diff --git a/include/libipc/imp/expected.h b/include/libipc/imp/expected.h
new file mode 100644
index 0000000..e5f7820
--- /dev/null
+++ b/include/libipc/imp/expected.h
@@ -0,0 +1,392 @@
+/**
+ * \file libimp/expected.h
+ * \author mutouyun (orz@orzz.org)
+ * \brief Provides a way to store either of two values.
+ */
+#pragma once
+
+#include <type_traits>
+#include <utility>  // std::exchange
+#include <array>
+#include <memory>   // std::addressof
+#include <cstddef>  // std::nullptr_t
+
+#include "libipc/imp/uninitialized.h"
+#include "libipc/imp/generic.h"
+#include "libipc/imp/byte.h"
+
+namespace ipc {
+
+/**
+ * \brief In-place construction tag for unexpected value in expected.
+ * \see https://en.cppreference.com/w/cpp/utility/expected/unexpect_t
+ */
+struct unexpected_t {
+  explicit unexpected_t() = default;
+};
+constexpr unexpected_t unexpected {};
+
+/**
+ * \class template <typename T, typename E> expected
+ * \brief Provides a way to store either of two values.
+ * \see https://en.cppreference.com/w/cpp/utility/expected
+ * \tparam T - the type of the expected value.
+ * \tparam E - the type of the unexpected value.
+ */
+template <typename T, typename E>
+class expected;
+
+namespace detail_expected {
+
+template <typename T, typename E>
+struct data_union {
+  using const_value_t = typename std::add_const<T>::type;
+  using const_error_t = typename std::add_const<E>::type;
+
+  union {
+    T value_;  ///< the expected value
+    E error_;  ///< the unexpected value
+  };
+
+  data_union(data_union const &) = delete;
+  data_union &operator=(data_union const &) = delete;
+
+  data_union(std::nullptr_t) noexcept {}
+  ~data_union() {}
+
+  template <typename... A>
+  data_union(in_place_t, A &&...args) : value_{std::forward<A>(args)...} {}
+  template <typename... A>
+  data_union(unexpected_t, A &&...args) : error_{std::forward<A>(args)...} {}
+
+  void destruct_value() noexcept { destroy(&value_); }
+  void destruct_error() noexcept { destroy(&error_); }
+
+  const_value_t & value() const &  noexcept { return value_; }
+  T &             value() &        noexcept { return value_; }
+  const_value_t &&value() const && noexcept { return std::move(value_); }
+  T &&            value() &&       noexcept { return std::move(value_); }
+
+  const_error_t & error() const &  noexcept { return error_; }
+  E &             error() &        noexcept { return error_; }
+  const_error_t &&error() const && noexcept { return std::move(error_); }
+  E &&            error() &&       noexcept { return std::move(error_); }
+};
+
+template <typename E>
+struct data_union<void, E> {
+  using const_error_t = typename std::add_const<E>::type;
+
+  alignas(E) std::array<byte, sizeof(E)> error_;  ///< the unexpected value
+
+  data_union(data_union const &) = delete;
+  data_union &operator=(data_union const &) = delete;
+
+  data_union(std::nullptr_t) noexcept {}
+
+  template <typename... A>
+  data_union(in_place_t, A &&...) noexcept {}
+  template <typename... A>
+  data_union(unexpected_t, A &&...args) {
+    construct<E>(&error_, std::forward<A>(args)...);
+  }
+
+  void destruct_value() noexcept {}
+  void destruct_error() noexcept { destroy(reinterpret_cast<E *>(&error_)); }
+
+  const_error_t & error() const &  noexcept { return *reinterpret_cast<E *>(error_.data()); }
+  E &             error() &        noexcept { return *reinterpret_cast<E *>(error_.data()); }
+  const_error_t &&error() const && noexcept { return std::move(*reinterpret_cast<E *>(error_.data())); }
+  E &&            error() &&       noexcept { return std::move(*reinterpret_cast<E *>(error_.data())); }
+};
+
+template <typename T, typename E>
+auto destruct(bool /*has_value*/, data_union<T, E> &/*data*/) noexcept
+  -> typename std::enable_if<std::is_trivially_destructible<T>::value &&
+                             std::is_trivially_destructible<E>::value>::type {
+  // Do nothing.
