Obtain Argument Index while Unpacking Argument List with Variadic Templates

Question

I need to obtain the index of an argument while unpacking and converting argument list. Is there any solution for the following problem:

Answer 1

This is how I would do it. First of all, you will need some machinery to create compile-time sequences of integers:

using namespace std;

//===========================================================================
// META-FUNCTIONS FOR CREATING INDEX LISTS

// The structure that encapsulates index lists
template 
struct index_list
{
};

// Collects internal details for generating index ranges [MIN, MAX)
namespace detail
{
    // Declare primary template for index range builder
    template 
    struct range_builder;

    // Base step
    template 
    struct range_builder
    {
        typedef index_list type;
    };

    // Induction step
    template 
    struct range_builder : public range_builder
    {
    };
}

// Meta-function that returns a [MIN, MAX) index range
template
using index_range = typename detail::range_builder::type;

//===========================================================================

Then, you could use that machinery for realizing the function call, exploiting the power of argument pack expansion:

#include 
#include 
#include 

void test(int a, std::string b, bool c)
{
    cout << a << "," << b << "," << c << endl ;
}

namespace detail
{
    // This is the function that does the real work.
    template
    void call_test(const vector& params, index_list)
    {
        test((*static_cast(params[Is]))...);
    }
}

// This function just creates the compile-time integer sequence and 
// forwards to another function that performs the real work.
// In other words, this is a proxy that hides the complexity of the
// machinery from the client.
template 
void call_test(const vector& params)
{
    detail::call_test(params, index_range<0, sizeof...(ARG)>());
}

int main(int argc, char **argv)
{
    int    a = 1;
    string b = "string";
    bool c   = false;
    vector v(3);
    v[0] = &a;
    v[1] = &b;
    v[2] = &c;

    call_test(v);
}