C/C++ program without header files

Question

I was wondering if there is any possible way to write C/C++ program without using header files at all.

#include <iostream.h> //I want this same code to work without including this line
#include <conio.h>    //I want this same code to work without including this line

int main ()
{
  clrscr();
  char str [80];
  int num;
  cout<<"Enter the string : "<<str;
  cin>>str;
  cout<<"Enter the number : "<<num;
  cin>>num;
  getch();  
  return 0;
}

I learned that in C we can use scanf() function without using stdio.h

Example:

#include <stdio.h>

int main ()
{
  char str [80];
  scanf ("%s",str);  
  return 0;
}

The above code can be written as

extern int scanf(const char *format, ...);

int main ()
{
  char str [80];
  scanf ("%s",str);  
  return 0;
}

Here by calling extern int scanf(const char *format, ...);, scanf() will work seamlessly.

I am a trainer, my students asked me about this weird doubt and its my reason. I need to how it can be achieved with some code, just like extern int scanf(const char *format, ...); for replacing stdio.h

Answer 1

I am not sure I understand the reasons behind your question but there is a (very ugly) way.

Simply use your preprocessor (and not the compiler) with the right options (which depend on your environment) and it will generate a header-free equivalent CPP file which can be compiled.

Doing the same thing manually is just very prone to errors... and in my opinion completely useless.

Below is an example based on provided code in the question.

#include <stdio.h>

int main ()
{
    char str [80];
      scanf ("%s",str);
        return 0;
}

Using the command gcc -E main.cpp|grep -v "^#" |grep -v "^$" i extracted the following code (note that lines starting with # and empty lines were removed by my grep commands) :

