It's just the way it's defined by the C++ Standard -- as it happens, map and stdio.h don't even have to be real files.
As a side-note, stdio.h is the header that was originally imported into C++ from the C standard libraries -- the C++ version is cstdio. In practical terms, this generally means that when you include cstdio instead, you get the stuff from stdio.h, but it's in namespace std.
Just to clarify: the stdio.h you include in C++ is the C++ version of what was originally a C header. But the C++ way of writing the include is cstdio.
