ut0expected.h

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/** @file include/ut0expected.h
 Minimal implementation of C++23 std::expected.
 
 https://en.cppreference.com/w/cpp/utility/expected
 
 ****************************************************************************/
 
#ifndef ut0expected_h
#define ut0expected_h
 
#include <type_traits>
#include <utility>
#include <variant>
 
#include "db0err.h"  // dberr_t
#include "ut0dbg.h"
 
namespace ut {
 
/** C++23 std::unexpected. */
template <class E>
class Unexpected {
 public:
  template <class Err = E>
  constexpr explicit Unexpected(Err &&e) : m_error(std::forward<Err>(e)) {}
 
  constexpr const E &error() const &noexcept { return m_error; }
 
  constexpr E &error() &noexcept { return m_error; }
 
  constexpr const E &&error() const &&noexcept { return std::move(m_error); }
 
  constexpr E &&error() &&noexcept { return std::move(m_error); }
 
 private:
  E m_error;
};
 
template <class E>
Unexpected(E) -> Unexpected<E>;
 
/* Replace with std::expected once it is available in all supported compilers -
the support for C++23 is not enough for some older gcc and clang that we still
support. */
 
/** C++23 std::expected. */
template <class T, class E = dberr_t>
class Expected : public std::variant<T, E> {
 public:
  using value_type = T;
  using error_type = E;
 
 public:
  constexpr Expected(const Expected &other) = default;
 
  constexpr Expected(Expected &&other) noexcept
      : std::variant<T, E>(std::move(other)) {}
 
  template <class U, std::enable_if_t<
                         !std::is_same_v<std::decay_t<U>, Expected>, int> = 0>
  constexpr Expected(U &&u) noexcept
      : std::variant<T, E>(std::in_place_index<0>, std::forward<U>(u)) {}
 
  template <class U>
  constexpr Expected(Unexpected<U> &&u)
      : std::variant<T, E>(std::in_place_index<1>, std::move(u).error()) {}
 
  constexpr Expected &operator=(const Expected &) = default;
  constexpr Expected &operator=(Expected &&) = default;
 
  constexpr const T *operator->() const noexcept {
    ut_a(has_value());
    return &std::get<0>(*this);
  }
 
  constexpr T *operator->() noexcept {
    ut_a(has_value());
    return &std::get<0>(*this);
  }
 
  constexpr const T &operator*() const &noexcept {
    ut_a(has_value());
    return std::get<0>(*this);
  }
 
  constexpr T &operator*() &noexcept {
    ut_a(has_value());
    return std::get<0>(*this);
  }
 
  constexpr const T &&operator*() const &&noexcept {
    ut_a(has_value());
    return std::move(std::get<0>(*this));
  }
 
  constexpr T &&operator*() &&noexcept {
    ut_a(has_value());
    return std::move(std::get<0>(*this));
  }
 
  constexpr explicit operator bool() const noexcept { return has_value(); }
 
  constexpr bool has_value() const noexcept { return this->index() == 0; }
 
  constexpr const T &value() const & {
    ut_a(has_value());
    return std::get<0>(*this);
  }
 
  constexpr T &value() & {
    ut_a(has_value());
    return std::get<0>(*this);
  }
 
  constexpr const T &&value() const && {
    ut_a(has_value());
    return std::move(std::get<0>(*this));
  }
 
  constexpr T &&value() && {
    ut_a(has_value());
    return std::move(std::get<0>(*this));
  }
 
  constexpr const E &error() const &noexcept {
    ut_a(!has_value());
    return std::get<1>(*this);
  }
 
  constexpr E &error() &noexcept {
    ut_a(!has_value());
    return std::get<1>(*this);
  }
 
  constexpr const E &&error() const &&noexcept {
    ut_a(!has_value());
    return std::move(std::get<1>(*this));
  }
 
  constexpr E &&error() &&noexcept {
    ut_a(!has_value());
    return std::move(std::get<1>(*this));
  }
};
 
} /* namespace ut */
 
#endif /* !ut0expected_h */