extern "C" {
typedef unsigned int size_t;
typedef unsigned char __u_char;
typedef unsigned short int __u_short;
typedef unsigned int __u_int;
typedef unsigned long int __u_long;
typedef signed char __int8_t;
typedef unsigned char __uint8_t;
typedef signed short int __int16_t;
typedef unsigned short int __uint16_t;
typedef signed int __int32_t;
typedef unsigned int __uint32_t;
__extension__ typedef signed long long int __int64_t;
__extension__ typedef unsigned long long int __uint64_t;
__extension__ typedef long long int __quad_t;
__extension__ typedef unsigned long long int __u_quad_t;
__extension__ typedef __u_quad_t __dev_t;
__extension__ typedef unsigned int __uid_t;
__extension__ typedef unsigned int __gid_t;
__extension__ typedef unsigned long int __ino_t;
__extension__ typedef __u_quad_t __ino64_t;
__extension__ typedef unsigned int __mode_t;
__extension__ typedef unsigned int __nlink_t;
__extension__ typedef long int __off_t;
__extension__ typedef __quad_t __off64_t;
__extension__ typedef int __pid_t;
__extension__ typedef struct { int __val[2]; } __fsid_t;
__extension__ typedef long int __clock_t;
__extension__ typedef unsigned long int __rlim_t;
__extension__ typedef __u_quad_t __rlim64_t;
__extension__ typedef unsigned int __id_t;
__extension__ typedef long int __time_t;
__extension__ typedef unsigned int __useconds_t;
__extension__ typedef long int __suseconds_t;
__extension__ typedef int __daddr_t;
__extension__ typedef long int __swblk_t;
__extension__ typedef int __key_t;
__extension__ typedef int __clockid_t;
__extension__ typedef void * __timer_t;
__extension__ typedef long int __blksize_t;
__extension__ typedef long int __blkcnt_t;
__extension__ typedef __quad_t __blkcnt64_t;
__extension__ typedef unsigned long int __fsblkcnt_t;
__extension__ typedef __u_quad_t __fsblkcnt64_t;
__extension__ typedef unsigned long int __fsfilcnt_t;
__extension__ typedef __u_quad_t __fsfilcnt64_t;
__extension__ typedef int __ssize_t;
typedef __off64_t __loff_t;
typedef __quad_t *__qaddr_t;
typedef char *__caddr_t;
__extension__ typedef int __intptr_t;
__extension__ typedef unsigned int __socklen_t;
struct _IO_FILE;
typedef struct _IO_FILE FILE;
typedef struct _IO_FILE __FILE;
typedef struct
{
  int __count;
  union
  {
    unsigned int __wch;
    char __wchb[4];
  } __value;
} __mbstate_t;
typedef struct
{
  __off_t __pos;
  __mbstate_t __state;
} _G_fpos_t;
typedef struct
{
  __off64_t __pos;
  __mbstate_t __state;
} _G_fpos64_t;
typedef int _G_int16_t __attribute__ ((__mode__ (__HI__)));
typedef int _G_int32_t __attribute__ ((__mode__ (__SI__)));
typedef unsigned int _G_uint16_t __attribute__ ((__mode__ (__HI__)));
typedef unsigned int _G_uint32_t __attribute__ ((__mode__ (__SI__)));
typedef __builtin_va_list __gnuc_va_list;
struct _IO_jump_t; struct _IO_FILE;
typedef void _IO_lock_t;
struct _IO_marker {
  struct _IO_marker *_next;
  struct _IO_FILE *_sbuf;
  int _pos;
};
enum __codecvt_result
{
  __codecvt_ok,
  __codecvt_partial,
  __codecvt_error,
  __codecvt_noconv
};
struct _IO_FILE {
  int _flags;
  char* _IO_read_ptr;
  char* _IO_read_end;
  char* _IO_read_base;
  char* _IO_write_base;
  char* _IO_write_ptr;
  char* _IO_write_end;
  char* _IO_buf_base;
  char* _IO_buf_end;
  char *_IO_save_base;
  char *_IO_backup_base;
  char *_IO_save_end;
  struct _IO_marker *_markers;
  struct _IO_FILE *_chain;
  int _fileno;
  int _flags2;
  __off_t _old_offset;
  unsigned short _cur_column;
  signed char _vtable_offset;
  char _shortbuf[1];
  _IO_lock_t *_lock;
  __off64_t _offset;
  void *__pad1;
  void *__pad2;
  void *__pad3;
  void *__pad4;
  size_t __pad5;
  int _mode;
  char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
};
struct _IO_FILE_plus;
extern struct _IO_FILE_plus _IO_2_1_stdin_;
extern struct _IO_FILE_plus _IO_2_1_stdout_;
extern struct _IO_FILE_plus _IO_2_1_stderr_;
typedef __ssize_t __io_read_fn (void *__cookie, char *__buf, size_t __nbytes);
typedef __ssize_t __io_write_fn (void *__cookie, __const char *__buf,
     size_t __n);
typedef int __io_seek_fn (void *__cookie, __off64_t *__pos, int __w);
typedef int __io_close_fn (void *__cookie);
typedef __io_read_fn cookie_read_function_t;
typedef __io_write_fn cookie_write_function_t;
typedef __io_seek_fn cookie_seek_function_t;
typedef __io_close_fn cookie_close_function_t;
typedef struct
{
  __io_read_fn *read;
  __io_write_fn *write;
  __io_seek_fn *seek;
  __io_close_fn *close;
} _IO_cookie_io_functions_t;
typedef _IO_cookie_io_functions_t cookie_io_functions_t;
struct _IO_cookie_file;
extern void _IO_cookie_init (struct _IO_cookie_file *__cfile, int __read_write,
        void *__cookie, _IO_cookie_io_functions_t __fns);
extern "C" {
