ABSTRACTION

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Abstraction is a process of identifying the relevant qualities and behvaiors an object should possess.It gives the option to represent the needed information in program without disclosing all details. Also this feature gives the possibility of creating user defined data types increases the power of programming language.

Example :- A Laptop consists of many things such as processor, motherboard, RAM, keyboard, LCD screen, wireless antena, web camera, usb ports, battery, speakers etc. To use it, you don't need to know how internally LCD screens, keyboard, web camera, battery, wireless antena, speakers works. You just need to know how to operate the laptop by switching it on. The intrinsic details are invisitble. Think about if you would have to call to the engineer who knows all internal details of the laptop before operating it. This would have highly expensive as well as not easy to use everywhere by everyone. So here the Laptop is an object that is designed to hide its complexity.

In object-oriented software, complexity is managed by using abstraction. Abstraction is a process that involves identifying the critical behavior of an object and eliminating irrelevant and complex detilals. A well thought-out abstraction is usually simple, and easy to use in the perspective of the user, the person who is using your object.

Now if we talk in terms of C++ Programming, C++ classes provides great level of data abstraction. They provide sufficient public methods to the outside world to play with the functionality of the object and to manipulate object data ie. state without actually knowing how class has been implemented internally.

Example :- Your program can make a call to the sort() function without knowing what algorithm the function actually uses to sort the given values. In fact, the underlying implementation of the sorting functionality could change between releases of the library, and as long as the interface stays the same, your function call will still work.
In C++ we use classes to define our own abstract data types (ADT). You can use the cout object of class ostream to stream data to standard output like this:

#include < iostream.h > 
using namespace std;

int main( )
{
   cout << "Hello C++" << endl; 
   return 0; 
}

Here you don't need to understand how cout displays the text on the user's screen. You need only know the public interface and the underlying implementation of cout is free to change.