Passing variable arguments in a C++ variadic template

Question

Say I have a template function that accepts variable arguments and calls other functions with those arguments...

template<typename... A> func(int i, A... args)
{
  // do something common

  switch (i)
  {
    case 0: x(args...); break;
    case 1: y(args...); break;
    case 2: z(args...); break;

    default: break; 
  }

  // do something common
}

...and the other functions are defined as...

void x(int a, int b);
void y(int a, int b);  // note: takes same args as x()
void z(std::string a);

This doesn't compile because calling func() with a std::string argument will not find a match for x() and y() that takes a std::string argument.

Note that func() uses some external criteria (the value of parameter i in this example) to decide which other function to pass parameter args to.

Is there a way to pull this off without resorting to something like stuffing parameter args into a std:tuple and passing that?

[EDIT]

Ok, I figured out an approach that works well enough for my purposes. Here's an example class to illustrate...

class X
{
  private:

    bool a(int) { return true; }
    bool b(int) { return true; }

    template<typename... A> bool a(const A&...) { return false; }
    template<typename... A> bool b(const A&...) { return false; }

  public:

    template<typename... A> void x(int i, const A&... args)
    {
        if (i == 0)
        {
            a(args...);
        }
        else
        {
            b(args...);
        }
    }
};

The variadic template methods for a() and b() will catch any calls made to X.x() where the arguments are not a single int. This resolves the compiler error I was receiving and enables the simpler class API that I was looking for.

Kerrek SB Kerrek SB · Accepted Answer · 2013-01-14T23:04:37

Yes, make separate overloads:

template <typename A1>
func(A1 arg1)
{
    x(arg1);
}

template <typename A1, typename A2>
func(A1 arg1, A2 arg2)
{
    y(arg1, arg2);
}

template <typename A1, typename A2, typename A3>
func(A1 arg1, A2 arg2, A3 arg3)
{
    z(arg1, arg2, arg3);
}

(Any additional, common code can of course be factored into a separate, common function, possibly a variadic function template.)

Passing variable arguments in a C++ variadic template

2 Answers