iostream vs ostream what is different?

Question

As the book says (Exploring C++: The Programmer\'s Introduction to C++):

The istream header declares input operators (>>), and ostream declares outp

Answer 1

I can perfectly run that code without adding #include ostream;

You happen to be able to on your specific installation. Upgrade your toolchain and that may no longer be the case.

As of C++11 you can assume it for iostream/ostream, but there are other similar scenarios which C++11 does not cover.

So, a general rule of thumb: whenever you use a standard library feature, include the header where it's declared/defined, rather than making assumptions and shortcuts.