A Deeper Look at Classes

1. A Deeper Look at Classes CS-2303System Programming Concepts (Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie and from C: How to Program, 5th and 6th editions, by Deitel and Deitel) A Deeper Look at Classes

2. const (Constant) Objects andconst Member Functions • Principle of Least Privilege • “Allow access to data only when absolutely needed” • Fundamental principle of good software engineering • const objects • Keyword const • Specifies that an object is not modifiable • Attempts to modify const object  compilation errors • Example • const Time noon (12, 0, 0); A Deeper Look at Classes

3. const Member Functions • Only const member functions can be called for const objects • Member functions declared const may not modify the object in any way • A function is specified as const both prototype and definition. • Constructors and destructors are not const • By definition! A Deeper Look at Classes

4. const Example A promise that these functionsdo not modify the object at all A Deeper Look at Classes

5. Same here! const Example (continued) A Deeper Look at Classes

6. Notconst const Example (continued) A Deeper Look at Classes

7. constkeyword in function definition, as well as in function prototype const Example (continued) A Deeper Look at Classes

8. And here! Same here! const Example (continued) A Deeper Look at Classes

9. Purpose of const • To enlist the aid of the compiler in detecting unwanted changes to objects • Widely used in large programming projects • Helps maintain sanity, teamwork, etc. A Deeper Look at Classes

10. Questions? A Deeper Look at Classes

11. Deitel & Deitel, §20.10 Default Memberwise Assignment • Assignment operator (=) • Can be used to assign an object to another object of the same type. • Each data member of the right object is assigned to the same data member in the left object. • Not usually want you want to do! • Often causes serious problems when data members contain pointers to dynamically allocated memory !! • Because pointers are simply copied A Deeper Look at Classes

12. Default Memberwise Assignment – Example Default initialization of data members A Deeper Look at Classes

13. Default Memberwise Assignment (continued) A Deeper Look at Classes

14. Default Memberwise Assignment (continued) memberwise assignment assigns data members of date1 to date2 date2now stores the same date as date1 A Deeper Look at Classes

15. Default Memberwise Assignment (concluded) • Many classes must provide their own assignment operator • To intelligently assign one object to another • More about assignment of objects to each other when we get to Operator Overloading • The '=' operator can be overloaded, just like any other A Deeper Look at Classes

16. Copy Constructor • A constructor that copies another object of the same type • Example:– class TreeNode { public: ... /* other methods */ TreeNode(const TreeNode &nodeToBeCopied); // Copy Constructor private: const string word; int count; TreeNode *left, *right; }; A Deeper Look at Classes

17. Default Copy Constructor • Compiler provides a default copy constructor • Copies each member of the original object into the corresponding member of the new object • i.e., memberwise assignment • Enables pass-by-value for objects • Used to copy original object’s values into new object to be passed to a function or returned from a function • Not usually want you want to do! • Often causes serious problems when data members contain pointers to dynamically allocated memory !! • Just like default memberwise assignment A Deeper Look at Classes

18. Why do we need '&'? Copy Constructor • Many classes must provide an explicit Copy Constructor • To intelligently copy one object to another • Example:– string(const string &stringToBeCopied); • Allocates new memory for the new string • Copies characters from one to other • Does not blindly copy members (include internal pointers, etc.) A Deeper Look at Classes

19. The string copy constructor Usage of Copy Constructor • In Initializer list • Example:– TreeNode::TreeNode(const string &newWord) :word(newWord), //initialize word count(1), //initialize count left(NULL), right(NULL) {/* rest of constructor body */ } // TreeNode constructor A Deeper Look at Classes

20. Questions? A Deeper Look at Classes

21. New Topics friends and this A Deeper Look at Classes

22. Ordinary Member Functions • Function can access the private members of the class • Function is in the scope of the class • Function must be invoked on a specific object of the class – e.g., • ptr -> func() • obj.func() A Deeper Look at Classes

23. static Member Function • Function can access the private members of the class • Function is in the scope of the class • Function must be invoked on a specific object of the class – e.g., • ptr -> func() • obj.func() • But only the static members • Members that exist independently of any objects A Deeper Look at Classes

