5.11. Functions with Variable-Length Argument Lists

5.11. Functions with Variable-Length Argument Lists
	Chapter 5. Functions

[ 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, ...)
```
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. src/ellipsis/ellipsis.cpp

#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;
}

	First parameter is number of args.
	Sequentially points to each unnamed arg.
	Now, ap points to first unnamed arg.
	Clean up before returning.


5.10.1. Inlining Versus Macro Expansion		5.12. Exercise: Encryption