我在vc 6.0中#include <cstdio>，没有用using namespace std;为什么也可以啊

#if <TRADITIONAL C HEADERS>
#include <stdio.h>

#undef clearerr
#undef feof
#undef ferror
#undef getc
#undef getchar
#undef putc
#undef putchar

namespace std {
using ::fclose;
using ::feof;
using ::ferror;
using ::fflush;
using ::fgetc;
using ::fgetpos;
using ::fgets;
using ::FILE;
using ::clearerr;
using ::fopen;
using ::fprintf;
using ::fpos_t;
using ::fputc;
using ::fputs;
using ::fread;
using ::freopen;
using ::fscanf;
using ::fseek;
using ::fsetpos;
using ::ftell;
using ::fwrite;
using ::getc;
using ::getchar;
using ::gets;
using ::perror;
using ::putc;
using ::putchar;
using ::printf;
using ::puts;
using ::remove;
using ::rename;
using :