The #include pre-processor directive is seen in three different forms for standard library header files:
1) The C++ specific header files were declared of the form <header>, dropping the earlier syntax of <header.h>
2) The C library files included in ANSI C++ were prefixed with the character 'c' and the .h suffix was dropped
3) The C library files included in ANSI C++ usually implement their functions as templates rather than macros.
4) All names in both sets of these files were placed within the std namespace