iostream header

From Cppwiki

iostream is a C++ Standard Library #include. It is responsible for definiting the global stream objects std::cout, std::cin, std::cerr, std::clog, std::wcout, std::wcin, std::wcerr and std::wclog. Note that iostream is not technically required to define std::istream, std::ostream etc, but due to practical considerations those classes will be defined by including this header.