01cppClasses - Classes in C++ C++ originally called...

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Unformatted text preview: Classes in C++ C++ originally called "C with classes": Swedish connection: Bjarne Stoustrup borrowed from Simula ('67) Simulating classes of real world objects 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 C++ continues to evolve: 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 May 12, 2009 1 C structs A data structure for dates in C: 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 May 12, 2009 2 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); May 12, 2009 3 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 May 12, 2009 4 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; } }; Pros and cons of this approach? inline function is efficient, though it tends to break information hiding cout << today.getMonth(); //outside class Date 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? May 12, 2009 5 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); myChecking.getBalance(); May 12, 2009 //Checking constructor //Account function 6 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 //... } May 12, 2009 7 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; public: Circle(Point a, int r) : center(a), radius(r) {} draw(); //draw a circle using center and radius }; May 12, 2009 8 Calling a virtual function { //Construct some shapes Shape aShape; Circle c(Point(20,30),7); //illegal--why? //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? May 12, 2009 9 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? May 12, 2009 10 ...
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