24. friend Function • Function can access the private members of the class • Function is in the scope of the class • Function must be invoked on a specific object of the class – e.g., • ptr -> func() • obj.func() A Deeper Look at Classes

25. Deitel & Deitel, §21.4 friend Function of a Class • Defined outside that class’s scope. • Not a member function of that class. • Has right to access non-public and public members of that class. • Often appropriate when a member function cannot be used for certain operations. • Can enhance performance. A Deeper Look at Classes

26. friend Functions and friend Classes • To declare a function as a friend of a class:– • Provide the function prototype in the class definition preceded by keyword friend • To declare a class as a friend of another class: • Place a declaration of the form • friend class ClassTwo;in the definition of class ClassOne • All member functions of class ClassTwo are friends of class ClassOne A Deeper Look at Classes

27. friend Functions and friend Classes(continued) • Friendship is granted, not taken. • For class B to be a friend of class A, class A must explicitly declare that class B is its friend. • Friendship relation is neither symmetric nor transitive • If class A is a friend of class B, and class B is a friend of class C, cannot infer that • class B is a friend of class A, • class C is a friend of class B, • class A is a friend of classC. • … A Deeper Look at Classes

28. friend Functions and friend Classes(continued) • … • It is possible to specify overloaded functions as friends of a class. • Each overloaded function intended to be a friend must be explicitly declared as a friend of the class. A Deeper Look at Classes

29. friend Function Example friendfunction declaration (can appear anywhere in the class) A Deeper Look at Classes

30. friend Function Example (continued) friendfunction can modify Count’s private data Calling a friend function; note that we pass the Countobject to the function A Deeper Look at Classes

31. Questions? A Deeper Look at Classes

32. The this Pointer • Member functions know which object’s data members to manipulate • Every object has access to its own address through a pointer called this(a C++ keyword) • An object’s this pointer is not part of the object itself • The this pointer is passed (by the compiler) as an implicit argument to each of the object’s non-static member functions A Deeper Look at Classes

33. Using the this Pointer • Objects may use the this pointer implicitly or explicitly. • this is used implicitly when accessing members directly. • It is used explicitly when using keyword this. • Type of the this pointer depends on • type of the object, and • whether member function is declared const. A Deeper Look at Classes

34. this Example A Deeper Look at Classes

35. this Example Implicitly using the thispointer to access member x Explicitly using the this pointer to access member x Using the dereferenced thispointer and the dot operator A Deeper Look at Classes

36. Another Example Using this class TreeNode { public: ... /* other methods */ TreeNode(const string &newWord, TreeNode *parent); //constructor private: const string word; int count; TreeNode *left, *rightTreeNode *const myParent; }; A Deeper Look at Classes

37. Example Using this (continued) TreeNode::TreeNode(const string &newWord, TreeNode *parent) :word(newWord), //initialize word count(1), //initialize count left(NULL), right(NULL), myParent(parent) //initialize myParent {/* rest of constructor body */ } // TreeNode constructor A Deeper Look at Classes

38. Contructor of TreeNode with pointerback to parent node! Example Using this (continued) TreeNode *TreeNode::AddNode(const string &newWord){if (newWord == word){ incr(); return this;}else if (newWord < word) { if (left) return left->AddNode(newWord); else return left = new TreeNode(newWord, this);} else { /* same for right */} } // AddNode A Deeper Look at Classes

39. Cascaded member-function calls Multiple functions are invoked in the same statement. Enabled by member functions returning the dereferenced this pointer. Example t.setMinute( 30 ).setSecond( 22 ); Calls t.setMinute( 30 ); Then calls t.setSecond( 22 ); See Deitel & Deitel, Figures 21.18–21.20 A Deeper Look at Classes

40. Questions? New Programming Assignmentwill be posted this weekend! A Deeper Look at Classes

41. Next Week • Subclasses • Needed for next Programming Assignment • See Deitel & Deitel Chapter 23 • Operator Overloading • Deitel & Deitel Chapter 22 A Deeper Look at Classes