ENCAPSULATION

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To make parts of a class public (i.e., accessible to other parts of your program), you must declare them after the public keyword. All variables or functions defined after the public specifier are accessible by all other functions in your program. Making one class a friend of another exposes the implementation details and reduces encapsulation. The ideal is to keep as many of the details of each class hidden from all other classes as possible.
Any C++ program where you implement a class with public and private members is an example of data encapsulation and data abstraction.

Example :-

#include <iostream.h > 
using namespace std; 
class Adder 
{ 
   public: 
      		// constructor 
      Adder(int i = 0) 
      { 
        total = i; 
      } 
      	       // interface to outside world 
      void addNum(int number) 
      { 
          total += number; 
      } 
      	      // interface to outside world 
      int getTotal() 
      { 
          return total; 
      };
   private: 
             // hidden data from outside world 
      int total; 
}; 
int main( ) 
{ 
   Adder a; 
   a.addNum(10); 
   a.addNum(20);
   a.addNum(30);
   cout << "Total " << a.getTotal() << endl; 
   return 0;
}

When the above code is compiled and executed, it produces following result:
Total 60
Above class adds numbers together, and returns the sum. The public members addNum and getTotal are the interfaces to the outside world and a user needs to know them to use the class. The private member total is something that is hidden from the outside world, but is needed for the class to operate properly.