C++ Programming/Code/Standard C Library/Functions/va arg

va_arg

Syntax

#include <cstdarg>
type va_arg( va_list argptr, type );
void va_end( va_list argptr );
void va_start( va_list argptr, last_parm );

The va_arg() macros are used to pass a variable number of arguments to a function.

First, you must have a call to va_start() passing a valid va_list and the name of the last argument variable before the ellipsis ("..."). This first argument can be anything; one way to use it is to have it be an integer describing the number of parameters being passed.
Next, you call va_arg() passing the va_list and the type of the argument to be returned. The return value of va_arg() is the current parameter.
Repeat calls to va_arg() for however many arguments you have.
Finally, a call to va_end() passing the va_list is necessary for proper cleanup.

int sum( int num, ... ) {
  int answer = 0;
  va_list argptr;            

  va_start( argptr, num );            

  for( ; num > 0; num-- ) {
    answer += va_arg( argptr, int );
  }           

  va_end( argptr );           

  return( answer );
}             
                

int main( void ) {            

  int answer = sum( 4, 4, 3, 2, 1 );
  printf( "The answer is %d\n", answer );           

  return( 0 );
}

This code displays 10, which is 4+3+2+1.

Here is another example of variable argument function, which is a simple printing function:

void my_printf( char *format, ... ) {
  va_list argptr;             

  va_start( argptr, format );          

  while( *format != '\0' ) {
    // string
    if( *format == 's' ) {
      char* s = va_arg( argptr, char * );
      printf( "Printing a string: %s\n", s );
    }
    // character
    else if( *format == 'c' ) {
      char c = (char) va_arg( argptr, int );
      printf( "Printing a character: %c\n", c );
      break;
    }
    // integer
    else if( *format == 'd' ) {
      int d = va_arg( argptr, int );
      printf( "Printing an integer: %d\n", d );
    }          

    *format++;
  }            

  va_end( argptr );
}              
                

int main( void ) {             

  my_printf( "sdc", "This is a string", 29, 'X' );         

  return( 0 );
}

This code displays the following output when run:

Printing a string: This is a string
Printing an integer: 29
Printing a character: X