, etc.

inheritance15 Using a derived class Derived class objects can call base class member functions as if they were member functions of the derived class An object of the derived class may be used in place of a base class object - for example, if a function takes a base class object as a parameter, a derived class object may be substituted

inheritance16 Derived classes & assignment Assignments are allowed form a derived class to a base class if the base class is public However, a base class object cannot be used as if it were an object of the derived class - so you can’t assign a base class object to a derived class variable

inheritance17 Inheritance & Overriding A derived class inherits all members of the base class, but often a derived object needs to perform a particular operation a different way than that defined in an inherited function When a new function body is provided for an inherited member function within a derived class, the function is overridden

inheritance18 Inheritance & Overriding Sometimes overriding a function involves adding functionality, rather than simply replacing an original base class function When this is the case, the overriding function can call the base class function as its first action, then proceed with additional operations

inheritance19 Example int Clock24::get_hour() const // overrides inherited function { int ohr; ohr = Clock::get_hour() // call to overridden function if (is_morning()) { if (ohr == 12) return 0; else return ohr; } etc.

inheritance20 Using inheritance with ADTs Throughout the semester, we have studied various container classes In many ways, one container is very much like another; inheritance can be used to derive many different types of containers by placing common characteristics in a base class

inheritance21 Example: Set derived from Bag Both Bags and Sets contain unordered collection of elements The basic difference between them is that all elements in a Set must be unique We can create a Set class by inheriting from Bag, and overriding the insert function to eliminate insertion of duplicates

inheritance22 Example: Generic List Class Suppose you are provided with a generic ADT to hold an ordered list of items The data part of the List class is unknown to you, but you are given a set of member functions and a description of their actions, as shown on the slides that follow

inheritance23 List member functions List() // postcondition: initializes an empty List bool is_item() const // postcondition: returns true if there is a valid current // item; false if there is no valid current item Item current() const // precondition: is_item returns true // postcondition: returns current List item void start() // postconditon: first item on List becomes current item; // if list is empty, there is no current item

inheritance24 List member functions void advance() // precondition: is_item returns true // postcondition: if current item was the last on List, there is // now no current item; otherwise, the new current item is the // one immediately following the original current item void insert (const Item &entry) // postcondition: a new copy of entry has been inserted before // the current item; if no current item existed, entry was inserted // at the front of the List. Item containing entry is now the // current item

inheritance25 List member functions void attach (const Item &entry) // postcondition: a new copy of enty has been inserted in list // after the current item; if there was no current item, entry was // inserted at the end of the list. Item containing entry is now the // current item void remove_current() // precondition: is_item returns true // postcondition: current item has been removed; item following // current (if one existed) is now current item size_t size() const // postcondition: returns count of items in List

inheritance26 Deriving Stack from List Stack is defined by its push and pop functions, respectively inserting and removing items from the same end of the structure Because List has many functions that won’t (and shouldn’t) be used directly by users of Stack code, List is inherited as a private base class

inheritance27 Stack class definition template class Stack : private List { public: void push(const Item &entry); Item pop(); Item peek() const; List ::size; // changes inherited private function size to public // Stack function available to Stack user - this is an // example of exempting a member from private base // class bool is_empty() const {return size() == 0;} };

inheritance28 Stack member functions template void push (const Item &entry) { insert(entry); } template Item pop() { Item answer; assert(size() > 0); answer = current(); remove_current(); return answer; } template Item peek() { assert (size() > 0); return current(); }

inheritance29 Deriving Queue from List A child queue class could be derived from List in a similar fashion List would be inherited as a private base class, and queue functions enqueue and dequeue would be defined using inherited private functions from List

inheritance30 Inheritance Software re-use with derived classes - ends -