+}
+
+template <typename T, typename E>
+auto destruct(bool has_value, data_union<T, E> &data) noexcept
+  -> typename std::enable_if<!std::is_trivially_destructible<T>::value &&
+                              std::is_trivially_destructible<E>::value>::type {
+  if (has_value) data.destruct_value();
+}
+
+template <typename T, typename E>
+auto destruct(bool has_value, data_union<T, E> &data) noexcept
+  -> typename std::enable_if< std::is_trivially_destructible<T>::value &&
+                             !std::is_trivially_destructible<E>::value>::type {
+  if (!has_value) data.destruct_error();
+}
+
+template <typename T, typename E>
+auto destruct(bool has_value, data_union<T, E> &data) noexcept
+  -> typename std::enable_if<!std::is_trivially_destructible<T>::value &&
+                             !std::is_trivially_destructible<E>::value>::type {
+  if (has_value) {
+    data.destruct_value();
+  } else {
+    data.destruct_error();
+  }
+}
+
+template <typename S, typename T, typename E>
+struct value_getter : data_union<T, E> {
+  using data_union<T, E>::data_union;
+
+  template <typename U>
+  value_getter(U &&other) : data_union<T, E>(nullptr) {
+    if (other) {
+      construct<data_union<T, E>>(this, in_place, std::forward<U>(other).value());
+    } else {
+      construct<data_union<T, E>>(this, unexpected, std::forward<U>(other).error());
+    }
+  }
+
+  T const *operator->() const noexcept { return std::addressof(this->value()); }
+  T *      operator->()       noexcept { return std::addressof(this->value()); }
+
+  T const & operator*() const &  noexcept { return this->value(); }
+  T &       operator*()       &  noexcept { return this->value(); }
+  T const &&operator*() const && noexcept { return std::move(this->value()); }
+  T &&      operator*()       && noexcept { return std::move(this->value()); }
+
+  template <typename U>
+  T value_or(U &&def) const & {
+    return bool(*static_cast<S *>(this)) ? **this : static_cast<T>(std::forward<U>(def));
+  }
+  template <typename U>
+  T value_or(U &&def) && {
+    return bool(*static_cast<S *>(this)) ? std::move(**this) : static_cast<T>(std::forward<U>(def));
+  }
+
+  template <typename... A>
+  T &emplace(A &&...args) {
+    static_cast<S *>(this)->reconstruct(in_place, std::forward<A>(args)...);
+    return this->value();
+  }
+
+  void swap(S &other) {
+    if (bool(*static_cast<S *>(this)) && bool(other)) {
+      std::swap(this->value(), other.value());
+    } else if (!*static_cast<S *>(this) && !other) {
+      std::swap(this->error(), other.error());
+    } else if (!*static_cast<S *>(this) && bool(other)) {
+      E err(std::move(this->error()));
+      this->emplace(std::move(other.value()));
+      other.reconstruct(unexpected, std::move(err));
+    } else /*if (bool(*this) && !other)*/ {
+      E err(std::move(other.error()));
+      other.emplace(std::move(this->value()));
+      static_cast<S *>(this)->reconstruct(unexpected, std::move(err));
+    }
+  }
+};
+
+template <typename S, typename E>
+struct value_getter<S, void, E> : data_union<void, E> {
+  using data_union<void, E>::data_union;
+
+  template <typename U>
+  value_getter(U &&other) : data_union<void, E>(nullptr) {
+    if (other) {
+      construct<data_union<void, E>>(this, in_place);
+    } else {
+      construct<data_union<void, E>>(this, unexpected, std::forward<U>(other).error());
+    }
+  }
+
+  void emplace() noexcept {
+    static_cast<S *>(this)->reconstruct(in_place);
+  }
+
+  void swap(S &other) {
+    if (bool(*static_cast<S *>(this)) && bool(other)) {
+      return;
+    } else if (!*static_cast<S *>(this) && !other) {
+      std::swap(this->error(), other.error());
+    } else if (!*static_cast<S *>(this) && bool(other)) {
+      E err(std::move(this->error()));
+      this->emplace();
+      other.reconstruct(unexpected, std::move(err));
+    } else /*if (bool(*this) && !other)*/ {
+      E err(std::move(other.error()));
+      other.emplace();
+      static_cast<S *>(this)->reconstruct(unexpected, std::move(err));
+    }
+  }
+};
+
+/**
+ * \brief Define the expected storage.
+ */
+template <typename T, typename E>
+struct storage : value_getter<storage<T, E>, T, E> {
+  using getter_t = value_getter<storage<T, E>, T, E>;
+
+  bool has_value_;
+
+  template <typename... A>
+  storage(in_place_t, A &&...args)
+    : getter_t(in_place, std::forward<A>(args)...)
+    , has_value_(true) {}
+
+  template <typename... A>
+  storage(unexpected_t, A &&...args)
+    : getter_t(unexpected, std::forward<A>(args)...)
+    , has_value_(false) {}
+
+  storage(storage const &other)
+    : getter_t(other)
+    , has_value_(other.has_value_) {}
+
+  storage(storage &&other)
+    : getter_t(std::move(other))
+    /// After construction, has_value() is equal to other.has_value().
