Newton's method in C++

newton.cc

#include <iostream>
#include <cmath>
#include <functional>

struct NewtonSolver {
  std::function<float(const float)> f;
  std::function<float(const float)> fprime;
  const float tol;
  NewtonSolver(
    float(& arg_f)(const float), float(& arg_fprime)(const float), const float arg_tol):
    f(arg_f), fprime(arg_fprime), tol(arg_tol) {
  }
  void operator()(float& x) const {
    float old_x = x;
    x -= f(x)/fprime(x);
    std::cout << x << std::endl;
    if (std::abs(x - old_x) < std::abs(x)*tol) {
      return;
    }
    else {
      (*this)(x);
    }
  }
};

float f(const float x) {
  return x*x + 2*x - 3;
}

float fprime(const float x) {
  return 2*x + 2;
}

int main() {
  const float tol = 0.001;
  float x = -1000;
  NewtonSolver newton(f, fprime, tol);
  newton(x);
  std::cout << "Final answer: " << x << std::endl;
  return 0;
}