member function pointer using dlib was not declared in this scope

Question

I'm searching to optimize an error of approximation using the dlib library... so let's say I have Point (x,y) and a vector of values, which are used to find the minimum and fit the error locally, so I implemented this class:

#include <iostream>
#include <vector>
#include <cmath>
#include <dlib/optimization.h>
class Point
{
   int _x, _y;
public:
    Point(int x, int y): _x(x), _y(y){}

    double findError(const double& psi)
    {
        auto err = std::erf(_x - psi) - std::erf(_y-psi);
        return std::pow(err,2);
    }
    double optimize(const Point& p, dlib::matrix<double,0,1> &psiValues)
    {
         auto err = p->*findError ;
         dlib::find_min_box_constrained(bfgs_search_strategy(),  
                             objective_delta_stop_strategy(1e-9),  
                             findError, derivative(findError), psiValues, {0.1,0.5,0.9},{0.1,0.4,0.8 });
        return psiValues(0);
    }
};

and this code don't compile so I tried to extract the optimizer in a static class like this:

#include <iostream>
#include <vector>
#include <cmath>
//#include <dlib/optimization.h>
class Point
{
   int _x, _y;
public:
   Point(int x, int y): _x(x), _y(y){}

    double findError(const double& psi)
   {
       auto err = std::erf(_x - psi) - std::erf(_y-psi);
       return std::pow(err,2);
   }
};

class Errorcalculator
{
public:

    static double optimize(const Point& p, std::vector<double> &psiValues)
    {
        auto err = p->*findError ;
        // dlib::find_min_box_constrained(bfgs_search_strategy(),  
        //                     objective_delta_stop_strategy(1e-9),  
        //                     p->*findError, derivative(p->*findError), psiValue, {0.1,0.9 },{0.1 0.8});
        return 0.0;
    }
 };

now I'm getting (the same compile error using dlib function with or without static keyword)

error: 'findError' was not declared in this scope

How is it possible if the point p is declared as parameter? How can I use the dlib function wrapped in the class?

Answer 1

findError is member method, but isn't pointer. For creating pointer to method use next syntax: return value (class_name::* pointer_name)(method arguments); Sample:

class widget
{
public:
    unsigned char foo(long long, double) {}
};
int main()
{
    // declaration of pointer
    unsigned char (widget::*ptr_to_method)(long long, double);
    // setting the pointer on certain method in class (not object!)
    ptr_to_method = &widget::foo;
    // we have some object of widget class
    widget obj{};
    // using the pointer
    unsigned char res = (obj.*ptr_to_method)(10L, 10.0); // arguments
    // if we have obj as widget*, we'll use ->* syntax to call method
}

In this sample we used local pointer, may be you would to use global pointer. Read more, for example, here