feat add status and status_or (#4)

Co-authored-by: tqcq <99722391+tqcq@users.noreply.github.com>
Reviewed-on: #4

CMakeLists.txt

@@ -1,6 +1,9 @@
 cmake_minimum_required(VERSION 3.10)
 
-project(ulib LANGUAGES CXX C VERSION 0.1.0)
+project(
+  ulib
+  LANGUAGES CXX C
+  VERSION 0.1.0)
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_STANDARD_REQUIRED ON)
 set(CMAKE_C_EXTENSIONS OFF)
@@ -23,14 +26,16 @@ set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 if(ULIB_SHARED_LIB)
     add_library(${PROJECT_NAME} SHARED "")
 else()
-    add_library(${PROJECT_NAME} STATIC ""
-            src/ulib/system/timer.cpp
+    add_library(${PROJECT_NAME} STATIC "" src/ulib/system/timer.cpp
                                      src/ulib/system/timer.h)
 endif()
-target_sources(${PROJECT_NAME} PRIVATE
-        3party/mongoose/mongoose.c
+target_sources(
+  ${PROJECT_NAME}
+  PRIVATE 3party/mongoose/mongoose.c
           src/ulib/base/location.h
           src/ulib/base/location.cpp
+          src/ulib/status.h
+          src/ulib/status.cpp
           src/ulib/concorrency/barrier.cpp
           src/ulib/concorrency/barrier.h
           src/ulib/concorrency/mutex.cpp
@@ -50,13 +55,12 @@ target_sources(${PROJECT_NAME} PRIVATE
           src/ulib/system/thread_pool.h
           src/ulib/system/thread_pool.cpp
           src/ulib/system/timer.h
-            src/ulib/system/timer.cpp
-)
+          src/ulib/system/timer.cpp)
 
 find_package(Threads REQUIRED)
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
-## CMP0063
+# CMP0063
 if(POLICY CMP0063)
     cmake_policy(SET CMP0063 NEW)
 endif()
@@ -64,9 +68,15 @@ if(POLICY CMP0048)
     cmake_policy(SET CMP0048 NEW)
 endif()
 
-set(FMT_USE_CPP11 OFF CACHE BOOL "Use C++11" FORCE)
-set(FMT_TEST OFF CACHE BOOL "Build tests" FORCE)
-set(FMT_USE_CPP11 OFF CACHE BOOL "Use C++11" FORCE)
+set(FMT_USE_CPP11
+    OFF
+    CACHE BOOL "Use C++11" FORCE)
+set(FMT_TEST
+    OFF
+    CACHE BOOL "Build tests" FORCE)
+set(FMT_USE_CPP11
+    OFF
+    CACHE BOOL "Use C++11" FORCE)
 add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/3party/fmt)
 add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/3party/sqlpp11)
 
@@ -77,24 +87,22 @@ set(BUILD_SHARED_LIBS OFF)
 set(BUILD_OBJECT_LIBS OFF)
 add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/3party/jsoncpp)
 
-target_sources(${PROJECT_NAME} PRIVATE
-        src/ulib/empty.cpp
-        src/ulib/log/logger.cpp
-        src/ulib/log/log.cpp
-        src/ulib/log/level.cpp
-)
-target_link_libraries(${PROJECT_NAME} PUBLIC fmt::fmt jsoncpp_static sqlpp11::sqlpp11)
+target_sources(
+  ${PROJECT_NAME} PRIVATE src/ulib/empty.cpp src/ulib/log/logger.cpp
+                          src/ulib/log/log.cpp src/ulib/log/level.cpp)
+target_link_libraries(${PROJECT_NAME} PUBLIC fmt::fmt jsoncpp_static
+                                             sqlpp11::sqlpp11)
 target_compile_definitions(${PROJECT_NAME} PRIVATE ULIB_LIBRARY_IMPL)
-target_include_directories(${PROJECT_NAME} PUBLIC
-    3party/inja
+target_include_directories(
+  ${PROJECT_NAME}
+  PUBLIC 3party/inja
          3party/mongoose
          3party/nlohmann
          3party/nonstd
          3party/sigslot
          3party/rxcpp/Rx/v2/src
          3party/rxcpp/Ix/CPP/src
-    src
-)
+         src)
 
