Classes: A Deeper Look, Part 2

1. Classes: A Deeper Look, Part 2 Chapter 10 Part II

2. Outline

4. 10.4 friend Functions and friend Classes • A non-member function that can access the private members of a class • To make a function a friend function • Include its prototype in the class • Precede it with the friend keyword • Member access specifiers are not relevant to friend declarations, so they can be placed anywhere in a class definition • Defined outside class’s scope • Not a member function. • Access two or more classes with less overhead. • Has the right to access the non-public (and public) members of the class.

5. friend function declaration (can appear anywhere in the class)

6. friend function can modify Count’s private data Calling a friend function; note that we pass the Count object to the function

7. 10.4 Friend Classes • Friend functions may be members of other classes • To declare such friend functions, use scope resolution friendvoidotherClass::func(inti); • We can make all member functions of class ClassTwo friends of class ClassOne • In ClassOne definition, place declaration of form friend classClassTwo;

8. 10.4 Friend Classes (Cont.) • Friendship is established, 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 not symmetric • If class A is a friend of class B, it does not imply that B is a friend of A • Friendship relation is not transitive • If class A is a friend of class B, and class B is a friend of class C, it does not imply that class A is a friend of class C

9. 10.5  Using the this Pointer • Every object has access to its own address through a pointer called this (a C++ keyword). • The this pointer is not part of the object itself. • passed as an implicit. • Used inside a member function; it is a pointer to the invoking • object • string getName() { return name; } • or equivalently • string getName() { return this->name; } • Objects use the this pointer implicitly or explicitly to reference their data members and member functions. • cascaded member-function calls

19. 10.6 staticClass Members • A property of a class shared by all instances, not a property of a specific object of the class • Only one copy of a variable shared by all objects of a class • Specific to a class, NOT object of a class • Exists even if no objects of the class exist • May seem like global variables, but have class scope • static members can be declared public, private or protected. • A class’s private and protected static members are normally accessed through the class’s public member functions or friends. • static const data members of integral or enum types can be initialized where they are declared • If you want a different initial value, a static data member can be initialized once. • Initialized to zero if no initialization is present

20. static Data Members (Cont.) • Initialized in the definition file of the class • All other static data members must be defined at global namespace scope and can be initialized only in those definitions. • If a static data member is an object of a class that provides a default constructor, the static data member need not be initialized because its default constructor will be called. • publicstatic variables • Can also be accessed using binary scope resolution operator Employee::count • private static variables • When no class member objects exist • Can only be accessed via public static member function

21. static Member Functions • Is a service of the class, not of a specific object of the class • A class’s static members exist even when no objects of that class exist. • Cannot access non-static data or functions • No this pointer for static functions • static data members and static member functions exist independent of objects • Can be accessed using class name and binary scope resolution operator(::) Employee::getCount() • To access a public static class member when no objects of the class exist, prefix the class name and the binary scope resolution operator (::) to the name of the data member. • To access a private or protected static class member when no objects of the class exist, provide a public static member function and call the function by prefix-ing its name with the class name and binary scope resolution operator.

27. 10.6  static Class Members (cont.) • A member function should be declared static if it does not access non-static data members or non-static member functions of the class. • A static member function does not have a this pointer, because static data members and static member functions exist independently of any ob-jects of a class.

29. 10.7  Data Abstraction and Information Hiding • Classes normally hide the details of their implementation from their clients. This is called information hiding. • The client cares about what functionality a stack offers, not about how that functionality is implemented. This concept is referred to as data abstraction.

30. This enables a particular class to be replaced with another version without affecting the rest of the system. • As long as the public services of the class do not the rest of the system is not affected.