using namespace std;you will have to explicitly add
std::before anything in the std namespace.
using namespace stdto use the members of that header. This:
|Additionally the 1990 C standard defined a number of header files with .h.|
These header files have been incorporated into the 1998 C++ standard and they still work.
However, with the new thought that standard C++ header files should not have .h, the same files can also be included without .h, but they do need a "c" prefix then.
Moreover, all standard C++ functions and symbols reside in the namespace "std" now, although the old-style header files still reside in the global namespace.
For instance in C++ the following 2 code fragments are equivalent:
Note that non-standard C++ header files are conventionally still written with .h, such as <windows.h> and any header file that you introduce yourself.