extern int __underflow (_IO_FILE *);
extern int __uflow (_IO_FILE *);
extern int __overflow (_IO_FILE *, int);
extern int _IO_getc (_IO_FILE *__fp);
extern int _IO_putc (int __c, _IO_FILE *__fp);
extern int _IO_feof (_IO_FILE *__fp) throw ();
extern int _IO_ferror (_IO_FILE *__fp) throw ();
extern int _IO_peekc_locked (_IO_FILE *__fp);
extern void _IO_flockfile (_IO_FILE *) throw ();
extern void _IO_funlockfile (_IO_FILE *) throw ();
extern int _IO_ftrylockfile (_IO_FILE *) throw ();
extern int _IO_vfscanf (_IO_FILE * __restrict, const char * __restrict,
   __gnuc_va_list, int *__restrict);
extern int _IO_vfprintf (_IO_FILE *__restrict, const char *__restrict,
    __gnuc_va_list);
extern __ssize_t _IO_padn (_IO_FILE *, int, __ssize_t);
extern size_t _IO_sgetn (_IO_FILE *, void *, size_t);
extern __off64_t _IO_seekoff (_IO_FILE *, __off64_t, int, int);
extern __off64_t _IO_seekpos (_IO_FILE *, __off64_t, int);
extern void _IO_free_backup_area (_IO_FILE *) throw ();
}
typedef __gnuc_va_list va_list;
typedef __off_t off_t;
typedef __off64_t off64_t;
typedef __ssize_t ssize_t;
typedef _G_fpos_t fpos_t;
typedef _G_fpos64_t fpos64_t;
extern struct _IO_FILE *stdin;
extern struct _IO_FILE *stdout;
extern struct _IO_FILE *stderr;
extern int remove (__const char *__filename) throw ();
extern int rename (__const char *__old, __const char *__new) throw ();
extern int renameat (int __oldfd, __const char *__old, int __newfd,
       __const char *__new) throw ();
extern FILE *tmpfile (void) ;
extern FILE *tmpfile64 (void) ;
extern char *tmpnam (char *__s) throw () ;
extern char *tmpnam_r (char *__s) throw () ;
extern char *tempnam (__const char *__dir, __const char *__pfx)
     throw () __attribute__ ((__malloc__)) ;
extern int fclose (FILE *__stream);
extern int fflush (FILE *__stream);
extern int fflush_unlocked (FILE *__stream);
extern int fcloseall (void);
extern FILE *fopen (__const char *__restrict __filename,
      __const char *__restrict __modes) ;
extern FILE *freopen (__const char *__restrict __filename,
        __const char *__restrict __modes,
        FILE *__restrict __stream) ;
extern FILE *fopen64 (__const char *__restrict __filename,
        __const char *__restrict __modes) ;
extern FILE *freopen64 (__const char *__restrict __filename,
   __const char *__restrict __modes,
   FILE *__restrict __stream) ;
extern FILE *fdopen (int __fd, __const char *__modes) throw () ;
extern FILE *fopencookie (void *__restrict __magic_cookie,
     __const char *__restrict __modes,
     _IO_cookie_io_functions_t __io_funcs) throw () ;
extern FILE *fmemopen (void *__s, size_t __len, __const char *__modes)
  throw () ;
extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) throw () ;
extern void setbuf (FILE *__restrict __stream, char *__restrict __buf) throw ();
extern int setvbuf (FILE *__restrict __stream, char *__restrict __buf,
      int __modes, size_t __n) throw ();
extern void setbuffer (FILE *__restrict __stream, char *__restrict __buf,
         size_t __size) throw ();
extern void setlinebuf (FILE *__stream) throw ();
extern int fprintf (FILE *__restrict __stream,
      __const char *__restrict __format, ...);
extern int printf (__const char *__restrict __format, ...);
extern int sprintf (char *__restrict __s,
      __const char *__restrict __format, ...) throw ();
extern int vfprintf (FILE *__restrict __s, __const char *__restrict __format,
       __gnuc_va_list __arg);
extern int vprintf (__const char *__restrict __format, __gnuc_va_list __arg);
extern int vsprintf (char *__restrict __s, __const char *__restrict __format,
       __gnuc_va_list __arg) throw ();
extern int snprintf (char *__restrict __s, size_t __maxlen,
       __const char *__restrict __format, ...)
     throw () __attribute__ ((__format__ (__printf__, 3, 4)));
extern int vsnprintf (char *__restrict __s, size_t __maxlen,
        __const char *__restrict __format, __gnuc_va_list __arg)
     throw () __attribute__ ((__format__ (__printf__, 3, 0)));
extern int vasprintf (char **__restrict __ptr, __const char *__restrict __f,
        __gnuc_va_list __arg)
     throw () __attribute__ ((__format__ (__printf__, 2, 0))) ;
extern int __asprintf (char **__restrict __ptr,
         __const char *__restrict __fmt, ...)
     throw () __attribute__ ((__format__ (__printf__, 2, 3))) ;
extern int asprintf (char **__restrict __ptr,
       __const char *__restrict __fmt, ...)
     throw () __attribute__ ((__format__ (__printf__, 2, 3))) ;
extern int vdprintf (int __fd, __const char *__restrict __fmt,
       __gnuc_va_list __arg)
     __attribute__ ((__format__ (__printf__, 2, 0)));
extern int dprintf (int __fd, __const char *__restrict __fmt, ...)
