#include<boost/variant>
#include<memory>

struct Visitor : public boost::static_visitor<std::unique_ptr<int>> {
  template<class T>
  std::unique_ptr<int> operator()(T& operand) {
    return std::unique_ptr<int>(new int(5));
  }
};


int main() {
  boost::variant<int,double> a = int(5);
  boost::apply_visitor(Visitor(),a); // this doesn't compile
}