Class and Objects

C++ Classes and Objects

Class:- A class in C++ is the building block, that leads to Object-Oriented programming. It is a user-defined data type, which holds its own data members and member functions, which can be accessed and used by creating an instance of that class. A C++ class is like a blueprint for an object.

Class...

• Class is a collection of data member and member function in a single unit .

• Class is a keyword which is used for declare a class .

• The member of the class is private  by default.

• Class is used for providing high security.

• Class is used in large program and it takes more memory space.

• Syntax :

 Class class_name
 {

   Private :

     ...................

     ......…..........

   Public :

     ...................

     ...................

};

* Object...

• Object is an instance of a class .

• We can pass the value directly through the object .

• Syntax :  class_name object_name ;

Define C++ Objects

A class provides the blueprints for objects, so basically an object is created from a class. We declare objects of a class with exactly the same sort of declaration that we declare variables of basic types. Following statements declare two objects of class Box −

Box Box1;          // Declare Box1 of type Box
Box Box2;          // Declare Box2 of type Box

Both of the objects Box1 and Box2 will have their own copy of data members.

The classes are the most important feature of C++ that leads to Object Oriented programming. Class is a user defined data type, which holds its own data members and member functions, which can be accessed and used by creating instance of that class.

The variables inside class definition are called as data members and the functions are called member functions.

For Example: Consider the Class of Cars. There may be many cars with different names and brand but all of them will share some common properties like all of them will have 4 wheels, Speed Limit, Mileage range etc. So here, Car is the class and wheels, speed limits, mileage are their properties.

• A Class is a user defined data-type which has data members and member functions.
• Data members are the data variables and member functions are the functions used to manipulate these variables and together these data members and member functions defines the properties and behavior of the objects in a Class.
• In the above example of class Car, the data member will be speed limit, mileage etc and member functions can be apply brakes, increase speed etc.

An Object is an instance of a Class. When a class is defined, no memory is allocated but when it is instantiated (i.e. an object is created) memory is allocated.

Concept of class with example

Example of class in easy way to understand In C++.

#include 
#include
using namespace std;

// Class Declaration

class technical {
    
public:

    
    string name;
    int number;
};

int main() {
    // Object created
    technical obj;
    cout << "Enter the Name :";
    cin >> obj.name;

    cout << "Enter the Number :";
    cin >> obj.number;

    cout << obj.name <<" : "<< obj.number << endl;

    getch();
    return 0;
}

Output

Enter the Name :point
Enter the Number :100
point : 100

Defining Class and Declaring Objects

A class is defined in C++ using keyword class followed by the name of class. The body of class is defined inside the curly brackets and terminated by a semicolon at the end.

Declaring Objects: When a class is defined, only the specification for the object is defined; no memory or storage is allocated. To use the data and access functions defined in the class, you need to create objects.

Syntax:

ClassName ObjectName;

Accessing data members and member functions: The data members and member functions of class can be accessed using the dot(‘.’) operator with the object. For example if the name of object is obj and you want to access the member function with the name printName() then you will have to write obj.printName() .

Accessing Data Members

The public data members are also accessed in the same way given however the private data members are not allowed to be accessed directly by the object. Accessing a data member depends solely on the access control of that data member.
This access control is given by Access modifiers in C++. There are three access modifiers : public, private and protected.

// C++ program to demonstrate  // accessing of data members     #include using namespace std; class technical point {     // Access specifier     public:        // Data Members     string technicalpointname;        // Member Functions()     void printname()     {        cout << "technicalpoint is: " << technicalpoint;     } };    int main() {        // Declare an object of class geeks     technicalpoint obj1;        // accessing data member     obj1.technicalpointname = "Learning";        // accessing member function     obj1.printname();     return 0; }

Output:

technicalpointname is: Learning

Member Functions in Classes

There are 2 ways to define a member function:

• Inside class definition
• Outside class definition

To define a member function outside the class definition we have to use the scope resolution :: operator along with class name and function name.

C++ program to demonstrate function  // declaration outside class    #include using namespace std; class Geeks {     public:     string geekname;     int id;            // printname is not defined inside class definition     void printname();            // printid is defined inside class definition     void printid()     {         cout << "Geek id is: " << id;     } };    // Definition of printname using scope resolution operator :: void Geeks::printname() {     cout << "Geekname is: " << geekname;  } int main() {            Geeks obj1;     obj1.geekname = "xyz";     obj1.id=15;            // call printname()     obj1.printname();     cout << endl;            // call printid()     obj1.printid();     return 0; }

Output:

Geekname is: xyz
Geek id is: 15

Note that all the member functions defined inside the class definition are by default inline, but you can also make any non-class function inline by using keyword inline with them. Inline functions are actual functions, which are copied everywhere during compilation, like pre-processor macro, so the overhead of function calling is reduced.

Note: Declaring a friend function is a way to give private access to a non-member function.

c++ - Returning object from function

A function can also return objects either by value or by reference. When an object is returned by value from a function, a temporary object is created within the function, which holds the return value. This value is further assigned to another object in the calling function

The syntax for defining a function that returns an object by value is

class_name function_name (parameter_list)

{

// body of the function

}

To understand the concept of returning an object by value from a function, consider this example.