     __attribute__ ((__format__ (__printf__, 2, 3)));
extern int fscanf (FILE *__restrict __stream,
     __const char *__restrict __format, ...) ;
extern int scanf (__const char *__restrict __format, ...) ;
extern int sscanf (__const char *__restrict __s,
     __const char *__restrict __format, ...) throw ();
extern int vfscanf (FILE *__restrict __s, __const char *__restrict __format,
      __gnuc_va_list __arg)
     __attribute__ ((__format__ (__scanf__, 2, 0))) ;
extern int vscanf (__const char *__restrict __format, __gnuc_va_list __arg)
     __attribute__ ((__format__ (__scanf__, 1, 0))) ;
extern int vsscanf (__const char *__restrict __s,
      __const char *__restrict __format, __gnuc_va_list __arg)
     throw () __attribute__ ((__format__ (__scanf__, 2, 0)));
extern int fgetc (FILE *__stream);
extern int getc (FILE *__stream);
extern int getchar (void);
extern int getc_unlocked (FILE *__stream);
extern int getchar_unlocked (void);
extern int fgetc_unlocked (FILE *__stream);
extern int fputc (int __c, FILE *__stream);
extern int putc (int __c, FILE *__stream);
extern int putchar (int __c);
extern int fputc_unlocked (int __c, FILE *__stream);
extern int putc_unlocked (int __c, FILE *__stream);
extern int putchar_unlocked (int __c);
extern int getw (FILE *__stream);
extern int putw (int __w, FILE *__stream);
extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)
     ;
extern char *gets (char *__s) ;
extern char *fgets_unlocked (char *__restrict __s, int __n,
        FILE *__restrict __stream) ;
extern __ssize_t __getdelim (char **__restrict __lineptr,
          size_t *__restrict __n, int __delimiter,
          FILE *__restrict __stream) ;
extern __ssize_t getdelim (char **__restrict __lineptr,
        size_t *__restrict __n, int __delimiter,
        FILE *__restrict __stream) ;
extern __ssize_t getline (char **__restrict __lineptr,
       size_t *__restrict __n,
       FILE *__restrict __stream) ;
extern int fputs (__const char *__restrict __s, FILE *__restrict __stream);
extern int puts (__const char *__s);
extern int ungetc (int __c, FILE *__stream);
extern size_t fread (void *__restrict __ptr, size_t __size,
       size_t __n, FILE *__restrict __stream) ;
extern size_t fwrite (__const void *__restrict __ptr, size_t __size,
        size_t __n, FILE *__restrict __s);
extern int fputs_unlocked (__const char *__restrict __s,
      FILE *__restrict __stream);
extern size_t fread_unlocked (void *__restrict __ptr, size_t __size,
         size_t __n, FILE *__restrict __stream) ;
extern size_t fwrite_unlocked (__const void *__restrict __ptr, size_t __size,
          size_t __n, FILE *__restrict __stream);
extern int fseek (FILE *__stream, long int __off, int __whence);
extern long int ftell (FILE *__stream) ;
extern void rewind (FILE *__stream);
extern int fseeko (FILE *__stream, __off_t __off, int __whence);
extern __off_t ftello (FILE *__stream) ;
extern int fgetpos (FILE *__restrict __stream, fpos_t *__restrict __pos);
extern int fsetpos (FILE *__stream, __const fpos_t *__pos);
extern int fseeko64 (FILE *__stream, __off64_t __off, int __whence);
extern __off64_t ftello64 (FILE *__stream) ;
extern int fgetpos64 (FILE *__restrict __stream, fpos64_t *__restrict __pos);
extern int fsetpos64 (FILE *__stream, __const fpos64_t *__pos);
extern void clearerr (FILE *__stream) throw ();
extern int feof (FILE *__stream) throw () ;
extern int ferror (FILE *__stream) throw () ;
extern void clearerr_unlocked (FILE *__stream) throw ();
extern int feof_unlocked (FILE *__stream) throw () ;
extern int ferror_unlocked (FILE *__stream) throw () ;
extern void perror (__const char *__s);
extern int sys_nerr;
extern __const char *__const sys_errlist[];
extern int _sys_nerr;
extern __const char *__const _sys_errlist[];
extern int fileno (FILE *__stream) throw () ;
extern int fileno_unlocked (FILE *__stream) throw () ;
extern FILE *popen (__const char *__command, __const char *__modes) ;
extern int pclose (FILE *__stream);
extern char *ctermid (char *__s) throw ();
extern char *cuserid (char *__s);
struct obstack;
extern int obstack_printf (struct obstack *__restrict __obstack,
      __const char *__restrict __format, ...)
     throw () __attribute__ ((__format__ (__printf__, 2, 3)));
extern int obstack_vprintf (struct obstack *__restrict __obstack,
       __const char *__restrict __format,
       __gnuc_va_list __args)
     throw () __attribute__ ((__format__ (__printf__, 2, 0)));
extern void flockfile (FILE *__stream) throw ();
extern int ftrylockfile (FILE *__stream) throw () ;
extern void funlockfile (FILE *__stream) throw ();
}
int main ()
{
    char str [80];
      scanf ("%s",str);
        return 0;
}

Yes, it does hurt the eye a bit...