range.hpp

template <typename T>
struct range {
  T _a;
  T _b;

  [[nodiscard]] constexpr explicit range(T a, T b) noexcept : _a{a}, _b{b} {}

  struct end_sentinel {
    T _value;
  };

  struct iter {
    T _value;

    [[nodiscard]] constexpr bool operator!=(const end_sentinel& e) noexcept {
      return _value != e._value;
    }

    constexpr void operator++() noexcept { ++_value; }
    [[nodiscard]] constexpr T operator*() const noexcept { return _value; }
  };

  [[nodiscard]] constexpr iter begin() const noexcept { return {_a}; }
  [[nodiscard]] constexpr end_sentinel end() const noexcept { return {_b}; }
};

int main() {
  int ctr = 0;

  for (const auto i : range{0, 100}) {
    ctr += i;
  }

  return ctr;
}