How can I generate a tuple of N type T's?

Question

I want to be able to write generate_tuple_type which would internally have a type alias type which would be std::tuple

Answer 1

Fairly straightforward recursive formulation:

template
struct generate_tuple_type
{
 typedef typename generate_tuple_type::type type;
};

template
struct generate_tuple_type
{
  typedef std::tuple type;
};

Live example

[Update]

OK, so I was only thinking about modest values of N. The following formulation is more complex, but also significantly faster and less compiler-crushing for large arguments.

#include 

template
struct join_tuples
{
};

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

template
struct generate_tuple_type
{
  typedef typename generate_tuple_type::type left;
  typedef typename generate_tuple_type::type right;
  typedef typename join_tuples::type type;
};

template
struct generate_tuple_type
{
  typedef std::tuple type;
};

template
struct generate_tuple_type
{
  typedef std::tuple<> type;
};

int main()
{
  using gen_tuple_t = generate_tuple_type::type;
  static_assert( std::tuple_size::value == 30000, "wrong size" );
}

Live example

This version performs at most 2*log(N)+1 template instantiations, assuming your compiler memoizes them. Proof left as an exercise for the reader.