Objects classes Relationship between classes and objects

Objects and classes, Relationship between classes and objects

Object Oriented Programming is the most commonly used type of programming technique. This technique is very useful in writing long and complex programs easily. OOP technique provides advanced programming features because it uses real world modeling technique to solve a problem.

Object: Any thing that has state and behavior is called an objected. In real world modeling we observe the actual characteristics of an object and functions performed by the object. For example a person, teacher, vehicle, place is an object in real world. The name of the person is used to define the property of an object. Price of the vehicle is used to define the properties of an object. Same like that a teacher teaches a class so this is the behavior of that object. Vehicle is used to transport some articles from one place to another. These are the functions of an object.

In computer programming, the use of GUI (graphical user interface) is the combination of various objects like form, buttons, text boxes and many others. These components which are also considered as tools/controls and objects have properties like name, color, size, style, and caption. They also have behavior like click, double click, load and some others. They are also considered as functions of an object.

Class: Class is simply the combination of data and the functions applied on that data. Class is also considered as set of objects which share their common characteristics like all the buttons on a form belong to a single class of button as all the buttons surely share their common characteristics.

Classes are used in programming phase to define objects. Writing a class doesn’t mean that we are creating an object. Classes are just template or sketch that tells us that the object created by that class will how look like and what functions can be performed by these objects.

There is strong relationship between classes and objects that classes are used to define objects. And a program in execution with its functions and data members are called object of that classes. More than one object can be created from one class. Same like that so many houses can be built from one sketch of house.

Class is the template and sketch that combines data member and member functions applied on data. These data members are normally declared as private and member functions are declare public, so these data members may be access by these member functions within or outside the class. A class is written in the same way as structure is written. But structure only combines data members. But class combines data members and member functions also.

When we write a class it doesn’t create any data member of function in memory. To allocate memory space to the data members and to execute member function, we must have to create the object of that class. Object creation or class instantiation is required to allocate memory space to data members and to execute or call member functions. We can create as many object as we require for our program.

Access Specifiers in OOP

The use of class is very important feature of OOP. By using class we can restrict the use of member variables and member functions. This feature makes our programming more efficient and meets with our requirements. We can allow functions of same and child classes to access them easily.

Basically there are 2 types of access Specifiers private and public. Private access Specifiers are used to restrict the members of the class to be used with in the class, not outside of the class. It provides security that the members of the class cannot accessed or changed outside the class. This is the default behavior. It means if we don’t use the keyword private then the compiler automatically consider that the members of the class are private.

If we declare data members or member functions with public keyword it means that the members can be accessed within or outside the class anywhere in the program. They can also be used in main function which is the starting point of ever program. Normally the member functions of the class are declared with public keyword. These member functions can access private data members as well within or outside the class as well as in the main function where we call these functions for execution.

The private data members cannot be accessed outside the class. If we try to access them with private member functions then compiler will generate an error that we cannot access the private members outside the class. So member functions declared with public keyword can access them for data storing and manipulation with in the class, outside the class and in main function.

CONSTRUCTOR

When we create an object/instantiation of a class, then a member function of the class with the same as class will implicitly executed. A class is just a sketch and used for defining the data variables and functions of the object. So writing the code of class doesn’t create any object or allocate memory space for variables and functions. We have to create an object  

A special type of function which is called constructor is executed whenever we create an object of a class in main function. This function has no return type. The constructor can work as normal function. It allocates memory space for member variables and also execute member functions. We can use it in class to initialize some variables or just show a message on screen when the object is created.

DESTRUCTOR

When constructor is executed and allocated memory space to the variables declared, then after execution of that function that reserved space which is allocated to the variables in RAM must be released or evacuated so that this space may be utilized for other programs.

After the constructor is executed then destructor will also be executed to release that space. Whenever the constructor is executed, destructor will also be executed. If there are 2 or 3 or more objects of a class then destructor will also be executed as many times as objects are being used.  Destructor is written in the same manner as constructor but a ~ tilde sign is added with its name to indicate that this is a destructor.