 install(TARGETS ${PROJECT_NAME} DESTINATION lib)
 

src/ulib/status.cpp

@@ -0,0 +1,253 @@
+// Copyright 2018 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "status.h"
+#include <sstream>
+#include <string>
+#include <unordered_map>
+
+namespace ulib {
+
+namespace {
+std::string
+StatusWhat(Status const &status)
+{
+    std::ostringstream os;
+    os << status;
+    return std::move(os).str();
+}
+}// namespace
+
+std::string
+StatusCodeToString(StatusCode code)
+{
+    switch (code) {
+    case StatusCode::kOk:
+        return "OK";
+    case StatusCode::kCancelled:
+        return "CANCELLED";
+    case StatusCode::kUnknown:
+        return "UNKNOWN";
+    case StatusCode::kInvalidArgument:
+        return "INVALID_ARGUMENT";
+    case StatusCode::kDeadlineExceeded:
+        return "DEADLINE_EXCEEDED";
+    case StatusCode::kNotFound:
+        return "NOT_FOUND";
+    case StatusCode::kAlreadyExists:
+        return "ALREADY_EXISTS";
+    case StatusCode::kPermissionDenied:
+        return "PERMISSION_DENIED";
+    case StatusCode::kUnauthenticated:
+        return "UNAUTHENTICATED";
+    case StatusCode::kResourceExhausted:
+        return "RESOURCE_EXHAUSTED";
+    case StatusCode::kFailedPrecondition:
+        return "FAILED_PRECONDITION";
+    case StatusCode::kAborted:
+        return "ABORTED";
+    case StatusCode::kOutOfRange:
+        return "OUT_OF_RANGE";
+    case StatusCode::kUnimplemented:
+        return "UNIMPLEMENTED";
+    case StatusCode::kInternal:
+        return "INTERNAL";
+    case StatusCode::kUnavailable:
+        return "UNAVAILABLE";
+    case StatusCode::kDataLoss:
+        return "DATA_LOSS";
+    default: {
+        std::stringstream ss;
+        ss << "UNEXPECTED_STATUS_CODE=" << static_cast<int>(code);
+        return ss.str();
+    }
+    }
+}
+
+std::ostream &
+operator<<(std::ostream &os, StatusCode code)
+{
+    return os << StatusCodeToString(code);
+}
+
+bool
+operator==(ErrorInfo const &a, ErrorInfo const &b)
+{
+    return a.reason_ == b.reason_ && a.domain_ == b.domain_
+        && a.metadata_ == b.metadata_;
+}
+
+bool
+operator!=(ErrorInfo const &a, ErrorInfo const &b)
+{
+    return !(a == b);
+}
+
+// Encapsulates the implementation of a non-OK status. OK Statuses are
+// represented by a nullptr Status::impl_, as an optimization for the common
+// case of OK Statuses. This class holds all the data associated with a non-OK
+// Status so we don't have to worry about bloating the common OK case.
+class Status::Impl {
+public:
+    using PayloadType = std::unordered_map<std::string, std::string>;
+
+    explicit Impl(StatusCode code,
+                  std::string message,
+                  ErrorInfo info,
+                  PayloadType payload)
+        : code_(code),
+          message_(std::move(message)),
+          error_info_(std::move(info)),
+          payload_(std::move(payload))
+    {}
+
+    StatusCode code() const { return code_; }
+
+    std::string const &message() const { return message_; }
+
+    ErrorInfo const &error_info() const { return error_info_; }
+
+    PayloadType const &payload() const { return payload_; }
+
+    // Allows mutable access to payload, which is needed in the
+    // `internal::SetPayload()` function.
+    PayloadType &payload() { return payload_; }
+
+    friend inline bool operator==(Impl const &a, Impl const &b)
+    {
+        return a.code_ == b.code_ && a.message_ == b.message_
