Modern C++ idoms and recipes

Make the tests pass by refactoring function differentiate to return std::optional<double> instead of double. Does it help to make use of value_or()?

Refactor the code to use std::optional

#include <functional>
#include <sstream>
#include <string>
// symmetric difference quotient
double differentiate(std::function<double(double)> f, double h, double x)
{
    return (f(x + h) - f(x - h)) / (2 * h);
}
std::string estimated_velocity(std::function<double(double)> distance, double time, double precision)
{
    auto velocity = differentiate(distance, precision, time);
    std::stringstream output;
    output << "Estimated velocity after " << time << " seconds is " << velocity << " m/s.";
    return output.str();
}

Bonus exercise

Replace the type for time with something better than than double.

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