Variadic functions provide the ability to specify a variable number of arguments to a function, but they can be problematic. Variadic functions contain an implicit contract between the function writer and the function user that must be made to establish how many arguments are passed on an invocation. If care is not exercised when invoking a variadic function to ensure that it knows when to stop processing arguments and that the argument list is used incorrectly, there may be dangerous consequences. It should be noted that variadic functions must always have an ellipsis as a parameter, or the result is undefined.
Argument Processing
In the following code example, a variadic function called average() (taken from an article written by Robert Seacord for Linux World Magazine on variadic functions) is used to determine the average value of its passed integer arguments. The function will stop processing arguments when it sees that the argument is -1.
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