1. Classes in C++ C++ originally called "C with classes":
Swedish connection: Bjarne Stoustrup borrowed from Simula (’67)
Simulating classes of real world objects
C++ continues to evolve:
Version 1.0 released by AT&T in 1986
Version 2.0 in 1990 (added multiple inheritance)
Version 3.0 in 1992 (templates)
ANSI standard in 1996 (exception handling, run time type info)
C++ became dominant OOPL in early 90's
Now Java and C# challenge
A class extends the C++ type system:
class Account { //creates a new type
}; //Note: you need this semi-colon in C++
Account a1,a2; //define variables of type Account
September 18

2. C structs A data structure for dates in C: What’s the problem?
struct Date {
int month, day, year; };
int set(struct Date*, int m, int d, int y);
int print(struct Date*);
What’s the problem?
No information hiding
No way to control access to data (obscure side effects)
No way to prevent assigning an illegal value to month
Changing representation of Date breaks all client code
September 18

3. C++ structs C++ provides for closer coupling of data and functions:
struct Date {
int month, day, year;
void set(int m, int d, int y);
void print(); //implement elsewhere };
Invoke functions with variable . memberFunction():
Date today; //In C++, structs automatically define types
today.set(9,29,1953); today.print();
Now we have data abstraction:
Procedural abstraction hides details of code in functions
Data abstraction couples data structure and functions
Still, no information hiding: today.set(50,-10,0);
September 18

4. C++ adds class C++ adds new keywords to support information hiding:
class Date {
int month, day, year;
public:
void set(int m, int d, int y);
void print(); //implement elsewhere };
Members after public: are visible to clients:
Date today; today.print();
today.month = 50; //is this legal?
Members after class are by default private
September 18

5. Member functions What can we add to Date to allow access to month?
class Date {
int month, day, year;
public:
void set(int m, int d, int y);
void print(); //implement elsewhere
int getMonth() { return month; } };
cout << today.getMonth(); //outside class Date
Pros and cons of this approach?
inline function is efficient, though it tends to break information hiding
Let’s define set() and protects the month data:
void Date :: set(int m, int d, int y) {
assert(m >= 1 && m <= 31); //#include <assert.h>
month = m; }
How can we ensure that month data is set to valid values?
September 18

6. Inheritance (class derivation)
class Account generalizes many kinds of bank accounts: checking, savings, etc.
C++ class derivation captures this generation:
class Checking : public Account {
public:
Checking(float balance);
Checking(); //default constructor };
: denotes derivation—Checking inherits from Account
public derivation denotes subtype inheritance
Account’s public methods accessible to instances of Checking
Checking myChecking(200); //Checking constructor
myChecking.getBalance(); //Account function
September 18

7. OOP = Data abstraction + inheritance + dynamic binding
Polymorphism: a function can mean different things at runtime
Dynamic binding: defer function binding to a subtype until runtime
Suppose we want to draw a heterogeneous collection of shapes?
class Point { ... }; //a Point has x and y coordinates
class Shape {
protected: //accessible to subclasses but otherwise private
Point center; //all Shapes have a center Point
public:
Point where() { return center; }
virtual void move(Point to) //virtual can be overridden
{ center = to; draw(); } // by derived classes
virtual void draw()=0; //a "pure" virtual function
//draw() must be implemented by derived classes
//... }
September 18

8. Subclasses override virtual functions
class Triangle: public Shape {
Point sw, se, top; //Three points define triangle
public:
Triangle(Point a, Point b, Point c) : sw(a), se(b), top(c) {}
draw() //implementing pure virtual function
{ put_line(sw,top); //draw line from sw to top
put_line(top,se); //draw line from top to se
put_line(se,sw); //draw line from se to sw } };
class Circle : public Shape {
int radius;
Circle(Point a, int r) : center(a), radius(r) {}
draw(); //draw a circle using center and radius
September 18

9. Calling a virtual function
{ //Construct some shapes
Shape aShape; //illegal--why?
Circle c(Point(20,30),7); //legal--what does it do?
//Create an array of various shapes
Shape* shapes[10]; //Why is this legal?
shapes[0] = new Circle(Point(20,30),7); //assign a Circle
shapes[1] = new Triangle(Point(50,50),Point(30,30),Point(40,40));
//... maybe assign other shapes, Rectangles, Squares, etc.
for (int i=0; i < 10; i++) //draw all the shapes
shapes[i]->draw(); //each shape draws itself! }
Why do we say that elements of shapes array are polymorphic?
How does polymorphic design support Open-Closed principle?
September 18

10. Why dynamic binding? What kind of code does dynamic binding avoid?
Avoids lots of switch statements, e.g.:
switch (shapes[i]->isa)
//each Shape derived class has an isa data member
{ case(triangle) Triangle::draw(); //test enumeration
case(circle) Circle::draw(); //run specific draw()
// ... }
Why is the dynamic binding version better for big, growing programs?
September 18