boost::movelib::operator<
// In header: <boost/move/unique_ptr.hpp> template<typename T1, typename D1, typename T2, typename D2> bool operator<(const unique_ptr< T1, D1 > & x, const unique_ptr< T2, D2 > & y);
Returns: x.get() < y.get().
Remarks: This comparison shall induce a strict weak ordering betwen pointers.