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 output operators(<<).

I can perfectly run the code without adding #include <ostream>:

#include <iostream>
using namespace std;
int main()
{
    cout << "hello world"<< endl;
    return 0;
}

But, in book's example like:

#include <iostream>
#include <ostream>    //why?
using namespace std;
int main()
{
    cout << "hello world"<< endl;
    return 0;
}

So, iostream, ostream and istream are header files right?

If ostream is not necessary (iostream does the job) why does the author include it in the example? Or why does the ostream header file still exist?

Note: In Bruce Eckel's Vol 1 book (which is published in 2000) there is nothing about ostream or istream. Only one header file which is iostream.

Answer 1

As ildjarn noted in the comment, C++ standard from 2003 says that iostream does not necessary include istream and ostream. So, theoretically, the book is correct.

However, most major compiler vendors have added istream and ostream into iostream, so your code works on the compiler you are using. You might not have such luck on some other compiler.

If you want to write portable code that would compile on older compilers that only adhere to 2003 standard (or earlier), you should include both headers. OTOH, if you are the only one compiling your code and have control which compilers would be used, it's safe to use iostream only, because that is forward-compatible.