Eigen Map class : mapping C-array to VectorXd pointer in an efficient way

Question

I need to convert a plain double C-array to a VectorXd*. I found Map class that seems to do the job but there's something I didn't understand.

class A {
  private:
    VectorXd *jpos_;
  public:
    int init( double *v, int len )
    {
      // Here I would like to assign v to jpos_ in an efficient way
      // (something like a pointer assignment without allocation or vector iterations)
      // Note that length of v is len
    }
};

I tried the following but it doesn't work:

int A::init( double *v, int len )
{
  jpos_ = &VectorXd::Map( v, len );
}

or

int A::init( double *v, int len )
{
  jpos_ = &Map<VectorXd>( v, len );
}

What's wrong? Thanks.

Emanuele

Answer 1

If you want to map an array by means of Eigen::Map, your member jpos_ should be of type Eigen::Map:

class A
{
private:
    Eigen::Map<Eigen::VectorXd> jpos_;

public:    
    A(double* v, int len) : jpos_ {v, len} {}
};

If you need to keep the VectorXd, you need to copy the values to the vector. It is a bit strange to see VectorXd*. It's similar like std::vector<double>*. I would recommend not using VectorXd*.

class A
{
private:
    Eigen::VectorXd jpos_;

public:    
    A(double* v, int len) : jpos_ {v}
    {
        for (int i=0; i<len; ++i)
            jpos_(i) = v[i];
    }
};

EDIT:

If you really need VectorXd*, you can allocate one with new (and don't forget to delete it).