Define tuple with variable size - c++

Question

I want to define a boost fusion::vector in my class with the size defined by a template parameter. ATM I'm doing this with a specialization of a helper class, but I think there should be a way to do this with boost mpl/fusion or something else in just one line.
namespace detail
{
template<int dim, typename T>
struct DimensionTupleSize
{ };
template <typename T>
struct DimensionTupleSize<1>
{
enum { Dimension = 1 }
typedef boost::fusion::vector<T> type;
};
template <typename T>
struct DimensionTupleSize<2>
{
enum { Dimension = 2 }
typedef boost::fusion::vector<T, T> type;
};
template <typename T>
struct DimensionTupleSize<3>
{
enum { Dimension = 3 }
typedef boost::fusion::vector<T, T, T> type;
};
}
template<int Dim = 2>
class QuadTreeLevel
{
public:
detail::DimensionTupleSize<Dim>::type tpl;
};
Any ideas?

Answer 1

You can do it recursively :
template<int N, class T> struct DimensionTupleSizeImpl
{
typedef typename DimensionTupleSizeImpl<N-1,T>::type base;
typedef typename boost::fusion::result_of::push_back<base,T>::type type;
};
template<class T> struct DimensionTupleSizeImpl<0,T>
{
typedef boost::fusion::vector<> type;
};
template<int N, class T>
struct DimensionTupleSize
: boost::fusion::result_of::
as_vector<typename DimensionTupleSizeImpl<N,T>::type>
{};

Answer 2

If you really want a tuple rather than an array, and you're simply looking for the most succinct solution..,
#include <boost/array.hpp>
#include <boost/fusion/include/boost_array.hpp>
#include <boost/fusion/include/as_vector.hpp>
template<std::size_t DimN, typename T>
struct DimensionTupleSize : boost::fusion::result_of::as_vector<
boost::array<T, DimN>
>::type
{ };

Answer 3

You could use this:
template<int N, typename T>
struct create_tuple
{
private:
template<int i, int n, typename ...U>
struct creator;
template<typename ...U>
struct creator<N,N, U...>
{
typedef boost::fusion::vector<U...> type;
};
template<int i, typename ...U>
struct creator<i, N,T, U...>
{
typedef typename creator<i+1,N,T,U...>::type type;
};
public:
typedef typename creator<1,N,T>::type type;
};
template<int N, class T>
struct DimensionTupleSize
{
typedef typename create_tuple<N,T>::type type;
};