Trailing dots (...) within function argument list in C++

Question

I was looking into some C++ source code files which perform some parsing when I came across a rather curious looking function as shown below along with the Doxygen documentation.

My question is, in the arguments of the function ERROR what do the trailing dots (...) mean? This is the first time I am seeing this being used.

/**
 * Create an error with given message id and fill in given string in message
 * @PARAM row   row where the error occured (0 to neglect)
 * @PARAM col   column where the error occured (0 to neglect)
 * @PARAM id    id of the message
 * @PARAM arg   an argument which will be filled in in the message,
 *              replacing %s, %i, %f, %c
 */
Error::Error(const int row, const int col, const int id, ...)
: err_row(row), err_col(col), err_id(id)
{
    //sprintf(msg, msgdesc(id));
    const char* msg_desc = msgdesc(id);

    va_list args;
    va_start(args, msg_desc);
    vsnprintf(msg, sizeof(msg)-1, msg_desc, args);
    msg[sizeof(msg)-1] = '\0';
    va_end(args);
}

The gcc compiler which I use throws me the following warning (among others concerning other files)

parser_error.cpp: In constructor ‘Error::Error(int, int, int, ...)’:
parser_error.cpp:30: warning: second parameter of ‘va_start’ not last named argument

Answer 1

It's a variable length argument list. The function basically requires the user to pass in a minimum three arguments (row, col, id), but you can pass in more arguments. The additional arguments are then handled by the va_list/va_start/va_end code.

Answer 2

That is a variable length argument list. From man stdarg:

NAME
     stdarg -- variable argument lists

SYNOPSIS
     #include <stdarg.h>

     void
     va_start(va_list ap, last);

     type
     va_arg(va_list ap, type);

     void
     va_copy(va_list dest, va_list src);

     void
     va_end(va_list ap);

DESCRIPTION
     A function may be called with a varying number of arguments of varying types.  The include file <stdarg.h> declares a
     type (va_list) and defines three macros for stepping through a list of arguments whose number and types are not known to
     the called function.

     The called function must declare an object of type va_list which is used by the macros va_start(), va_arg(), va_copy(),
     and va_end().

     The va_start() macro must be called first, and it initializes ap, which can be passed to va_arg() for each argument to be
     processed.  Calling va_end() signals that there are no further arguments, and causes ap to be invalidated.  Note that
     each call to va_start() must be matched by a call to va_end(), from within the same function.

     The parameter last is the name of the last parameter before the variable argument list, i.e., the last parameter of which
     the calling function knows the type.

     Because the address of this parameter is used in the va_start() macro, it should not be declared as a register variable,
     or as a function or an array type.

     The va_arg() macro expands to an expression that has the type and value of the next argument in the call.  The parameter
     ap is the va_list ap initialized by va_start().  Each call to va_arg() modifies ap so that the next call returns the next
     argument.  The parameter type is a type name specified so that the type of a pointer to an object that has the specified
     type can be obtained simply by adding a * to type.

     If there is no next argument, or if type is not compatible with the type of the actual next argument (as promoted accord-
     ing to the default argument promotions), random errors will occur.

     The first use of the va_arg() macro after that of the va_start() macro returns the argument after last.  Successive invo-
     cations return the values of the remaining arguments.

     The va_copy() macro copies the state of the variable argument list, src, previously initialized by va_start(), to the
     variable argument list, dest, which must not have been previously initialized by va_start(), without an intervening call
     to va_end().  The state preserved in dest is equivalent to calling va_start() and va_arg() on dest in the same way as was
     used on src.  The copied variable argument list can subsequently be passed to va_arg(), and must finally be passed to
     va_end() when through with it.

     After a variable argument list is invalidated by va_end(), it can be reinitialized with va_start() or made a copy of
     another variable argument list with va_copy().

EXAMPLES
     The function foo takes a string of format characters and prints out the argument associated with each format character
     based on the type.

           void foo(char *fmt, ...)
           {
                   va_list ap, ap2;
                   int d;
                   char c, *s;

                   va_start(ap, fmt);
                   va_copy(ap2, ap);
                   while (*fmt)
                           switch(*fmt++) {
                           case 's':                       /* string */
                                   s = va_arg(ap, char *);
                                   printf("string %s\n", s);
                                   break;
                           case 'd':                       /* int */
                                   d = va_arg(ap, int);
                                   printf("int %d\n", d);
                                   break;
                           case 'c':                       /* char */
                                   /* Note: char is promoted to int. */
                                   c = va_arg(ap, int);
                                   printf("char %c\n", c);
                                   break;
                           }
                   va_end(ap);
                   ...
                   /* use ap2 to iterate over the arguments again */
                   ...
                   va_end(ap2);
           }

COMPATIBILITY
     These macros are not compatible with the historic macros they replace.  A backward compatible version can be found in the
     include file <varargs.h>.

STANDARDS
     The va_start(), va_arg(), va_copy(), and va_end() macros conform to ISO/IEC 9899:1999 (``ISO C99'').

BUGS
     Unlike the varargs macros, the stdarg macros do not permit programmers to code a function with no fixed arguments.  This
     problem generates work mainly when converting varargs code to stdarg code, but it also creates difficulties for variadic
     functions that wish to pass all of their arguments on to a function that takes a va_list argument, such as vfprintf(3).