C++ Classes and Objects - Tech Trone

C++ Classes and Objects

     Class: The building block of C++ that leads to Object Oriented programming is a Class. 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 class is like a blueprint for an object.

     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 have 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.
   
   
   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 <bits/stdc++.h>

  using namespace std;

  class Geeks

  {

      // Access specifier

      public:

   

      // Data Members

      string geekname;

   

      // Member Functions()

      void printname()

      {

         cout << "Geekname is: " << geekname;

      }

  };

   

  int main() {

   

      // Declare an object of class geeks

      Geeks obj1;

   

      // accessing data member

      obj1.geekname = "Sachin";

   

      // accessing member function

      obj1.printname();

      return 0;

  }

   

  
  Output:
  
  

  Geekname is: Sachin

  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 <bits/stdc++.h>

  using namespace std;

  class Geeks

  {

      public:

      string geekname;

      int id;

       

      // printname is not defined inside class defination

      void printname();

       

      // printid is defined inside class defination

      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 = "Sachin";

      obj1.id=15;

       

      // call printname()

      obj1.printname();

      cout << endl;

       

      // call printid()

      obj1.printid();

      return 0;

  }

   

  
  Output:
  

  Geekname is: Sachin
  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.