#include <string>
#include <iterator>

#include <boost/variant.hpp>


struct Leaf { };
struct Node;

using TreeBase = boost::variant<Leaf, boost::recursive_wrapper<Node>>;

struct Tree : TreeBase {
  using TreeBase::TreeBase;

  template <typename Iter>
  static Tree Create(Iter first, Iter last) { return Leaf{}; }
};


struct Node {
  Tree left, right;
};


int main() {
  std::string input{"abracadabra"};

  const auto root = Tree::Create(begin(input), end(input));
}