+            && a.error_info_ == b.error_info_ && a.payload_ == b.payload_;
+    }
+
+    friend inline bool operator!=(Impl const &a, Impl const &b)
+    {
+        return !(a == b);
+    }
+
+private:
+    StatusCode code_;
+    std::string message_;
+    ErrorInfo error_info_;
+    PayloadType payload_;
+};
+
+Status::Status() = default;
+Status::~Status() = default;
+Status::Status(Status &&) noexcept = default;
+Status &Status::operator=(Status &&) noexcept = default;
+
+// Deep copy
+Status::Status(Status const &other)
+    : impl_(other.ok() ? nullptr : new auto(*other.impl_))
+{}
+
+// Deep copy
+Status &
+Status::operator=(Status const &other)
+{
+    impl_.reset(other.ok() ? nullptr : new auto(*other.impl_));
+    return *this;
+}
+
+// OK statuses have an impl_ == nullptr. Non-OK Statuses get an Impl.
+Status::Status(StatusCode code, std::string message, ErrorInfo info)
+    : impl_(code == StatusCode::kOk
+                ? nullptr
+                : new Status::Impl{code, std::move(message), std::move(info),
+                                   Status::Impl::PayloadType()})
+{}
+
+StatusCode
+Status::code() const
+{
+    return impl_ ? impl_->code() : StatusCode::kOk;
+}
+
+std::string const &
+Status::message() const
+{
+    static auto const *const kEmpty = new std::string{};
+    return impl_ ? impl_->message() : *kEmpty;
+}
+
+ErrorInfo const &
+Status::error_info() const
+{
+    static auto const *const kEmpty = new ErrorInfo{};
+    return impl_ ? impl_->error_info() : *kEmpty;
+}
+
+bool
+Status::Equals(Status const &a, Status const &b)
+{
+    return (a.ok() && b.ok()) || (a.impl_ && b.impl_ && *a.impl_ == *b.impl_);
+}
+
+std::ostream &
+operator<<(std::ostream &os, Status const &s)
+{
+    if (s.ok()) return os << StatusCode::kOk;
+    os << s.code() << ": " << s.message();
+    auto const &e = s.error_info();
+    if (e.reason().empty() && e.domain().empty() && e.metadata().empty()) {
+        return os;
+    }
+    os << " error_info={reason=" << e.reason();
+    os << ", domain=" << e.domain();
+    os << ", metadata={";
+    char const *sep = "";
+    for (auto const &item : e.metadata()) {
+        os << sep << item.first << "=" << item.second;
+        sep = ", ";
+    }
+    return os << "}}";
+}
+
+namespace internal {
+
+void
+AddMetadata(ErrorInfo &ei, std::string const &key, std::string value)
+{
+    ei.metadata_[key] = std::move(value);
+}
+
+// Sets the given `payload`, indexed by the given `key`, on the given `Status`,
+// IFF the status is not OK. Payloads are considered in equality comparisons.
+// The keyspace used here is separate from other keyspaces (e.g.,
+// `absl::Status`), so we only need to coordinate keys with ourselves.
+void
+SetPayload(Status &s, std::string key, std::string payload)
+{
+    if (s.impl_) s.impl_->payload()[std::move(key)] = std::move(payload);
+}
+
+// Returns the payload associated with the given `key`, if available.
+ulib::optional<std::string>
+GetPayload(Status const &s, std::string const &key)
+{
+    if (!s.impl_) return ulib::nullopt;
+    auto const &payload = s.impl_->payload();
+    auto it = payload.find(key);
+    if (it == payload.end()) return ulib::nullopt;
+    return it->second;
+}
+
+}// namespace internal
+
+RuntimeStatusError::RuntimeStatusError(Status status)
+    : std::runtime_error(StatusWhat(status)),
+      status_(std::move(status))
+{}
+
+}// namespace ulib

