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I would like to integrate a function with gsl. Therefor I have to define a function f (the integrant, which has to be of the form double (*)(double, void*)). For the call of the gsl integration method I need to define a struct, which contains a pointer to a function (this struct is called gsl_function).

gsl_function F; F.function = &MyClass::my_f;

The function f must be implemented in a class (in the same class from which the integration procedure should be called). How can I assign the pointer above correctly, since the 2nd line is not compiling and leads to the error:

cannot convert ‘double (MyClass::*)(double, void*)’ to ‘double (*)(double, void*)’ in assignment.

Here the definition of my_f

 struct my_f_params { double a; double b;}; double my_f (double x, void * p) { struct my_f_params * params = (struct my_f_params *)p; double a = (params->a); double b = (params->b); return 1.0/(sqrt(a * (1.0 + x)*(1.0 + x)*(1.0 + x) + (1-a) * std::pow((1.0 + x), (3.0 * (1.0 + b))))); }
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  • You'll need to provide a static member function as callback there. I suppose the void* parameter can be (mis-)used to pass your this pointer as necessary. Commented Feb 11, 2017 at 16:02
  • Pointers to members are not normal pointers. You will have to wrap it inside a flat function. There are many examples of this technique on SO. Commented Feb 11, 2017 at 16:03
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    Think about the void* parameter of gsl_function::function. What could it be possibly used for? Commented Feb 11, 2017 at 16:12

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which has to be of the form double (*)(double, void*)

Non static member function declarations involve the implicit call scope qualifier as stated in the error message

double (MyClass::*)(double, void*) // ^^^^^^^^^

This is different from the callback function pointer definition.

What you probably can do with such interface, is to pass the this pointer through the void* argument of the callback function:

class MyClass { static double func(double d,void* thisPtr) { MyClass* myClass = (MyClass*)thisPtr; // do something } };

As mentioned in the documentation you can set the params like that in a wrapper class:

class gsl_function_wrapper { public: gsl_function_wrapper() { F.function = &func; F.params = this; } private: gsl_function F; double a; double b; static double func(double d,void* thisPtr) { gsl_function_wrapper* myWrapper = (gsl_function_wrapper*)thisPtr; // do something with a and b foo(d,myWrapper->a,myWrapper->b); } };
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Please provide correct citation to where you got this solution (it was not on the GSL website). Possible correct source - stackoverflow.com/a/18413206/2472169

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