doc/examples/make_circulant.cpp.expression

template <class ArgType>
class Circulant : public Eigen::MatrixBase<Circulant<ArgType> > {
 public:
  Circulant(const ArgType& arg) : m_arg(arg) {
    EIGEN_STATIC_ASSERT(ArgType::ColsAtCompileTime == 1, YOU_TRIED_CALLING_A_VECTOR_METHOD_ON_A_MATRIX);
  }

  typedef typename Eigen::internal::ref_selector<Circulant>::type Nested;

  typedef Eigen::Index Index;
  Index rows() const { return m_arg.rows(); }
  Index cols() const { return m_arg.rows(); }

  typedef typename Eigen::internal::ref_selector<ArgType>::type ArgTypeNested;
  ArgTypeNested m_arg;
};