Pointer to Member Function

[rahxephon]

Trying to built a program but meet some problem in it about the pointer to use.

I have created this

double runge_kutta_4th(double(*equation)(double,double), double initial, double t, double dt){
    double k1 = equation(initial, t);
    double k2 = equation(initial + 0.5 * k1 * dt, t + 0.5 *dt);
    double k3 = equation(initial + 0.5 * k2 * dt, t + 0.5 *dt);
    double k4 = equation(initial + k3 * dt, t + dt);
    return initial + (k1 + 2*k2 + 2*k3 + k4)/6 * dt;
}
 

which work fine with a function.

Now I try to implement it with a equation in a function
something like this

class equation{
    public:
        double a;
        double b;
        double c;
 
        equation(double a, double b, double c):
            a(a),
            b(b),
            c(c)
        {
        }
 
        ~equation(){
        }
 
        double equation1(double y, double t){
            return (a) * t -  b* y;
        }
 
        double equation2(double y, double t){
            return 3 * (a) * t -  b* y;
        }
};

A bit stuck on how to pass the equation to the receiving function. Anyone have any idea? Might have to pass equation 1 on some time and pass equation to on another time. Or should i implement the solution in the class rather than calling it externally?

[Banfa]

If you want to pass you class to your function double runge_kutta_4th you need to define you class as a functor. That is a class with the operator() implemented.

You will also need to change the called function a bit. The following is an example of a function that can take a predicate that is either a function or a functor.

#include <iostream>
#include <iomanip>
 
class MySquare
{
public:
    MySquare(int init=1): multiple(init) {}
    ~MySquare() {}
 
    int operator()(int x) const { return multiple * x * x; }
 
private:
    int multiple;
};
 
template< typename PREDICATE >
void print(PREDICATE predicate, int input)
{
    std::cout << std::setw(3) << input << ": " << predicate(input) << std::endl;
}
 
int Sum(int input)
{
    int sum = 0;
 
    for(;input > 0; input--)
    {
        sum += input;
    }
 
    return sum;
}
 
int main()
{
    int ix;
 
    for(ix=0; ix<10; ix++)
    {
        print(Sum, ix);
    }
 
    for(ix=0; ix<10; ix++)
    {
        print(MySquare(5), ix);
    }
}
 

[unauthorized]

If you don't mind passing around an extra pointer, you can also declare a static method with the first parameter being a pointer or reference to your class. A static member still can access private data through a pointer/reference to it's class but the syntax is a little ugly. It goes like this:

class my_class {
public:
     static int my_method(my_class* pthis)
     {
          return pthis->my_var;
     }
private:
     int my_var;
};

You could also use member function pointers like this:

class my_class {
     void my_method(double a, double b) { ... }
};
 
void(class ::*func_ptr)(double, double);
func_ptr = &my_class::my_method;
 
my_class obj;
(obj.*func_ptr)(1.0, 2.0);

[rahxephon]

Thanks for both of the reply. I have make used of the functor in my code. Thanks for the example Banfa.