C++: "error collect2: error: ld returned 1 exit status"

Question

I am writing a simple program to calculate the derivative of a function, but I always get the error:

collect2: error: ld returned 1 exit status

Here is my program:

#include <iostream>
#include <stdlib.h>
#include <math.h>

using namespace std;

double derivative2(double (fun), double step, double x);
double fun(double);

int main(int argc, char* argv[]) {
    double h = atof(argv[1]);
    double x = sqrt(2);
    cout << derivative2(fun(x), h, x) << endl;
    return 0;
}


double derivative2(double fun(double), double step, double x) {
    return ((fun(x + step) - fun(x))/step);
}


double fun(double x) {
    return atan(x);
}

I have found this post, but it is not useful in my case.

Answer 1

double derivative2(double (fun), double step, double x);

And

double derivative2(double fun(double), double step, double x)

Are different things. In the first declaration fun is double, and in the second fun is double(*)(double) (a pointer to a function).

Because this function calculates a derivative at a point, the right declaration is the one with the function pointer.

Fix:

double derivative2(double fun(double), double step, double x); // 'fun' is a function pointer.

// ...

cout << derivative2(fun, h, x) << endl; // Pass fun as a function pointer.