Classes & Objects

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The mechanism that allows you to combine data and the function in a single unit is called a class. Once a class is defined, you can declare variables of that type. A class variable is called object or instance. In other words, a class would be the data type, and an object would be the variable. Classes are generally declared using the keyword class, with the following format:

class class_name
{
   private:
      members1;
   protected:
      members2;
   public:
      members3;
};

Where class_name is a valid identifier for the class. The body of the declaration can contain members, that can be either data or function declarations, The members of a class are classified into three categories: private, public, and protected. Private, protected, and public are reserved words and are called member access specifiers. These specifiers modify the access rights that the members following them acquire.

private members of a class are accessible only from within other members of the same class. You cannot access it outside of the class.
protected members are accessible from members of their same class and also from members of their derived classes.
Finally, public members are accessible from anywhere where the object is visible.

By default, all members of a class declared with the class keyword have private access for all its members. Therefore, any member that is declared before one other class specifier automatically has private access.

Here is a complete example :

class student 
{
  private :
    int rollno;
    float marks; 
  public:
    void getdata()
    {
       cout<<"Enter Roll Number : ";
       cin>>rollno;
       cout<<"Enter Marks : ";
       cin>>marks;
    }
    void displaydata()
    {
       cout<<"Roll number : "<<rollno<<"\nMarks : "<<marks;
    }
};

Object Declaration

Once a class is defined, you can declare objects of that type. The syntax for declaring a object is the same as that for declaring any other variable. The following statements declare two objects of type student:

student st1, st2;

Accessing Class Members

Once an object of a class is declared, it can access the public members of the class.

st1.getdata();

Defining Member function of class

You can define Functions inside the class as shown in above example. Member functions defined inside a class this way are created as inline functions by default. It is also possible to declare a function within a class but define it elsewhere. Functions defined outside the class are not normally inline.
When we define a function outside the class we cannot reference them (directly) outside
of the class. In order to reference these, we use the scope resolution operator, ::
(double colon). In this example, we are defining function getdata outside the class:
void student :: getdata()
{
     cout<<"Enter Roll Number : ";
     cin>>rollno;
     cout<<"Enter Marks : ";
     cin>>marks;
}

The following program demostrates the general feature of classes. Member function initdata() is defined inside the class. Member funcitons getdata() and showdata() defined outside the class.

class student //specify a class
{
  private :
    int rollno; //class data members
    float marks; 
  public:
    void initdata(int r, int m)
    {
       rollno=r;
       marks=m;
    }
    void getdata(); //member function to get data from user
    void showdata();// member function to show data
};

void student :: getdata()
{
    cout<<"Enter Roll Number : ";
    cin>>rollno;
    cout<<"Enter Marks : ";
    cin>>marks;
}

void student :: showdata()
{
    cout<<"Roll number : "<<rollno<<"\nMarks : "<<marks;
}

int main()
{
    student st1, st2; //define two objects of class student
    st1.initdata(5,78); //call member function to initialize
    st1.showdata();
    st2.getdata(); //call member function to input data
    st2.showdata(); //call member function to display data
    return 0;
}



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