visit.example.cpp

visit_example.cpp

// https://www.youtube.com/watch?v=KRUnZa9OBAM
// Generic Programming Without (Writing Your Own) Templates - Tina Ulbrich [ ACCU 2021 ]

#include <cmath>
#include <iostream>
#include <utility>
#include <variant>

using roots = std::variant<std::pair<double, double>, double, std::monostate>;

roots calculate_roots(const double a, const double b, const double c) {
  const auto d = b * b - 4 * a * c;

  if (d > 0.0) {
    const auto p = std::sqrt(d) / (2 * a);
    return std::pair{-b + p, -b - p};
  }

  if (d == 0.0) {
    return -b / (2 * a);
  }

  return std::monostate();
}

template <class... Ts>
struct overload : Ts... {
  using Ts::operator()...;
};

template <class... Ts>
overload(Ts...) -> overload<Ts...>;

int main() {
  std::visit(
      overload{[](const std::pair<double, double>& arg) {
                 std::cout << "2 roots " << arg.first << " " << arg.second
                           << '\n';
               },
               [](const double& arg) { std::cout << "1 root " << arg << '\n'; },
               [](const std::monostate) { std::cout << "no roots found\n"; }},
      calculate_roots(10, -2.0, -5.0));
}