The <iostream> header declares the eight standard stream objects. For other declarations, see http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt11ch24.html and the I/O forward declarations
They are required by default to cooperate with the global C library's
FILE streams, and to be available during program startup and termination. For more information, see the HOWTO linked to above.
This is a Standard C++ Library header.
Definition in file iostream.