Select every even (or odd) argument in template parameter pack

Question

I would like to allow use of the class I'm writing to specify as a template parameters a list of types along with a list of allocators of those types in a manner that types are at odd positions and allocators are at even ones:

template<typename... T>
class MyClass {
  // Stuff inside
}

int main() {
  MyClass<SomeType1, AllocatorOfSomeType1> c1;
  MyClass<SomeType1, AllocatorOfSomeType1, 
          SomeType2, AllocatorOfSomeType2> c2;
  MyClass<SomeType1, AllocatorOfSomeType1, 
          SomeType2, AllocatorOfSomeType2,
          SomeType3, AllocatorOfSomeType3> c3;
  // And so on....
}

Internally it would make sense to have a tuple of vectors of types for storage:

std::tuple<std::vector<EveryOddTypeInParameterPack>...> m_storage_;

and a tuple of allocators for usage:

std::tuple<std::vector<EveryEvenTypeInParameterPack>...> m_storage_;

How can I actually declare those tuples in code? In theory I need to somehow select every odd/even type in parameter pack - is that possible?

Answer 1

Though the code got a little lengthy, I suppose the mechanism doesn't have unnecessary peculiarities.
If I understand the question correctly, probably the following code will meet the purpose:

// push front for tuple
template< class, class > struct PFT;

template< class A, class... T > struct PFT< A, tuple< T... > > {
  typedef tuple< A, T... > type;
};

// for even
template< class... > struct even_tuple;

template< class A, class B > struct even_tuple< A, B > {
  typedef tuple< A > type;
};
template< class A, class B, class... T > struct even_tuple< A, B, T... > {
  typedef typename PFT< A, typename even_tuple< T... >::type >::type type;
};
// As for odd elements, in the same way as even(please see the test on ideone)

// objective type
template< class > struct storage_type;

template< class... T > struct storage_type< tuple< T... > > {
  typedef tuple< vector< T >... > type;
};

template< class... T >
struct MyClass {
  typename storage_type< typename even_tuple< T... >::type >::type
    m_storage_even_;
  typename storage_type< typename  odd_tuple< T... >::type >::type
    m_storage_odd_;
};

Here is a test on ideone.