src/ulib/status.h

@@ -0,0 +1,401 @@
+// Copyright 2018 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_STATUS_H
+#define GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_STATUS_H
+
+#include "optional.h"
+#include <iostream>
+#include <memory>
+#include <string>
+#include <unordered_map>
+
+namespace ulib {
+
+/**
+ * Well-known status codes with `grpc::StatusCode`-compatible values.
+ *
+ * The semantics of these values are documented in:
+ *     https://grpc.io/grpc/cpp/classgrpc_1_1_status.html
+ */
+enum class StatusCode {
+    /// Not an error; returned on success.
+    kOk = 0,
+
+    /// `kCancelled` (gRPC code `CANCELLED`) indicates the operation was
+    /// cancelled, typically by the caller.
+    kCancelled = 1,
+
+    /// `kUnknown` (gRPC code `UNKNOWN`) indicates an unknown error occurred.
+    ///
+    /// In general, more specific errors should be raised, if possible. Errors
+    /// raised by APIs that do not return enough error information may be
+    /// converted to this error.
+    kUnknown = 2,
+
+    /// `kInvalidArgument` (gRPC code `INVALID_ARGUMENT`) indicates the caller
+    /// specified an invalid argument, such as a malformed filename.
+    ///
+    /// Note that use of such errors should be narrowly limited to indicate the
+    /// invalid nature of the arguments themselves. Errors with validly formed
+    /// arguments that may cause errors with the state of the receiving system
+    /// should be denoted with `kFailedPrecondition` instead.
+    kInvalidArgument = 3,
+
+    /// `kDeadlineExceeded` (gRPC code `DEADLINE_EXCEEDED`) indicates a deadline
+    /// expired before the operation could complete.
+    ///
+    /// For operations that may change state within a system, this error may be
+    /// returned even if the operation has completed successfully. For example, a
+    /// successful response from a server could have been delayed long enough for
+    /// the deadline to expire.
+    kDeadlineExceeded = 4,
+
+    /// `kNotFound` (gRPC code `NOT_FOUND`) indicates some requested entity (such
+    /// as a file or directory) was not found.
+    ///
+    /// `kNotFound` is useful if a request should be denied for an entire class of
+    /// users, such as during a gradual feature rollout or undocumented allow
+    /// list.
+    /// If a request should be denied for specific sets of users, such as through
+    /// user-based access control, use `kPermissionDenied` instead.
+    kNotFound = 5,
+
+    /// `kAlreadyExists` (gRPC code `ALREADY_EXISTS`) indicates that the entity a
+    /// caller attempted to create (such as a file or directory) is already
+    /// present.
+    kAlreadyExists = 6,
+
+    /// `kPermissionDenied` (gRPC code `PERMISSION_DENIED`) indicates that the
+    /// caller does not have permission to execute the specified operation.
+    ///
+    /// Note that this error is different than an error due to an
+    /// *un*authenticated caller. This error code does not imply the request is
+    /// valid or the requested entity exists or satisfies any other
+    /// pre-conditions.
+    ///
+    /// `kPermissionDenied` must not be used for rejections caused by exhausting
+    /// some resource. Instead, use `kResourceExhausted` for those errors.
+    /// `kPermissionDenied` must not be used if the caller cannot be identified.
+    /// Instead, use `kUnauthenticated` for those errors.
+    kPermissionDenied = 7,
+
+    /// `kResourceExhausted` (gRPC code `RESOURCE_EXHAUSTED`) indicates some
+    /// resource has been exhausted.
+    ///
+    /// Examples include a per-user quota, or the entire file system being out of
+    /// space.
+    kResourceExhausted = 8,
+
+    /// `kFailedPrecondition` (gRPC code `FAILED_PRECONDITION`) indicates that the
+    /// operation was rejected because the system is not in a state required for
+    /// the operation's execution.
+    ///
+    /// For example, a directory to be deleted may be non-empty, a "rmdir"
+    /// operation is applied to a non-directory, etc.
+    ///
+    /// Some guidelines that may help a service implementer in deciding between
+    /// `kFailedPrecondition`, `kAborted`, and `kUnavailable`:
+    ///
+    /// 1. Use `kUnavailable` if the client can retry just the failing call.
+    /// 2. Use `kAborted` if the client should retry at a higher transaction
+    ///    level (such as when a client-specified test-and-set fails, indicating
