Q:

using namespace std and library

Why we need both the "header file" and the using namespace tag for the any library function to get executed properly. For example cout will not work unless we use iostream. Also it will not work unless we use "using namespace std". My question is why do we need combination of both using namespace std as well as #include <iostream> for cout to execute successfully? 

A:

Including a library header makes the library feature visible to your program code. Without that, your program has no idea that the library even exists. This is the part that is necessary.
Writing using namespace std simply allows you to write cout rather than the full name which is std::cout. It's a convenience, that's all.

A:

cout is defined in the std namespace, and you can use it without adding a using namespace like
std::cout << "Hello, World" << std::endl;

A:

Thanks for the reply. But my question is why do we need it in the first place.
  Since once we exposed "iostream" then why can't we simply use cout. 
Why to use std::cout or using namespace std?

When you don't use the std namespace, compiler will try to call cout or cin as if it weren't defined in a namespace. Since it doesn't exist there, compiler  tries to call something that doesn't exist! Hence, an error occurs.

