Object Oriented Programming with C++/ Session 2/ 1 of 37 More on Classes Session 2 Object Oriented Programming with C++/ Session 2/ 2 of 37 Session Objectives  Use the scope resolution operator  Use dynamic memory allocation with • New • Delete  Use pointers to objects  Define and use Constructors  Define and use Destructors  Define the "Const" keyword Object Oriented Programming with C++/ Session 2/ 3 of 37 Session Objectives (Contd.)  Define and use the "this" pointer  Describe how objects and functions are arranged in memory • Static Data Members • Static member Functions  Describe type conversions using • Converting by assignment • Type casting Object Oriented Programming with C++/ Session 2/ 4 of 37 Scope resolution operator  Function can be defined outside the class specifier using a scope resolution operator :: (double colon symbol) with the function definition. . • General syntax: return_type class_name ::member_functions(arg1, arg2,. . .,argn) • The type of member function arguments must exactly match with the type declared in the class specifier.  Important for defining the member functions outside the class declaration. Object Oriented Programming with C++/ Session 2/ 5 of 37 Scope resolution operator (Contd.)  The left-hand operator of :: must be the name of the class.  Only the scope operator identifies the function as a member of a particular class.  Is also used to refer to global variable names in cases where a global variable and a local variable share the same name. • The syntax used is: ::global_variable  More freedom in naming variables. • If two variables have different purposes, their names should reflect the difference. Object Oriented Programming with C++/ Session 2/ 6 of 37 Dynamic memory allocation  An object is created when its definition appears in a program and it is destroyed when its name goes out of scope or the program terminates.  Useful to create a new object that will exist only as long as it is needed.  new creates such objects and the operator delete can be used to destroy them later.  Objects allocated by new and delete are said to be on the free store. Object Oriented Programming with C++/ Session 2/ 7 of 37 New  The new operator is used to create a memory space for an object of a class  The general syntax of the new operator is: data_type pointer_variable = new data_type;  For example, int *p; //pointer to integer type float *f; //pointer to a float type p = new int; //allocates memory for an integer f = new float; //allocates memory for a float  If call to new is successful, it returns a pointer to the space that is allocated. Object Oriented Programming with C++/ Session 2/ 8 of 37 New (Contd.)  Returns zero if the space is not available or if some error is detected.  Same syntax for an object. For example, Student *stu_ptr; //pointer to an object of type Student stu_ptr = new Student; //points to new Student object  The new operator is similar to the malloc() function used in C. Object Oriented Programming with C++/ Session 2/ 9 of 37 Delete  Object created by new exists until it is explicitly destroyed by delete. delete pointer_variable;  Example of new and delete. int *ptr; ptr = new int; *ptr = 12; cout << *ptr; delete ptr; • Always a good practice to delete memory when you are through with it. • Be careful that you do not use pointers to memory that has been deleted. Object Oriented Programming with C++/ Session 2/ 10 of 37 Allocatng Arrays  Allocate blocks consisting of arrays of varying length using a similar technique. int *ptr; ptr = new int[100]; delete [] ptr;  Any time you allocate an array of objects using new, you must use [] in the delete statement.  Error to delete a variable that has been malloc'ed and it is an error to free a variable that was allocated with new. [...]... Oriented Objects, data members and member functions in memory Object 1 Object 3 data 1 data 1 mem_function1() data 2 data 2 mem_function2() Object 2 data 1 data 2 Object Oriented Static Data Members Useful when all objects of the same class must share a common item of information If a data item in a class is defined as static, then only one such item is created for the entire class, no matter how many objects... objects involved One is the pointer itself and the other is object pointed to Prefixing a declaration of a pointer with const makes the object, but not the pointer, a constant Example int num =10; const int *iptr = # *iptr = 25 ; //error num = 25 ; //valid, num is not constant int xyz = 20 0; iptr = &xyz; //iptr can point anywhere else *iptr = 305; //error Object Oriented Const with pointers (Contd.) Also... A constant is an entity whose value does not change during the execution of a program The keyword const can be added to the declaration of an object to make that object a constant rather than a variable A constant cannot be assigned to, so it must be initialised const int num=100; num =20 0; //error num++; //error Object Oriented Const with pointers When we use const with a pointer, there are two objects... scope-resolution operator as shown: alpha::display_count(); Object Oriented Type Conversions Type conversion is done to convert a variable of a declared type to some other required type int count = 7; float average = 15.5; double totalsum = count * average; Converting data types can be done in two ways: • Converting by assignment • Type casting Object Oriented Converting by assignment Typical way of converting... to an object of type date today_ptr = new date; //points to the new date object Since today_ptr is a pointer to an object use arrow operator (-> ) today_ptr->getdate(); Object Oriented Constructors A constructor is a special member function for automatic initialisation of an object Has the same name as the class it belongs to Can declare and define constructors within the class, or declare them within... functions Object Oriented Objects and functions in memory Each object has its own copy of the data members of the class All the objects in a given class use the same member functions The member functions are created and placed in memory only once - when they are defined in the class specifier Data is therefore placed in memory when each object is defined, so there is a set for each object Object Oriented. .. Constructors (Contd.) class date{ int month, day, year; public: date() //default constructor {day=1; month=1; year=1999;} date(int x) //only day is specified {day=x; month=1; year=1999;} date(int x, int y, int z) //day month year {day=x; month=y; year=z;} }; Object Oriented Constructors (Contd.) As long as the constructors differ sufficiently in their argument types the compiler can select the correct one for... functions Object Oriented Constructors (Contd.) class username { public: username(); //constructor }; username::username() { } No return type is used for constructors Also invoked when local or temporary objects of a class are created Several constructors provide several ways of initialising a class object • A default constructor is a constructor that does not have any arguments Object Oriented Constructors... like any other pointer to an object Can be used to access the members of the object it points to with the use of the arrow operator this->age = 5; this->getdata(); Object Oriented Use of this class Person{ private: int age; public: void display(); }; void Person :: display() { this->age = 25 ; cout . Object Oriented Programming with C++/ Session 2/ 1 of 37 More on Classes Session 2 Object Oriented Programming with C++/ Session 2/ 2 of 37 Session Objectives. initialised. const int num=100; num =20 0; //error num++; //error Object Oriented Programming with C++/ Session 2/ 20 of 37 Const with pointers  When we use const with