How could I pass std::function as function pointer?

Question

I am trying to write a class template and internally it use a C function (implementation of BFGS optimization, provided by the R environment) with

Answer 1

An alternative solution could be to have the optim class do its magic with two (possibly pure) virtual functions, and then inherit to define a new class Rosen which implements them. This could look like

class optim {
    public:
        // ...

        virtual double fn(int n, double *par, void *ex) = 0;
        virtual void gr(int n, double *par, double *gr, void *ex) = 0;

        void minimize(arma::vec &dpar, void *ex) {
            vmmin(... , &fn, &gr, ...);
            // ...
        }
};

class Rosen : public optim {
    public:
        // ...
        double fn(int n, double *par, void *ex);
        void gr(int n, double *par, double *gr, void *ex);

    private:
        // ...
};

// main.cc    
Rosen obj;
obj.minimize(dpar, ex);