eigen/doc/examples/CustomizingEigen_Inheritance.cpp

#include <Eigen/Core>
#include <iostream>

class MyVectorType : public Eigen::VectorXd
{
public:
    MyVectorType(void):Eigen::VectorXd() {}

    // This constructor allows you to construct MyVectorType from Eigen expressions
    template<typename OtherDerived>
    MyVectorType(const Eigen::MatrixBase<OtherDerived>& other)
        : Eigen::VectorXd(other)
    { }

    // This method allows you to assign Eigen expressions to MyVectorType
    template<typename OtherDerived>
    MyVectorType& operator=(const Eigen::MatrixBase <OtherDerived>& other)
    {
        this->Eigen::VectorXd::operator=(other);
        return *this;
    }
};

int main()
{
  MyVectorType v = MyVectorType::Ones(4);
  v(2) += 10;
  v = 2 * v;
  std::cout << v.transpose() << std::endl;
}