How could a cpp program run without having any header file (not even iostream and so)?

Question

I was typing a prog of printing hello on console without giving any header files. When i compiled the prog it showed me warnings but when i run the prog it showed the output.
How is it possible to run a prof without giving any header files?

Answer 1

I think it's not possible..

Answer 2

I typed a prog and it was running on my system.
But i don't know how?