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Lesson 17: Functions with Variable Argument Lists in C and C++ using va_list

Perhaps you would like to have a function that will accept any number of values and then return the average. You don't know how many arguments will be passed in to the function. One way you could make the function would be to accept a pointer to an array. Another way would be to write a function that can take any number of arguments. So you could write avg(4, 12.2, 23.3, 33.3, 12.1); or you could write avg(2, 2.3, 34.4); Some library functions can accept a variable list of arguments (such as the venerable printf).



To use a function with variable number of arguments, or more precisely, a function without a set number of arguments, you would use the cstdarg header file. There are four parts needed: va_list, which stores the list of arguments, va_start, which initializes the list, va_arg, which returns the next argument in the list, and va_end, which cleans up the variable argument list. Whenever a function is declared to have an indeterminate number of arguments, in place of the last argument you should place an ellipsis (which looks like '...'), so, int a_function ( int x, ... ); would tell the compiler the function should accept however many arguments that the programmer uses, as long as it is equal to at least one, the one being the first, x.

va_list is like any other variable. For example,
va_list a_list; 
va_start is a macro which accepts two arguments, a va_list and the name of the variable that directly precedes the ellipsis (...). So, in the function a_function, to initialize a_list with va_start, you would write va_start ( a_list, x );

va_arg takes a va_list and a variable type, and returns the next argument in the list in the form of whatever variable type it is told. It then moves down the list to the next argument. For example, va_arg ( a_list, double ) will return the next argument, assuming it exists, in the form of a double. The next time it is called, it will return the argument following the last returned number, if one exists.

To show how each of the parts works, take an example function:
#include <cstdarg>
#include <iostream>

using namespace std;

// this function will take the number of values to average
// followed by all of the numbers to average
double average ( int num, ... )
{
  va_list arguments;                     // A place to store the list of arguments
  double sum = 0;

  va_start ( arguments, num );           // Initializing arguments to store all values after num
  for ( int x = 0; x < num; x++ )        // Loop until all numbers are added
    sum += va_arg ( arguments, double ); // Adds the next value in argument list to sum.
  va_end ( arguments );                  // Cleans up the list

  return sum / num;                      // Returns the average
}
int main()
{
    // this computes the average of 13.2, 22.3 and 4.5 (3 indicates the number of values to average)
  cout<< average ( 3, 12.2, 22.3, 4.5 ) <<endl;
    // here it computes the average of the 5 values 3.3, 2.2, 1.1, 5.5 and 3.3
  cout<< average ( 5, 3.3, 2.2, 1.1, 5.5, 3.3 ) <<endl;
}
It isn't necessarily a good idea to use a variable argument list at all times, because the potential exists for assuming a value is of one type, while it is in fact another, such as a null pointer being assumed to be an integer. Consequently, variable argument lists should be used sparingly.

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