Applying callback to call gsl_integration - c++

Question

I am a newbie in c++ and I am trying to modify a code and use gsl_integration library of c in a class called Cosmology. In order to assign member functions to to form a pointer for gsl, I used callback procedure I found by looking up in internet
Update: Cosmology.h
#include <cmath>
#include <gsl/gsl_integration.h>
struct CCallbackHolder
{
Cosmology* cls;
void* data;
};
class Cosmology {
private:
static const double c = 299792458.0, Mpc2Km = 3.08567758e+19, Yrs2Sec = 3.15569e7;
double H0 = 67.77, OmegaM = (0.022161+0.11889)/(H0*H0), OmegaL = 0.6914, OmegaG = 8.24e-5, OmegaK = 0.0009;
double Ez(double z);
double Hz(double z, void* params);
static double CCallback(double z,void* param)
{
CCallbackHolder* h = static_cast<CCallbackHolder*>(param);
return h->cls->Hz(h->data);
}
public:
double distH, timeH;
Cosmology();
Cosmology(double);
Cosmology(double , double );
Cosmology(double , double , double );
Cosmology(double , double , double , double );
Cosmology(double , double , double , double , double );
double distC(double);
} cosmo;
Cosmology.cpp
#include <cmath>
#include <gsl/gsl_integration.h>
#include "Cosmology.h"
#include<iostream>
using namespace std;
double Cosmology::Hz(double z, void* params) {
double result = 1.0/pow(OmegaL + pow(1.0+z,3.0)*OmegaM + pow(1.0+z,4.0)*OmegaG + pow(1.0+z,2.0)*OmegaK, 0.5);
return result;
}
double Cosmology::distC(double z) {
double lower_limit = 0.0, abs_error = 1.0e-8, rel_error = 1.0e-8, alpha = 0.0, result, error;
gsl_integration_workspace *work_ptr = gsl_integration_workspace_alloc(1000);
gsl_function Hz_function;
void* params_ptr = α
Hz_function.function = &Cosmology::CCallback;
Hz_function.params = params_ptr;
gsl_integration_qags(&Hz_function, lower_limit, z, abs_error, rel_error, 1000, work_ptr, &result, &error);
return distH*result;
}
using namespace std;
int main () {
Cosmology cosmo;
cout << "Comoving Distance: " << cosmo.distC (0.3);
return 0;
}
I am getting the following errors when I tried to compile the code:
Cosmology.h:10: error: ISO C++ forbids declaration of ‘Cosmology’ with no type
Cosmology.h:10: error: expected ‘;’ before ‘*’ token
Cosmology.h:16: error: ISO C++ forbids declaration of ‘constexpr’ with no type
Cosmology.h:16: error: expected ‘;’ before ‘double’
Cosmology.h:17: error: ISO C++ forbids declaration of ‘constexpr’ with no type
Cosmology.h:17: error: expected ‘;’ before ‘double’
In file included from Universe.cpp:3:
Cosmology.h: In static member function ‘static double Cosmology::CCallback(double, void*)’:
by compiling with this line : g++ -Wall -pedantic Cosmology.cpp -o Cosmology std=c++0x. How could I fix the code?

Answer 1

The main problem with your program is that you initialize non-static variables that are class members while declaring them, which is not a legal C++ operation. To be specific, the problem are these lines:
static constexpr double c = 299792458.0, Mpc2Km = 3.08567758e+19, Yrs2Sec = 3.15569e7;
double H0 = 67.77, OmegaM = (0.022161+0.11889)/(H0*H0), OmegaL = 0.6914, OmegaG = 8.24e-5, OmegaK = 0.0009;
c, Mpc2Km, Yrs2Sec, H0, OmegaM, OmegaL, OmegaG, OmegaK are non-static, non-const and double. If you want to initialize them (i.e. assign values to them), you have to do it within the Cosmology class constructor or move them outside of the class declaration and thus make them global, but note that you should avoid global variables unless you know what you do and there's no other way.
If you want to initialize them in the header file like you tried to do, they must be of integral type - won't work for doubles.
Shortly: the first line won't work because the type is double (only integral static const types can be initialized inside class declaration); the second won't work since the type is non-const, non-static and double.
The rest of the errors will probably vanish as soon as these lines are fixed.