A function with default parameter values can be regarded as having a variable number of user-provided parameters, where you know at compile time the maximum number of parameters and their values if the caller chooses not to provide values. C++ also allows you to write functions where there is less certainty about the number of parameters, and the values passed to the function.
There are three ways to have a variable number of parameters: initializer lists, C-style variable argument lists, and variadic templated functions. The latter of these three will be addressed later in the chapter once templated functions have been covered.
