gsl_odeiv2 in c++ class: Template wrapper for int(

Question

I am currently using gsl_odeiv2 methods inside my classes to solve differential equations. But because of the well known memberfunction problem I can not define my ode-system inside the class. I am currently using a workaround: I define my ode in a global namespace:

ODE.hpp:
#include "EoS.hpp"

#include <gsl/gsl_math.h>
#include <gsl/gsl_errno.h>

namespace ODEs
{
    struct tov_eq_params {EoS *eos;};
    int tov_eq(double, const double *, double *, void *);
}

ODE.cpp:
namespace ODEs {
    int tov_eq(double r, const double *PM, double *dPdM, void *p) {
        struct tov_eq_params * params = (struct tov_eq_params *)p;
        EoS *eos = (params->eos);
        ...
    return GSL_SUCCESS
    }
}

and use a pointer to an object of a coustom type (class EoS) as parameter. Inside the class that solves the ode I use:

...
struct tov_eq_params comp_tov_params = {(this->star_eos)};
gsl_odeiv2_evolve *comp_tov_evolve = gsl_odeiv2_evolve_alloc(3);
gsl_odeiv2_system comp_tov_system = {tov_eq, NULL, 3,&comp_tov_params};
...

to initalise my system. This works fine but is a bit messy because I need to declare my differential equations in a global namespace.

I know that it is possible to use template wrappers for gsl_functions stackoverflow.com/questions/.../how-to-avoid-static-member-function-when-using-gsl-with-c/... to use them in C++ Classes. I actually use the wrapper described there to define functions for gsl_integration Methods within my classes and it works perfectly and is much cleaner and less code to write. For example: I can use my star_eos Object from above direcly inside the function:

  auto dBf = [=](double r)->double{
        return 4 * M_PI * gsl_pow_2(r) * (this->star_eos)->nbar(this->P(r)) * sqrt(this->expLambda(r))* 1e54;
    };
    gsl_function_pp<decltype(dBf)> dBfp(dBf);
    gsl_function *dB = static_cast<gsl_function*>(&dBfp);

I tried to write such a template wrapper for the int(double r, const double *PM, double *dPdM, void *p) functions that gsl_odeiv2_system needs but I failed because I am new to C++ and did not fully understand its template/static_cast mechanisms.

Is there someone who uses gsl_odeiv methods and its ode systems with a template wrapper? Or can someone come up with a template similar to the one described above for gsl_functions but for the int(...) ode.

Answer 1

Thinking about how I got it working with the differential equations set in a global namespace I found a solution for my problem. I now have a working wrapper. In a global namespace I have the following:

//gsl_wrapper.hpp
#include <iostream>
#include <vector>
#include <functional>

#include <gsl/gsl_math.h>
#include <gsl/gsl_errno.h>

namespace gsl_wrapper {

    class ode_System{
    public:
        ode_System(int);
        int dim;
        std::function<double (double, const double *, double *, int)> *df;

    };

    struct ode_struct {ode_System *ode;};
    int ode(double, const double *, double *, void *);
}

//gsl_wrapper.cpp
#include "../../include/gsl_wrapper.hpp"

namespace gsl_wrapper {

    ode_System::ode_System(int dim) {
        this->dim=dim;
    }

    int ode(double r, const double *f, double *df, void *p) {
        struct ode_struct * params = (struct ode_struct *)p;
        ode_System *odeFunc = (params->ode);

        int dim = odeFunc->dim;
        std::function<double (double, const double *, double *, int)> dfeq=*(odeFunc->df);

        for(int i=0;i<dim;i++){
            df[i] = dfeq(r,f,df,i);
        }

        return GSL_SUCCESS;
    }

};

So I bassically have all my specific information stored in my new class ode_System, which has an int dim to specify the systems dimensions and a pointer so a std::function object. This object represents the mathematica differential equation system.

Inside my class, where I want to solve a differential equation using gsl_odeiv2, I define that system using a lambda function:

std::function<double (double, const double *, double *, int)> dPMeq = [=](double r , const double * PM, double *dPdM, int i)->double{
    double df;
    switch ( i )
    {
        case 0:
            df = ... // df_1/dr
            break;
        case 1: 
            df = ... // df_2/dr
            break;
        case 2: 
            df = ... // df_3/dr
            break;
        default:
            GSL_ERROR_VAL ("comp_tov_eq:", GSL_FAILURE,GSL_FAILURE);
            df = 0;
    }
    return df;
};

The above system represents a coupled system of 3 differential equations. I then declare a ode_System object with the right dimension and set its function pointer df to my defined system. Then I only need a structure with a reference to that system and done: I can use my differential equation defined inside my class with gsl_odeiv2_system:

 ode_System tov(3);
 tov.df= &dPMeq;
 struct ode_struct comp_tov_params = {&tov};
 gsl_odeiv2_evolve *comp_tov_evolve = gsl_odeiv2_evolve_alloc(3);
 gsl_odeiv2_system comp_tov_system = {ode, NULL, 3, &comp_tov_params};
 ...

As far as I can tell this works just as well (or bad) as the implementation I presented in my question. It can use some clean up but in principle this works fine for me.

But if someone knows a better way to do this please feel free to share it!