Question

I wish to do something like this:

template<typename ...T> struct foo
{
  bar<0 /*to index through types in pack*/ ,T...>::type var1;
  bar<1 /*to index through types in pack*/ ,T...>::type var2;
  ...
}

But how would I define bar? No recursion technique comes to mind to do this.

I want a general technique so that I could typedef any particular type from a type pack, not just the two shown in the example.

Answer 1

#include <iostream>
#include <typeinfo>

template<class T, class T2, class... Args>
class C
{
public:
   typedef T type1;
   typedef T2 type2;
};

int main()
{
   typedef C<int, double, int, float> c;
   std::cout << typeid(c::type1).name() << " " << typeid(c::type2).name() << std::endl;
}

or mb something like this.

#include <iostream>
#include <typeinfo>

template<int N, class T, class... Args>
struct expand
{
public:
   typedef typename expand<N - 1, Args...>::type type;
};

template<class T, class... Args>
struct expand<1, T, Args...>
{
public:
   typedef T type;
};

template<class... Args>
class argsExpander
{
public:
   typedef typename expand<1, Args...>::type type1;
   typedef typename expand<2, Args...>::type type2;
};

template<class... Args>
class C
{
public:
    typename argsExpander<Args...>::type1 var1;
    typename argsExpander<Args...>::type2 var2;
};

int main()
{
   C<int, double, int, float> c;
   std::cout << typeid(c.var1).name() << " " << typeid(c.var2).name() << std::endl;
}

http://liveworkspace.org/code/7de289f128e86eb6006f576cbaf98991