Variable number of parameters in function in C++

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;
}

The real C++ solution is variadic templates. You'll need a fairly recent compiler and enable C++11 support if needed.

Two ways to handle the "do the same thing with all function arguments" problem: recursively, and with an ugly (but very very Standards compliant) solution.

The recursive solution looks somewhat like this:

template<typename... ArgTypes>
void print(ArgTypes... args);
template<typename T, typename... ArgTypes>
void print(T t, ArgTypes... args)
{
  std::cout << t;
  print(args...);
}
template<> void print() {} // end recursion

It generates one symbol for each collection of arguments, and then one for each step into the recursion. This is suboptimal to say the least, so the awesome C++ people here at SO thought of a great trick abusing the side effect of a list initialization:

struct expand_type {
  template<typename... T>
  expand_type(T&&...) {}
};
template<typename... ArgTypes>
void print(ArgTypes... args)
{ 
  expand_type{ 0, (std::cout << args, 0)... };
}

Code isn't generated for a million slightly different template instantiations, and as a bonus, you get preserved order of you function arguments. See the other answer for the nitty gritty details of this solution.

In C++11 and later you can also use initializer lists.

int sum(const initializer_list<int> &il)
{
    int nSum = 0;
    for (auto x: il) 
        nSum += x;
    return nsum;
}

cout << sum( { 3, 4, 6, 9 } );

Aside from the other answers, if you're just trying to pass an array of integers, why not:

void func(const std::vector<int>& p)
{
    // ...
}

std::vector<int> params;
params.push_back(1);
params.push_back(2);
params.push_back(3);

func(params);

You can't call it in parameter, form, though. You'd have to use any of the variadic function listed in your answers. C++0x will allow variadic templates, which will make it type-safe, but for now it's basically memory and casting.

You could emulate some sort of variadic parameter->vector thing:

// would also want to allow specifying the allocator, for completeness
template <typename T> 
std::vector<T> gen_vec(void)
{
    std::vector<T> result(0);
    return result;
}

template <typename T> 
std::vector<T> gen_vec(T a1)
{
    std::vector<T> result(1);

    result.push_back(a1);

    return result;
}

template <typename T> 
std::vector<T> gen_vec(T a1, T a2)
{
    std::vector<T> result(1);

    result.push_back(a1);
    result.push_back(a2);

    return result;
}

template <typename T> 
std::vector<T> gen_vec(T a1, T a2, T a3)
{
    std::vector<T> result(1);

    result.push_back(a1);
    result.push_back(a2);
    result.push_back(a3);

    return result;
}

// and so on, boost stops at nine by default for their variadic templates

Usage:

func(gen_vec(1,2,3));

See Variadic functions in C, Objective-C, C++, and D

You need to include stdarg.h and then use va_list, va_start, va_arg and va_end, as the example in the Wikipedia article shows. It's a bit more cumbersome than in Java or C#, because C and C++ have only limited built-in support for varargs.