GSL integration from class

#include <iostream>
#include <gsl/gsl_integration.h>
#include <functional>

class A
{
    public :
        A(const double & _z = 1.0) : z(_z) {;}
        double f( double x ) { return z*x*x ; } 
        double test( double x, void * y ) { return f(x) ; }
        friend double test2( double x, void * y ) ;
    private :
        const double z ;
};

//cplusplus example
struct MyPair {
  double a,b;
  double multiply() {return a*b;}
  };



double test2( double x , void * v ) 
{
    return ((A*)v)->f(x) ;
}


int main(int argc, char *argv[])
{

    //cplusplus example
    MyPair ten_two {10,2};
    // binding members:
    auto bound_member_fn = std::bind (&MyPair::multiply,std::placeholders::_1); // returns x.multiply()
    std::cout << bound_member_fn(ten_two) << '\n';           // 20
    auto bound_member_data = std::bind (&MyPair::a,ten_two); // returns ten_two.a
    std::cout << bound_member_data() << '\n';     
    

    A a(2) ;
    double alpha = 2.0 ;
    double beta = 4.0 ;
    auto f1 = std::bind(&A::f, a, std::placeholders::_1);
    //
    std::cout << f1(alpha) << std::endl ;
    //
    //
    auto f2 = std::bind(&A::test, a, std::placeholders::_1, std::placeholders::_2);
    std::cout << f2(alpha,&beta) << std::endl ;
    //
    double result, error;
    double expected = 2.0 * 1./3. ; 
    gsl_integration_workspace * w = gsl_integration_workspace_alloc (1000);
    gsl_function F;
    //fails the bind
    //F.function = &f2;
    //F.params = &alpha;
    // friend functions
    F.function = &test2;
    F.params = &a;

    //func, min, max, error min, error max, limit, workspace, result, error
    gsl_integration_qags (&F, 0, 1, 0, 1e-7, 1000,
            w, &result, &error); 

    printf ("result          = % .18f\n", result);
    printf ("exact result    = % .18f\n", expected);
    printf ("estimated error = % .18f\n", error);
    printf ("actual error    = % .18f\n", result - expected);
    printf ("intervals =  %d\n", w->size);
    gsl_integration_workspace_free (w);
    return 0;
}