AndreyAkinshin AndreyAkinshin - 1 month ago 6
C Question

Variable number of parameters in function in C++

How I can have variable number of parameters in my function in C++.

Analog in C#:

public void Foo(params int[] a) {
for (int i = 0; i < a.Length; i++)
Console.WriteLine(a[i]);
}

public void UseFoo() {
Foo();
Foo(1);
Foo(1, 2);
}


Analog in Java:

public void Foo(int... a) {
for (int i = 0; i < a.length; i++)
System.out.println(a[i]);
}

public void UseFoo() {
Foo();
Foo(1);
Foo(2);
}

Answer

These are called Variadic functions. Wikipedia lists example code for C++.

To portably implement variadic functions in the C programming language, the standard stdarg.h header file should be used. The older varargs.h header has been deprecated in favor of stdarg.h. In C++, the header file cstdarg should be used.

To create a variadic function, an ellipsis (...) must be placed at the end of a parameter list. Inside the body of the function, a variable of type va_list must be defined. Then the macros va_start(va_list, last fixed param), va_arg(va_list, cast type), va_end(va_list) can be used. For example:

#include <stdarg.h>

double average(int count, ...)
{
    va_list ap;
    int j;
    double tot = 0;
    va_start(ap, count); //Requires the last fixed parameter (to get the address)
    for(j=0; j<count; j++)
        tot+=va_arg(ap, double); //Requires the type to cast to. Increments ap to the next argument.
    va_end(ap);
    return tot/count;
}