C++ for Java Programmers

1. C++ for Java Programmers Chapter 5 Class Definitions C++ for Java Programmers

2. ClassesSimilarities & Minor Differences Java C++ End with a semicolon. Divided into major sections by private, protected, and public. class A : B, C, Dto indicate inheritance All classes are public. Member function implementation can be external to class • No semicolon. • Modifiers to each data field or method individually. • extends to indicate Inheritance from a parent class. • Class contains member function implementation C++ for Java Programmers

3. Example class box { // Java public box (int v) { val = v; }public int value() { return val; }private int val; } class box { // C++ public: box (int v) { val = v; } int value() { return val; }private: int val; }; C++ for Java Programmers

4. Separation of Class & Implementation • In C++, method bodies are typically not placed in the class definition. Usually prototypes only • External method body must use fully qualified name void Link::addBefore(int value, List* theList) { … } C++ for Java Programmers

5. Separation of Class & Implementation • Methods defined in class are called inline definitions • Use inline definitions only for methods that are short. • An optimizing compiler may choose to expand an inline method directly, without the cost of a function call C++ for Java Programmers

6. Interface & Implementation Files • Interface file: extension .h is used in the interface file name. • Implementations will often be in a different file from the class definition. • In C++, a class need not be defined in a file with the same name. C++ for Java Programmers

7. Example of Interface # include <libClass.h> # include "myClass.h" # include "/users/…/…/tom/myClass.h" • Angle bracket indicate “system” interface files. • Quotation marks are used for immediate interface files. C++ for Java Programmers

8. The inline Directive • Can be used to indicate that the function can be expanded inline at a point of call, exactly as if the method had been written in a class description. • If the method body is sufficiently short, it can be marked as inline. • Virtual methods should not be declared as inline, as compiler is not able to produce inline code even if requested by the user. C++ for Java Programmers

9. Virtual Functions class Animal { public: virtual void eat() { cout << "I eat like a generic Animal.”; } }; class Wolf : public Animal { public: void eat() { cout << "I eat like a wolf!”; } }; class Fish : public Animal { public: void eat() { cout << "I eat like a fish!”; } }; class GoldFish : public Fish { public: void eat() { cout << "I eat like a goldfish!“; } }; C++ for Java Programmers

10. Virtual Functions Animal* anAnimal[4]; anAnimal[0] = new Animal(); anAnimal[1] = new Wolf(); anAnimal[2] = new Fish(); anAnimal[3] = new GoldFish(); for (int i = 0; i < 5; i++) { anAnimal[i]->eat(); // what happens when delete anAnimal[i]; // i == 4 ?? } C++ for Java Programmers

11. Prototypes • Function type signature must be known before a function can be invoked. • Function type signature describes the argument and return types of a function. • Must happen before first time a function is used (note, different than in Java) C++ for Java Programmers

12. Examples of Prototypes int max (int, int); int min (int a, int b); // argument names are optional complex abs (complex &); // can use user defined types bool operator < (distance &, distance &); // prototype for operator C++ for Java Programmers

13. External Declarations • The extern modifier to a declaration indicates that a global variable is defined in another file but will be used in the current file. • The declaration informs the linker that the value being named is used in two or more files but that it should nevertheless refer to only one object. C++ for Java Programmers

14. Example of extern extern int size; extern char buffer[ ]; // array limits don't have to be given extern ObjectType anObject; // declare strcmp is written in C, not C++ extern "C" int strcmp (const char *, const char *); C++ for Java Programmers

15. Forward References • Both function and class names must be defined before they can be used. • A forward declaration asserts that a particular name represents a class but gives no further information. • It permits pointers to be declared to the class but not to invoke methods defined by the class or the creation of instances of the class. C++ for Java Programmers

16. Forward Reference A list Link Link Link Link C++ for Java Programmers

17. Example of Forward References class Link; // forward declaration class List {public:...private: Link * firstLink; // permitted, since class Link is declared void push_front (int val); }; C++ for Java Programmers

18. Example of Forward References class Link { // now provide the full link implementationpublic: // constructor defined in-line Link (int v, Link * f, Link * b) { value = v; forwardLink = f; backwardLink = b; } // data fields are public int value; Link * forwardLink; Link * backwardLink; // prototype, definition given elsewhere // requires knowledge of class List void addBefore (int val, List * theList); }; C++ for Java Programmers

19. Example of Forward References inline void List::push_front (int val) {if (firstElement == 0) // adding to empty list firstElement = new Link(val, 0, 0);else // else add before first element firstElement->addBefore(val, this); } C++ for Java Programmers

20. Constructors and Initialization • Constructors tie together the tasks of creation and initialization. • Ensuring that no value is created without being initialized and no value is initialized more than once. C++ for Java Programmers

21. Default & Copy Constructors • All class definitions should include both a default and a copy constructor. • The default constructor takes no arguments. • Used to initialize object data fields with default values when no other arguments are specified. • A copy constructor takes an instance of the same class as a constant reference argument. (Clone constructor) • Used internally when functions return Class values, or are passed Class parameters by value C++ for Java Programmers

22. Example of Constructors class box { public: box () // default constructor { val = 1; } // give data field some default value box (int x) // ordinary constructor { val = x; } box (const box & a) // copy constructor { val = a.val; } // clone argument valueprivate: int box_val; }; box one; // default constructor box two (7); // ordinary constructor box three (two); // copy constructor box four = merge_boxes(one, two) // copy constructor C++ for Java Programmers