Example of Passing object & Returning object

#include
 #include

 class Rectangle
 {
  int L,B;

 Rectangle(int l,int b)
 {
  L = l;
  B = b;
 }

 Rectangle()
 {
  L = 0;
  B = 0;
 }

 Rectangle Sum(Rectangle Rec)        //function  1
 {

  Rectangle temp;

  temp.L = L + Rec.L;     //Statement  1
  temp.B = B + Rec.B;     //Statement  2

  return temp;
 }

 void Display()
 {
  cout<<"\nLength : "<//Statement  3

cout<<"\n\nValues of Rectangle 3 : "; R3.Display(); } Output : Values of Rectangle 1 : Length : 5 Breadth : 7 Values of Rectangle 2 : Length : 2 Breadth : 3 Values of Rectangle 3 : Length : 7 Breadth : 10

In statement 3, object R1 is calling member function Sum and passing object R2 as argument. Function 1 is receiving values of R2 in Rec. In statement 1 L is length of calling object R1 and Rec.L is length of object R2.

Both statement 1 and 2 is adding the length and breadth of Rectangle 1 and 2 respectively and assigning the result to object temp.

Statement 3 is receiving the values of object temp in object R3.

Structure vs class in C++

In C++, a structure is the same as a class except for a few differences. The most important of them is security. A Structure is not secure and cannot hide its implementation details from the end user while a class is secure and can hide its programming and designing details. Following are the points that expound on this difference:

1) Members of a class are private by default and members of a struct are public by default.
For example program 1 fails in compilation and program 2 works fine.

// Program 1

#include

  

class Test {

    int x; // x is private

};

int main()

{

  Test t;

  t.x = 20; // compiler error because x is private

  getchar();

  return 0;

}




// Program 2 #include    struct Test {     int x; // x is public }; int main() {   Test t;   t.x = 20; // works fine because x is public   getchar();   return 0; }

2) When deriving a struct from a class/struct, default access-specifier for a base class/struct is public. And when deriving a class, default access specifier is private.
For example program 3 fails in compilation and program 4 works fine.

   

// Program 3

#include

  

class Base {

public:

    int x;

};

  

class Derived : Base { }; // is equilalent to class Derived : private Base {}

  

int main()

{

  Derived d;

  d.x = 20; // compiler error becuase inheritance is private

  getchar();

  return 0;

}




// Program 4

#include

  

class Base {

public:

    int x;

};

  

struct Derived : Base { }; // is equilalent to struct Derived : public Base {}

  

int main()

{

  Derived d;

  d.x = 20; // works fine becuase inheritance is public

  getchar();

  return 0;

}




difference between class and structure




Class

1. To define class we will use “class” keyword.
2. When we initiate a class it will be allocate into heap memory.
3. To create an object for class we have to use new operator.
4. Class will support inheritance.
5. Class can contain destructor.
6. Class are reference type.

Structure

1. To define structure we will use “struct” keyword.
2. When we initiate a structure it will be allocate into stack memory.
3. To create an object for structure we don't require to use new operator.
4. Structure will not support inheritance.
5. Structure can't contain destructor.
6. Structure are value type.




Array of Object in C++




An object of class represents a single record in memory, if we want more than one record of class type, we have to create an array of class or object. As we know, an array is a collection of similar type, therefore an array can be a collection of class type.

Syntax for Array of object

class class-name
              {
                     datatype var1;
                     datatype var2;
                     - - - - - - - - - -
                     datatype varN;

                     method1();
                     method2();
                     - - - - - - - - - -
                     methodN();
              };

              class-name obj[ size ];

Example for Array of object

       #include
       #include

       class Employee
       {
              int Id;
              char Name[25];
              int Age;
              long Salary;

              public:
              void GetData()           //Statement 1 : Defining GetData()
              {
                    cout<<"\n\tEnter Employee Id : ";
                    cin>>Id;

                    cout<<"\n\tEnter Employee Name : ";
                    cin>>Name;

                    cout<<"\n\tEnter Employee Age : ";
                    cin>>Age;

                    cout<<"\n\tEnter Employee Salary : ";
                    cin>>Salary;
              }

              void PutData()           //Statement 2 : Defining PutData()
              {
                    cout<<"\n"< Output :

Enter details of 1 Employee
 Enter Employee Id : 101
 Enter Employee Name : Mohsin
 Enter Employee Age : 20
 Enter Employee Salary : 45000
 Enter details of 2 Employee
 Enter Employee Id : 102
 Enter Employee Name : Naim
 Enter Employee Age : 20
 Enter Employee Salary : 51000
 Enter details of 3 Employee
 Enter Employee Id : 103
 Enter Employee Name : Mirza Ali
 Enter Employee Age : 21
 Enter Employee Salary : 47000
 Details of Employees
 101 Mohsin 20 45000 1
 02 Naim 20 51000
 103 Mirza Ali 28 47000