header file to SO asnwer http://stackoverflow.com/questions/19450198/calling-gsl-function-inside-a-class-in-a-shared-library

MyClass.h

#include <gsl/gsl_roots.h>
#include <gsl/gsl_interp.h>
#include <gsl/gsl_spline.h>
#include <gsl/gsl_errno.h>
#include <iostream>


// forward definition of param struct
struct gsl_f_pars; 


// Class Definition
// ================

class MyClass {

	private:
		gsl_f_pars *p;

	public: 
		double obj(double x, void * pars);
		double GetSolution( void );
		void setPars( gsl_f_pars * xp ) { p = xp; };
		double getC( void )  ; 

};


// param struct
	struct gsl_f_pars {
		double a;
		double b;
		double c;
		MyClass * pt_MyClass;
	};


	// Wrapper that points to member function
	// Trick: MyClass is an element of the gsl_f_pars struct
	// so we can tease the value of the objective function out
	// of there.
	double gslClassWrapper(double x, void * pp) {
		gsl_f_pars *p = (gsl_f_pars *)pp;
		return p->pt_MyClass->obj(x,p);
	}



// helper function for GSL root finder
// ===================================
  
double find_root(gsl_root_fsolver *RootFinder,  gsl_function * F, double x_lo, double x_hi) {
  gsl_root_fsolver_set (RootFinder, F, x_lo , x_hi );

	//std::cout << "objective function value is " << GSL_FN_EVAL(F,2.0) << std::endl;
	
  int iter = 0,status;
  int maxiter = 100;
  double r;
		printf ("%5s [%9s, %9s] %9s %9s\n",
          "iter", "lower", "upper", "root", 
           "err(est)");
  do {
        iter++;
        status = gsl_root_fsolver_iterate (RootFinder);
        r      = gsl_root_fsolver_root    (RootFinder);
        x_lo   = gsl_root_fsolver_x_lower (RootFinder);
        x_hi   = gsl_root_fsolver_x_upper (RootFinder);
        status = gsl_root_test_interval (x_lo, x_hi,0, 0.0001);


		      printf ("%5d [%.7f, %.7f] %.7f %.7f\n",
              iter, x_lo, x_hi,
              r,  
              x_hi - x_lo);
  } while (status == GSL_CONTINUE && iter < maxiter);
  return(r);
}