result_storage.hpp

#ifndef MRS_LIB_CORO_INTERNAL_RESULT_STORAGE_HPP_
#define MRS_LIB_CORO_INTERNAL_RESULT_STORAGE_HPP_
 
 
#include <cstddef>
#include <exception>
#include <variant>
 
 
namespace mrs_lib::coro::internal
{
 
  template <typename T>
  class ResultStorage
  {
  private:
    // Not enum class to allow usage in functions like std::get
    enum State : size_t
    {
      empty = 0,
      value = 1,
      exception = 2,
    };
 
  public:
    constexpr ResultStorage() noexcept : data_()
    {
    }
 
    constexpr void set_value(T&& val) noexcept(std::is_nothrow_move_constructible_v<T>)
    {
      assert(data_.index() == State::empty);
      data_.template emplace<State::value>(std::move(val));
    }
 
    void set_exception(std::exception_ptr eptr) noexcept
    {
      assert(data_.index() == State::empty || data_.valueless_by_exception());
      data_.template emplace<State::exception>(std::move(eptr));
    }
 
    constexpr T get_value() &&
    {
      size_t state = data_.index();
      if (state == State::exception)
      {
        std::rethrow_exception(std::get<State::exception>(data_));
      }
      assert(state == State::value);
      return std::get<State::value>(std::move(data_));
    }
 
  private:
    std::variant<std::monostate, T, std::exception_ptr> data_;
  };
 
} // namespace mrs_lib::coro::internal
 
 
#endif // MRS_LIB_CORO_INTERNAL_RESULT_STORAGE_HPP_