Question

I saw some programs including <iostream> and <ostream> simultaneously. Why?

Thanks for your kind reply.

Answer 1

<iostream> is not the combination of <istream> and <ostream>. It merely defines std::cin, std::cout and related objects. To actually do anything useful with std::cout, you still need <ostream>. Now, by C++ rules it’s possible that some implementations actually do include <ostream> in <iostream>, but you shouldn’t rely on this.