Define custom stepper for dynamic array in boost

How would I whip up a custom stepper for the ODE integrator in boost? I know how to do that for an array whose size is known at compile time. A simple implementation is like this

#include <boost/array.hpp>
#include <boost/numeric/odeint.hpp>

using namespace std;
using namespace boost::numeric::odeint;

typedef boost::array<double, 2> state_type;

template <size_t N>
class euler_stepper {
public:
    typedef double value_type;
    typedef double time_type;
    typedef unsigned short order_type;

    typedef boost::numeric::odeint::stepper_tag stepper_category;

    static order_type order(void) { return(1); }

    template<class System>
    void do_step(System system, state_type &x, time_type t, time_type dt) const {
        state_type der;
        system(x , der);
        for(size_t i=0 ; i<x.size(); i++) {
            x[i] += dt*der[i];
        }
    }
};

struct rhs {
    void operator()(const state_type &x, state_type &dxdt) const {
        for(int i=0; i < x.size(); i++) dxdt[i] = -x[i];
    }
};

struct observer {
    void operator()(const state_type &x , double t) const {
        for(int i=0; i < x.size(); i++) cout << x[i] << '\t';
        cout << endl;
    }
};

int main(int argc , char **argv) {  
    double dt = 0.01;
    state_type x = {{2.0, 1.0}};
    integrate_const(euler_stepper<2>(), rhs(), x, 0.0, 0.1, dt, observer());
    return 0;
}

But how should I change the implementation, if I wanted to work with a state_type like this

typedef vector<complex<double>> state_type;

Basically, I would like to pass the size of the state vector as an argument to main.

Thanks,

Zoltán

Try this:

template < typename State >
class euler_stepper {
public:
    using state_type = State;
    using value_type = typename state_type::value_type;
    using time_type = value_type;
    using order_type = unsigned short;

    using stepper_category = boost::numeric::odeint::stepper_tag;

    static order_type order(void) { return(1); }

    template<class System>
    void do_step( System system , state_type &x , time_type t , time_type dt ) const
    {
        state_type der;
        system(x , der);
        for(size_t i=0 ; i<x.size(); i++) {
            x[i] += dt*der[i];
        }
    }
};

You can also fix the state type for your solver, for example:

class euler_stepper
{
public:
    using state_type = vector< complex< double > >;
    using value_type = double;
    using time_type = double;
    // ...
};