Pointer to member function of an object [duplicate] - c++

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Function pointer to member function
(8 answers)
Closed 5 years ago.
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)))));
}

Answer 1

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);
}
};