23. Initializers • In Java, if a data member is initialized with a value that is independent of the constructor arguments, simply written as an initial assignment at the point of declaration, otherwise, an explicit assignment statement. • C++ does not allow the initialization of data members at the point of declaration. • All data members must be initialized in a constructor. • Performed either in an explicit assignment or in an initializer. C++ for Java Programmers

24. Example of Initialization class Link {public: Link(int v, Link * f, Link * b) : value(v), forwardLink(f), backwardLink(b) { } Link(int v, Link * f, Link * b) // without initializers { value = v; forwardLink = f; backwardLink = b } }; C++ for Java Programmers

25. Example of Initializers class A { // class with initialization error public: void A (box & aBox) : boxOne(aBox) // copy constructor { boxTwo = aBox; } // default constructor, then assignprivate: box boxOne; box boxTwo; }; class B { // a better way,public: void B (box & aBox) : boxOne(aBox), boxTwo(aBox) { }private: box & boxOne; const box boxTwo; }; C++ for Java Programmers

26. Example of Initializers class bigBox extends box { // Java codepublic bigBox (int x, double d) { super(x); // initialize parent dvalue = d; // initialize self}private double dvalue; // private data field } class bigBox : public box { // C++ codepublic: bigBox (int x, double d) : box(x), //init base class dvalue(d) //init derived class {}private: double dvalue; }; C++ for Java Programmers

27. Example of Initializers class order { // Warning - Initializations done // in order of method declaration // Not in order of initializerpublic: order (int i) : one(i), two(one) { } int test() { return two; }private: int two; // initialized first int one; // initialized second }; C++ for Java Programmers

28. Order of Initialization • In C++, the initialization of parent classes occurs before the initialization of child class. • Methods that are invoked are matched only to functions in the parent class, even if these methods have been declared as virtual. C++ for Java Programmers

29. Initialization in Java class A { // Java classes illustrating initializationpublic A () { System.out.println("in A constructor"); init(); }public void init () { System.out.println ("in A init"); } } class B extends A {public B () { System.out.println ("in B constructor");}public void init () { super.init(); System.out.println ("in B init"); } } C++ for Java Programmers

30. Output of Java in A constructor in A init in B init in B constructor C++ for Java Programmers

31. Initialization in C++ class A { // C++ classes illustrating initializationpublic: A () { printf("in A constructor\n"); init(); } virtual void init () { printf("in A init\n"); } }; class B : public A {public: B () { printf("in B constructor\n"); } virtual void init () { A::init(); printf("in B init\n"); } }; C++ for Java Programmers

32. Output of C++ in A constructor in A init in B constructor C++ for Java Programmers

33. Combining Constructors • In C++, you cannot invoke one constructor form within another. class box { // error -- does not work as expectedpublic: box (int i) : x(i) { } box (int i, int j) : y(j) { box::box(i); } int x, y; }; C++ for Java Programmers

34. Example of Constructors // C++ class with default arguments in constructor class newClass { public: newclass (int i, int j = 7) { // do object initialization ... } }; C++ for Java Programmers

35. Example of Constructors // C++ class with factored constructors class newClass {public: newClass (int i) { initialize(i); // do common initialization } newClass (int i, int j) { initialize(i); ... // then do further initialization }private: void initialize (int i) { ... // common initialization actions } }; C++ for Java Programmers

36. The Orthodox Canonical Class Form • A default constructor: used internally to initialize objects and data members when no other value is available. • A copy constructor: used in the implementation of call-by-value parameters. • An assignment operator: used to assign one value to another. • A destructor: Invoked when an object is deleted. C++ for Java Programmers

37. Visibility Modifiers • In C++, the modifiers designate a section of a class definition rather than being applied item by item as in Java. • The modifiers cannot be applied to entire classes. • A subclass is permitted to change the visibility of attributes inherited from a parent class. C++ for Java Programmers

38. Example of Modifiers class parent {public: virtual void test () { printf("in parent test\n"); } }; class child : public parent {private: void test () { printf("in parent test\n"); } }; parent * p = new child; p->test(); child * c = (parent *) p; c->test(); // compile error, cannot invoke private method C++ for Java Programmers

39. Inner Classes vs. Nested Classes • An inner class in Java is linked to a specific instance of surrounding class, and is permitted access to data fields and methods in this object. • A nested class in C++ is simply a naming device; it restricts the visibility of features associated with the inner class, but otherwise the two are not related. C++ for Java Programmers

40. static Initialization • C++ does not use the message-passing syntax of invoking static functions. d = Math.sqrt (d); // Java -- invoke static function sqrt Date::setDefault(12,7,42); // C++ -- use qualified name C++ for Java Programmers

41. Example of static class box { public: box (int v) : value(v) { boxCount++; if (v == 0) zeroCount++; } private: static int boxCount = 0; static int zeroCount; }; // global initialization is separate from class int box::zeroCount = 0; C++ for Java Programmers

42. Example of static constants class coloredBox : public box { public: // define the range of color values static const int Red = 1; static const int Yellow = 2; static const int Blue = 3; coloredBox (int v, int c) : box(v), ourColor(c) { } private: int ourColor; }; C++ for Java Programmers

43. Test your understanding • What are some superficial similarities between class definitions in Java and C++? What are some differences? • What does it mean to say that C++ separates class definition and implementation? C++ for Java Programmers

44. Test your understanding • What does it mean to say a method is inline? • How does (may) the compiler treat an inline method differently? • What are two different ways to create an inline method? C++ for Java Programmers

45. Test your understanding • What is a function prototype? When is it necessary to provide one? • What does the extern modifier on a declaration mean? C++ for Java Programmers

46. Test your understanding • What two tasks are tied together by a constructor? • What is a default constructor? • What is a copy constructor? C++ for Java Programmers