+    , has_value_(other.has_value_) {}
+
+  template <typename T_, typename E_>
+  storage(storage<T_, E_> const &other)
+    : getter_t(other)
+    , has_value_(other.has_value_) {}
+
+  template <typename T_, typename E_>
+  storage(storage<T_, E_> &&other)
+    : getter_t(std::move(other))
+    /// After construction, has_value() is equal to other.has_value().
+    , has_value_(other.has_value_) {}
+
+  bool has_value() const noexcept {
+    return has_value_;
+  }
+
+  explicit operator bool() const noexcept {
+    return this->has_value();
+  }
+
+protected:
+  friend getter_t;
+
+  template <typename... A>
+  void reconstruct(A &&...args) {
+    destroy(this);
+    construct<storage>(this, std::forward<A>(args)...);
+  }
+};
+
+/// \brief The invoke forwarding helper.
+
+template <typename F, typename... A>
+auto invoke(F &&f, A &&...args) noexcept(
+     noexcept(std::forward<F>(f)(std::forward<A>(args)...)))
+  -> decltype(std::forward<F>(f)(std::forward<A>(args)...)) {
+  return std::forward<F>(f)(std::forward<A>(args)...);
+}
+
+template <typename F, typename... A>
+auto invoke(F &&f, A &&...args) noexcept(
+     noexcept(std::forward<F>(f)()))
+  -> decltype(std::forward<F>(f)()) {
+  return std::forward<F>(f)();
+}
+
+/// \brief and_then helper.
+
+template <typename E, typename F,
+          typename R = decltype(invoke(std::declval<F>(), *std::declval<E>()))>
+R and_then(E &&exp, F &&f) {
+  static_assert(is_specialized<expected, R>::value, "F must return an `expected`.");
+  return bool(exp) ? invoke(std::forward<F>(f), *std::forward<E>(exp))
+                   : R(unexpected, std::forward<E>(exp).error());
+}
+
+/// \brief or_else helper.
+
+template <typename E, typename F,
+          typename R = decltype(invoke(std::declval<F>(), std::declval<E>().error()))>
+R or_else(E &&exp, F &&f) {
+  static_assert(is_specialized<expected, R>::value, "F must return an `expected`.");
+  return bool(exp) ? std::forward<E>(exp)
+                   : invoke(std::forward<F>(f), std::forward<E>(exp).error());
+}
+
+} // namespace detail_expected
+
+/**
+ * \class template <typename T, typename E> expected
+ * \brief Provides a way to store either of two values.
+ */
+template <typename T, typename E>
+class expected : public detail_expected::storage<typename std::remove_cv<T>::type, E> {
+public:
+  using value_type = typename std::remove_cv<T>::type;
+  using error_type = E;
+
+  using detail_expected::storage<value_type, E>::storage;
+
+  expected(expected const &) = default;
+  expected(expected &&) = default;
+
+  expected()
+    : detail_expected::storage<value_type, E>(in_place) {}
+
+  expected &operator=(expected other) {
+    this->swap(other);
+    return *this;
+  }
+
+  // Monadic operations
+
+  template <typename F>
+  auto and_then(F &&f) & {
+    return detail_expected::and_then(*this, std::forward<F>(f));
+  }
+
+  template <typename F>
+  auto and_then(F &&f) const & {
+    return detail_expected::and_then(*this, std::forward<F>(f));
+  }
+
+  template <typename F>
+  auto and_then(F &&f) && {
+    return detail_expected::and_then(std::move(*this), std::forward<F>(f));
+  }
+
+  template <typename F>
+  auto or_else(F &&f) & {
+    return detail_expected::or_else(*this, std::forward<F>(f));
+  }
+
+  template <typename F>
+  auto or_else(F &&f) const & {
+    return detail_expected::or_else(*this, std::forward<F>(f));
+  }
+
+  template <typename F>
+  auto or_else(F &&f) && {
+    return detail_expected::or_else(std::move(*this), std::forward<F>(f));
+  }
+};
+
+// Compares
+
+template <typename T1, typename E1, typename T2, typename E2>
+bool operator==(expected<T1, E1> const &lhs, expected<T2, E2> const &rhs) {
+  return (lhs.has_value() == rhs.has_value())
+      && (lhs.has_value() ? *lhs == *rhs : lhs.error() == rhs.error());