Inheritance and it feature and benefits

Object oriented programming technique provides us different features for writing long programs with efficient way. One of the most important features of object oriented programming is inheritance. When we write a class which has several member functions and data members and we have to use the features of the existing class in another class without rewriting the same code again then we inherit the class. All the features of the inherited class will be added to the new class which is inheriting. The class which is inherited is called super class, base class or parent class. The class which is inheriting the properties and functions of an existing class is known as subclass, derived class, child class.

Inheritance is one of the most powerful features of object oriented programming. The basic use of inheritance is that each sub class shares common properties with the base class. Child class can add its own capabilities also. The features of inheritance are given below.

REUSABILITY

The features of Inheritance allow the programmer to reuse the existing code in many situations without rewriting the code again. A class can be created one time and its functionality can be reused again and again in other classes.

SAVE TIME AND EFFORT

We inherit a base class to reuse its features in child class. It also saves our time and effort in doing the same thing. Lots of time and effort can be saved.

INCREASED PROGRAM STRUCTURE AND RELIABILITY

The main class or super class is already compiled and tested properly. This class can be used in a new application without compiling it again. It plays very important role in visual programming where we use many graphical objects which are derived from one main class. If a class has some important feature for input, processing and output then we don’t have to do the same task again. We just use it by inheriting its code and can focus on developing a well structure program.

Different types of inheritance, levels of inheritance

TYPES OF INHERITANCE

A parent class can be inherited using private, public and protected type of inheritance. These 3 keywords define the access of the parent class members in child class.

PUBLIC INHERITANCE

In public inheritance, the child or derived class access the parent or base class in such a way that the member functions or data members declared in parent class with public keyword will become the public members of derived class.

The members of parent class declared with protected class will become the protected members of the derived class.

The members of the parent class declared with private keyword will become the private members of the child class.

The derived class can access the public members of the parent class.

The derived class can access the protected members of the parent class.

The derived class cannot access the private members of the parent class.

PROTECTED INHERITANCE

When we inherit a class in protected way of inheritance, the public members of the parent class become the protected members of the derived class. The protected and private members of the parent class will remain protected and private in child class.

The derived class can access the public members of the parent class. Public members will become protected.

The derived class can access the protected members of the parent class.

The derived class cannot access the private members of the parent class.

PRIVATE INHERITANCE

In private inheritance, the private, protected and public members of the parent class will become the private members of the derived class. Their access will be restricted as private member may not be accessed outside the class.

CATEGORIES AND LEVELS OF INHERITANCE

There are different categories of inheritance like single inheritance and multiple inheritances.

Single Inheritance

A type of inheritance in which a child class is derived from single parent class is called single inheritance. In which one base, super, parent, base class is derived by a child, dependent class.

Multiple inheritances

A type of inheritance in which a child class is derived from multiple parent or base classes is called multiple inheritance. The child class in this inheritance inherits all the data members and member functions of the parent classes.  The child class can also add its own functionalities in it as well the functionalities of the base and parent classes.

Level of inheritance

level of inheritance means that a class inherits a base class and the child class is further inherited by another child class.  This type of inheritance is considered as multiple level of inheritance.  The members of parent class are inherited to the child class and the members of child class are inherited to the grand child class. The members of parent and child class are combined in grand child class so they can be used easily in the grand child class.

Function overriding by using classes with example

Answer: As we know that classes are the combination of data members and member functions. We can use these member functions and data members by inheriting these functions in the child class. Some time we can declare two functions separately in parent and child classes with same name. When we use same name and same signatures (means parameters in argument list) for declaring 2 functions then this type of method is called function overriding.

The object of the derived class cannot access that function of parent class because they have same name and signatures. But we can access that function of the parent class by using scope resolution operator with the name of the parent class in the body of the child class.

 class a

{

Protected:

int m;

public:

 void c ( )                                  //function of main class with same name as in child class.

{

                                                 //body of the function, define the feature of the function. 

}

};

 

Class b

{

Private:

int n;

public:

void c ( )  //function with same name in child class. c is the name of the function which is overridden

{

a::c ( );                         //calling of parent class function with scope resolution operator

}

};