[ fromfile: ellipsis.xml id: ellipsis ]

In C and in C++ it is possible to define functions that have parameter lists ending with the ellipsis (...). The ellipsis enables the caller to specify the number of parameters and their types. The usual example of such a function is from <stdio.h>:

 int printf(char* formatStr, ...)

This flexible mechanism permits calls such as

printf("Eschew Obfuscation!\n");
printf("%d days hath %s\n", 30, "September");

To define a function that uses the ellipsis, you need to include the cstdarg library, which adds a set of macros for accessing the items in the argument list to the std namespace. There must be at least one parameter other than the ellipsis in the parameter list. A variable, usually named ap (argument pointer), of type va_list, is used to traverse the list of unnamed arguments. The macro

va_start(ap, p)

where p is the last-named parameter in the list, initializes ap so that it points to the first of the unnamed arguments. The macro

va_arg(ap, typename)

returns the argument that ap is pointing to and uses the typename to determine (i.e., with sizeof) how large a step to take to find the next argument. The macro

va_end(ap)

must be called after all the unnamed arguments have been processed. It cleans up the stack of unnamed arguments and ensures that the program will behave properly after the function has terminated.

Example 5.20 shows how to use these features.

#include <cstdarg>
#include <iostream>
using namespace std;

double mean(int n ...) {    
    va_list ap;             
    double sum(0);
    int count(n);
    va_start(ap, n);        
    for (int i = 0; i < count; ++i) {
        sum += va_arg(ap, double);
    }
    va_end(ap);             
    return sum / count;
}

int main() {
    cout << mean(4, 11.3, 22.5, 33.7, 44.9) << endl;
    cout << mean (5, 13.4, 22.5, 123.45, 421.33, 2525.353) << endl;
}