Classes and objects in c++

Class

A class is a template that defines the form of an object.
A class specifies both code and data.
C++ uses a class specification to construct objects.
Objects are instances of a class.
Thus, a class is essentially a set of plans that specify how to build an object.
A class can contain private as well as public members. By default, all items defined in a class are private.
When you define a class, you declare the data that it contains and the code that operates on that data.
While very simple classes might contain only code or only data, most real-world classes contain both.
Data is contained in instance variables defined by the class, and code is contained in functions. The code and data that constitute a class are called members of the class.

Syntax

class-name
 {
  private data and functions
  public:
  public data and functions
 } object-list;

Defining the Class

class smallobj //define a class
 {
  private:
   int somedata; //class data
 public:
   void setdata(int d) //member function to set data
    {
      somedata = d;
    }
   void showdata() //member function to display data
    {
      cout << “
Data is “ << somedata;
    }
};

Object

An Object is a bit of self-contained Code and Data.
A key aspect of the Object approach is to break the problem into smaller understandable parts (divide and conquer).
Objects have boundaries that allow us to ignore un-needed detail.
We have been using objects all along: String Objects, Integer Objects, Dictionary Objects, List Objects.

Example

// A program that uses the Vehicle class.

#include 
using namespace std;

// Declare the Vehicle class.
class Vehicle
{
 public:
   int passengers; // number of passengers
   int fuelcap; // fuel capacity in gallons
   int mpg; // fuel consumption in miles per gallon
};

int main()
{
  Vehicle minivan; // create a Vehicle object
  int range;
  // Assign values to fields in minivan.
  minivan.passengers = 7;
  minivan.fuelcap = 16;
  minivan.mpg = 21;

  // Compute the range assuming a full tank of gas.
  range = minivan.fuelcap * minivan.mpg;

  cout << "Minivan can carry " << minivan.passengers << " with a range of " << range << "";
  return 0;
}

Output

Minivan can carry 7 with a range of 336