Function With Variable Arguments

May 16, 2012 2 min read C++variable arguments

To define a function with a variable number of arguments. One solution is to use the cstdarg header file. There are four parts needed:

  1. va_list, stores the list of arguments,
  2. va_start, initializes the argument list,
  3. va_arg, returns the next argument in the list,
  4. va_end, cleans up the variable argument list.

Whenever a function is declared to have an indeterminate number of arguments, in the place of the last argument you should use an ellipsis, int a_function (int x, ...); would tell the compiler the function should accept a list of variable arguments, as long as the caller should pass at least one, the one being the first, x. va_start is a macro which accepts two arguments, a va_list and the name of the variable that directly precedes the ellipsis. 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:

 1#include <cstdarg>
 2#include <iostream>
 3using namespace std;
 4
 5// this function will take the number of values to average
 6// followed by all of the numbers to average
 7double average(int num, ...)
 8{
 9    va_list arguments;                     // A place to store the list of arguments
10    double sum = 0;
11
12    va_start(arguments, num);              // Store all values after num to arguments
13    for (int x = 0; x < num; x++)          // Loop until all numbers are added
14        sum += va_arg(arguments, double);  // Adds the next value in argument list to sum
15
16    va_end (arguments);                    // Cleans up the list
17
18    return sum / num;                      // Returns the average
19}
20
21int main()
22{
23    // this computes the average of 13.2, 22.3 and 4.5
24    cout << average (3, 12.2, 22.3, 4.5) << endl;
25    // here it computes the average of the 5 values 3.3, 2.2, 1.1, 5.5 and 3.3
26    cout << average (5, 3.3, 2.2, 1.1, 5.5, 3.3) << endl;
27}

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