Variadic typedefs, or “Bimaps done the C++0x way”

Question

Short question: Can I typedef a variadic argument pack? I need template struct Forward { typedef T... args; };.

---

Long version

Answer 1

You can achieve what you want by encapsulating the variadic argument pack in a tuple and later using the following two helper template structs to forward the actual variadic arguments:

template
struct cat;

template
struct cat, std::tuple>
{
        typedef std::tuple type;
};

and

template class Receiver>
struct Unpack;

template class Receiver>
struct Unpack, Receiver>
{
        typedef Receiver type;
};

your code sample would look like this:

#include 
#include 
#include 

template
struct Cat;

template
struct Cat, std::tuple>
{
        typedef std::tuple type;
};

template class Receiver>
struct Unpack;

template class Receiver>
struct Unpack, Receiver>
{
        typedef Receiver type;
};

template
struct Forward
{    
        //typedef Args... args; // Problem here!!
        typedef std::tuple args; // Workaround

        static const std::size_t size = sizeof...(Args);
};

template class SCont, 
        template class TCont, 
        typename SForward = Forward<> ,
        typename TForward = Forward<>>
class Bimap
{
        //typedef SCont left_type;
        //typedef TCont right_type;
        typedef typename Unpack, typename SForward::args>::type, SCont>::type left_type; //Workaround
        typedef typename Unpack, typename TForward::args>::type, TCont>::type right_type; //Workaround

        template struct Relation; // to be implemented

        typedef Relation relation_type;

};

int main()
{
    Bimap> , Forward>> x;
}

which compiles just fine under gcc 4.6.0