[Help-gsl] Integration with C++

[Ivan Liu]

Hi,

I would like to use the numerical integration routine in a C++ class
environment, and I wrote the following test routine


#include <iostream>
#include <gsl/gsl_integration.h>
using namespace std;

/************************/
// Integrand

/* #(1)

struct f_params{ double a; double b; };

double f (double x, void * params) {
   struct f_params * pList = (struct f_params *) params;
   double a = (pList->a);
   double b = (pList->b);


   return a+b*x;

};
*/
/***************************/

class Testo
{
private:

   gsl_integration_workspace * w;
   gsl_function * pIntegrand;

public:
   Testo():
       w( gsl_integration_workspace_alloc(1000)),
       pIntegrand( new gsl_function() )
       {  ;};

/********************/
/* #(2) */

   struct f_params{ double a; double b; };

   double f (double x, void * params) {
       struct f_params * pList = (struct f_params *) params;
       double a = (pList->a);
       double b = (pList->b);


       return a+b*x;

   };

/********************/

   double Integrate()
   {
       double resIntegral, abserr;
       struct f_params list= {0.0,1.0};

       double x_start=0.0;
       double x_end=1.0;
       int limit=999;
       double epsabs=0.0;
       double epsrel=1e-6;

       pIntegrand->function = &f;
       pIntegrand->params = &list;

       gsl_integration_qag (pIntegrand, x_start, x_end, epsabs, epsrel,
limit, 1,
           w, &resIntegral, &abserr);

       return resIntegral;
   };

};




int main(void)
{
   Testo G;

   cout <<  G.Integrate() << endl;

   return 0;
}



The program doesn't compile and the message is:

test.cpp: In member function 'double Testo::Integrate()':
test.cpp:62: error: ISO C++ forbids taking the address of an unqualified or
parenthesized non-static member function to form a pointer to member
function.  Say '&Testo::f'
test.cpp:62: error: cannot convert 'double (Testo::*)(double, void*)' to
'double (*)(double, void*)' in assignment

If I comment out the section marked #(2) and use #(1) instead, the program
works no problem,
but I'd like to put the integrand into the class.

How do I do that?

thanks,
Ivan Liu