+}
+
+template <typename E1, typename E2>
+bool operator==(expected<void, E1> const &lhs, expected<void, E2> const &rhs) {
+  return (lhs.has_value() == rhs.has_value())
+      && (lhs.has_value() || lhs.error() == rhs.error());
+}
+
+template <typename T1, typename E1, typename T2, typename E2>
+bool operator!=(expected<T1, E1> const &lhs, expected<T2, E2> const &rhs) {
+  return !(lhs == rhs);
+}
+
+} // namespace ipc
diff --git a/test/imp/test_imp_expected.cpp b/test/imp/test_imp_expected.cpp
new file mode 100644
index 0000000..cb96e54
--- /dev/null
+++ b/test/imp/test_imp_expected.cpp
@@ -0,0 +1,142 @@
+
+#include "test.h"
+
+#include "libipc/imp/expected.h"
+
+namespace {
+
+class test_val {
+public:
+  int val = 0;
+  int dc_ = 0;
+  int cc_ = 0;
+  int mv_ = 0;
+
+  test_val() {
+    dc_ += 1;
+    std::printf("test_val construct.\n");
+  }
+  test_val(test_val const &o)
+    : val(o.val) {
+    cc_ = o.cc_ + 1;
+    std::printf("test_val copy construct.\n");
+  }
+  test_val(test_val &&o) noexcept
+    : val(std::exchange(o.val, 0)) {
+    mv_ = o.mv_ + 1;
+    std::printf("test_val move construct.\n");
+  }
+  ~test_val() {
+    std::printf("test_val destruct.\n");
+  }
+
+  test_val &operator=(test_val &&o) noexcept {
+    mv_ = o.mv_ + 1;
+    val = std::exchange(o.val, 0);
+    return *this;
+  }
+
+  test_val(int v)
+    : val(v) {
+    std::printf("test_val value initialization.\n");
+  }
+
+  bool operator==(test_val const &rhs) const {
+    return val == rhs.val;
+  }
+};
+
+class test_err {
+public:
+  int dc_ = 0;
+  int cc_ = 0;
+  int mv_ = 0;
+  std::int64_t val = 0;
+
+  test_err() {
+    dc_ += 1;
+    std::printf("test_err construct.\n");
+  }
+  test_err(test_err const &o)
+    : val(o.val) {
+    cc_ = o.cc_ + 1;
+    std::printf("test_err copy construct.\n");
+  }
+  test_err(test_err &&o) noexcept
+    : val(std::exchange(o.val, 0)) {
+    mv_ = o.mv_ + 1;
+    std::printf("test_err move construct.\n");
+  }
+  ~test_err() {
+    std::printf("test_err destruct.\n");
+  }
+
+  test_err &operator=(test_err &&o) noexcept {
+    mv_ = o.mv_ + 1;
+    val = std::exchange(o.val, 0);
+    return *this;
+  }
+
+  test_err(int v)
+    : val(v) {
+    std::printf("test_err value initialization.\n");
+  }
+
+  bool operator==(test_err const &rhs) const {
+    return val == rhs.val;
+  }
+};
+
+} // namespace
+
+TEST(expected, in_place) {
+  ipc::expected<test_val, test_err> e1;
+  EXPECT_TRUE(e1);
+  EXPECT_EQ(e1.value().dc_, 1);
+  EXPECT_EQ(e1.value().val, 0);
+
+  ipc::expected<test_val, test_err> e2 {ipc::in_place, 123};
+  EXPECT_TRUE(e2);
+  EXPECT_EQ(e2.value().dc_, 0);
+  EXPECT_EQ(e2.value().val, 123);
+}
+
+TEST(expected, unexpected) {
+  ipc::expected<test_val, test_err> e1 {ipc::unexpected};
+  EXPECT_FALSE(e1);
+  EXPECT_EQ(e1.error().dc_, 1);
+  EXPECT_EQ(e1.error().val, 0);
+
+  ipc::expected<test_val, test_err> e2 {ipc::unexpected, 321};
+  EXPECT_FALSE(e2);
+  EXPECT_EQ(e2.error().dc_, 0);
+  EXPECT_EQ(e2.error().val, 321);
+}
+
+TEST(expected, copy_and_move) {
+  ipc::expected<test_val, test_err> e1 {ipc::in_place, 123};
+  ipc::expected<test_val, test_err> e2 {e1};
+  EXPECT_TRUE(e1);
+  EXPECT_TRUE(e2);
+  EXPECT_EQ(e1, e2);
+  EXPECT_EQ(e2.value().cc_, 1);
+  EXPECT_EQ(e2.value().val, 123);
+
+  ipc::expected<test_val, test_err> e3 {ipc::unexpected, 333};
+  ipc::expected<test_val, test_err> e4 {e3};
+  EXPECT_FALSE(e3);
+  EXPECT_FALSE(e4);
+  EXPECT_EQ(e3, e4);
+  EXPECT_EQ(e4.error().cc_, 1);
+  EXPECT_EQ(e4.error().val, 333);
+
+  ipc::expected<test_val, test_err> e5;
+  e5 = e1;
+  EXPECT_EQ(e1, e5);
+  EXPECT_EQ(e5.value().val, 123);
+
+  ipc::expected<test_val, test_err> e6;
+  e6 = std::move(e5);
+  EXPECT_EQ(e1, e6);
+  EXPECT_EQ(e5.value().val, 0);
+}