dynamically allocate memeory for Eigen Vector - c++

Question

I am using the Eigen linear algebra library. I struggling trying to allocate Eigen Vectors in the constructor in a class, and then calling the elements of those vectors.
For example,
#include <Eigen/Dense>
using Eigen::VectorXd;
#include <memory>
using std::unique_ptr;
class Cl
{
public:
unique_ptr<VectorXd> v;
Cl(const int n);
~Cl(void);
}
Cl::Cl(const int n)
{
auto v= unique_ptr<VectorXd>(new VectorXd(n));
}
Cl::~Cl(void)
{
v= nullptr;
}
main(void)
{
Cl cl(10);
/* call an element of v how? */
}
For example, using "cl.v(2)" gives me the compiler error (I am using clang++)
error: type 'unique_ptr<VectorXd>' (aka 'unique_ptr<Matrix<double, Dynamic, 1> >') does
not provide a call operator
while using "cl.(*v)(2)" gives me
error: expected unqualified-id
cout << cl.(*v)(2) << endl;
I am new to c++, so I may be missing something very basic here.

Answer 1

Why are you trying to dynamically allocate the Eigen::VectorXd v; itself? Unless you would like to extend the lifetime of v beyond the lifetime of cl (in which case you would indeed have to do so), I would recommend to follow the following simple example:
#include <Eigen/Dense>
using Eigen::VectorXd;
class Cl
{
public:
VectorXd v;
Cl(int n) : v(n) {}
~Cl() {}
}
int main()
{
Cl cl(10);
for (int i=0; i<10; ++i)
cl.v(i) = i;
}

Answer 2

I believe in addition to the answers already regarding std::vector you are also misusing your 'unique_ptr', if indeed you actually require the use of one (reference ggael answer). Please see below example on the use of unique_ptr:
#include <iostream>
#include <memory>
#include <vector>
class Cl {
public:
std::unique_ptr<std::vector<int>> v;
Cl(const int size, int default_values);
~Cl(void);
int Size();
};
Cl::Cl(const int size, int default_values = 0) {
v.reset(new std::vector<int>(size, default_values));
}
Cl::~Cl(void) {
// do not require to reset/destroy an auto_ptr so this can be ommitted
}
int Cl::Size() {
return v->size();
}
int main(int argc, char* argv[]) {
Cl blob(10);
int size = blob.Size();
std::cout << size << std::endl;
}
In your provided code you're declaring a new auto in the constructor rather than using the variable you've defined in your Public class definition. I've included a 'Size' method so you can see the scope extends beyond the constructor.