2. Classes: Abstraction & Encapsulation

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1 2. Classes: Abstraction & Encapsulation Objects & Classes What is an Object? Concept, abstraction, or thing with crisp boundary & meaning for a problem Sam, Laura, mywatch, frontwheel

2 Objects & Classes What is a Class? Group of Objects with similar properties (attributes) behavior relationships to other objects semantics Blueprints of Objects Person Watch Wheel Classes in C++ What does a class in C++ provide? Abstraction Encapsulation Ease of Use (like built-in-type)

3 An Example C++ Class - Interface class Switch // Class Interface - Switch.h file public: // Indicates that the members following this line may be accessed // by any function void turnon(); void turnoff(); bool getposition() const; private: bool position; ; This function does not change // Indicates member following any this data line of may the be implicit accessed Switch only object by //member functions of this class Example Class Implementation #include Switch.h // Implementation goes into Switch.cpp file void Switch::turnOn() position = true; // Accessing private object member void Switch::turnOff() position = false; bool Switch::getPosition() const return position;

4 Creating & Using Switch Objects Object Creation Switch obj; // Created on stack Switch* ptr = new Switch; // Created on heap Object destruction obj created on stack, automatically destroyed Object created on heap needs manual deletion delete ptr; Calling member functions obj.turnon(); ptr->turnon(); Accessing member data Only public members accessible obj.position = true; // ERROR ptr->position = true; // ERROR Switch Objects and Memory Switch obj1; // object on the stack Switch* ptr1 = &obj1; Switch* ptr2 = new Switch; // Object on the heap 0 obj1 obj1.position ptr1 ptr2 0

5 Access Control public: Accessible by any function of any class private: Accessible only by member functions of the same class Make most variables private Safe guard against change in variable name type of variable implementation of data structure - like array to linked list closely monitor change in values Make most functions public Provides access to functionality defined by class Exhibits the prescribed behavior of class Helper or Utility Functions You may write functions that help you write other functions of a class These functions perform some tasks that are subtasks in other functions Example: class SalesClerk public: void completesales(double amount) updateregister(); void processrefund(double amount) updateregister(); private: void updateregister() ; Does not make sense for user of class to call these functions Make these functions private

6 const Functions A member function of a class may read modify any member variable of an object If I have an object, I may call methods on the object Switch obj; obj.turnon(); bool currentstate = obj.getposition(); What if I have const object - an object I can t change Not be allowed to call methods that modify the object only const functions may be called const functions guarantee that state of object will not change when called non-const object: const functions and non-const functions may be called const Functions... void Switch::turnOn() // Not const - may modify members position = true; // Modifying private object member bool Switch::getPosition() const if (somecondition) //position = true; // ERROR: can t modify member turnon();// ERROR: can t call methods that may modify members Switch someotherobject; someotherobject.turnon(); //OK, because it is not this object someotherobject.position = false; // OK, because it is not this object return position;

7 Coding Standard For Classes Interface defines what your class will do Implementation tells how you do it Interface given to others in readable form Implementation given to others in binary form Each function must explain its purpose requirement promise exception This convention illustrated in online examples Not shown on slides to save space A Date Class Interface class Date public: void set(int rmonth, int rday, int ryear) // Implementation provided with the interface. Better not to. // You may validate rmonth, rday and ryear before setting month = rmonth; day = rday; year = ryear; void get(int& rmonth, int& rday, int& ryear) const; private: int month; int day; int year; ;

8 Data class Implementation void Date::get(int& rmonth, int& rday, int& ryear) const rmonth = month; rday = day; ryear = year; What is the memory organization of the Date Objects? A String Class Strings are mere array of characters in C Tedious to deal with String class Abstracts strings for easy use class String char* str; public: String() str = 0; void set(const char* val); const char* get() const; ; H I \0 str

9 Implementation of get() and set() const char* get() const; return str; void String::set(const char* val) if (val!= 0) // Problem?! str = new char [strlen(val) + 1]; strcpy(str, val); Correct Implementation Clean char array before allocating new array void String::set(const char* val) if (str!= NULL) delete[] str; str = 0; if (val!= 0) str = new char [strlen(val) + 1]; strcpy(str, val);

10 Lab Work: Details provided on-line.

Pointers, Dynamic Data, and Reference Types

Pointers, Dynamic Data, and Reference Types Review on Pointers Reference Variables Dynamic Memory Allocation The new operator The delete operator Dynamic Memory Allocation for Arrays 1 C++ Data Types simple