What happen when we not use iostream.h in c++?

Answer 1

Let me correct you..
Cin and cout are objects of iostream class.

Answer 2

In C++, iostream.h is header file which contains input-output functions like 'cin' and 'cout'. If you exclude that file then you will not be able to use 'cin' and 'cout' functions thus no programming can be done as writing code requires atleast something to output (and input in some cases if you require input from the user).

Answer 3

Iostream is a header which includes very important functions for user input. Most common ones are cin and cout, but there are others such as cerr,clog, wcout (for wide Unicode characthers I believe).
You really don't have to include it, you know!
You can easily use cstdio which derives from C and replace input commands cin and cout with scanf and printf respectively.