+    ///    the client should restart a read-modify-write sequence).
+    /// 3. Use `kFailedPrecondition` if the client should not retry until the
+    ///    system state has been explicitly fixed. For example, if a "rmdir" fails
+    ///    because the directory is non-empty, `kFailedPrecondition` should be
+    ///    returned since the client should not retry unless the files are deleted
+    ///    from the directory.
+    kFailedPrecondition = 9,
+
+    /// `kAborted` (gRPC code `ABORTED`) indicates the operation was aborted.
+    ///
+    /// This is typically due to a concurrency issue such as a sequencer check
+    /// failure or a failed transaction.
+    ///
+    /// See the guidelines above for deciding between `kFailedPrecondition`,
+    /// `kAborted`, and `kUnavailable`.
+    kAborted = 10,
+
+    /// `kOutOfRange` (gRPC code `OUT_OF_RANGE`) indicates the operation was
+    /// attempted past the valid range, such as seeking or reading past an
+    /// end-of-file.
+    ///
+    /// Unlike `kInvalidArgument`, this error indicates a problem that may
+    /// be fixed if the system state changes. For example, a 32-bit file
+    /// system will generate `kInvalidArgument` if asked to read at an
+    /// offset that is not in the range [0,2^32-1], but it will generate
+    /// `kOutOfRange` if asked to read from an offset past the current
+    /// file size.
+    ///
+    /// There is a fair bit of overlap between `kFailedPrecondition` and
+    /// `kOutOfRange`.  We recommend using `kOutOfRange` (the more specific
+    /// error) when it applies so that callers who are iterating through
+    /// a space can easily look for an `kOutOfRange` error to detect when
+    /// they are done.
+    kOutOfRange = 11,
+
+    /// `kUnimplemented` (gRPC code `UNIMPLEMENTED`) indicates the operation is
+    /// not implemented or supported in this service.
+    ///
+    /// In this case, the operation should not be re-attempted.
+    kUnimplemented = 12,
+
+    /// `kInternal` (gRPC code `INTERNAL`) indicates an internal error has
+    /// occurred and some invariants expected by the underlying system have not
+    /// been satisfied.
+    ///
+    /// While this error code is reserved for serious errors, some services return
+    /// this error under overload conditions.
+    kInternal = 13,
+
+    /// `kUnavailable` (gRPC code `UNAVAILABLE`) indicates the service is
+    /// currently unavailable and that this is most likely a transient condition.
+    ///
+    /// An error such as this can be corrected by retrying with a backoff scheme.
+    /// Note that it is not always safe to retry non-idempotent operations.
+    ///
+    /// See the guidelines above for deciding between `kFailedPrecondition`,
+    /// `kAborted`, and `kUnavailable`.
+    kUnavailable = 14,
+
+    /// `kDataLoss` (gRPC code `DATA_LOSS`) indicates that unrecoverable data loss
+    /// or corruption has occurred.
+    ///
+    /// As this error is serious, proper alerting should be attached to errors
+    /// such as this.
+    kDataLoss = 15,
+
+    /// `kUnauthenticated` (gRPC code `UNAUTHENTICATED`) indicates that the
+    /// request does not have valid authentication credentials for the operation.
+    ///
+    /// Correct the authentication and try again.
+    kUnauthenticated = 16,
+};
+
+/// Convert @p code to a human readable string.
+std::string StatusCodeToString(StatusCode code);
+
+/// Integration with `std::iostreams`.
+std::ostream &operator<<(std::ostream &os, StatusCode code);
+
+class Status;
+class ErrorInfo;
+
+namespace internal {
+void AddMetadata(ErrorInfo &, std::string const &key, std::string value);
+void SetPayload(Status &, std::string key, std::string payload);
+ulib::optional<std::string> GetPayload(Status const &, std::string const &key);
+}// namespace internal
+
+/**
+ * Describes the cause of the error with structured details.
+ *
+ * @see https://cloud.google.com/apis/design/errors#error_info
+ */
+class ErrorInfo {
+public:
+    /**
+   * Default constructor.
+   *
+   * Post-condition: the `reason()`, `domain()`, and `metadata()` fields are
+   * empty.
+   */
+    ErrorInfo() = default;
+
+    /**
+   * Constructor.
+   *
+   * @param reason initializes the `reason()` value.
+   * @param domain initializes the `domain()` value.
+   * @param metadata initializes the `metadata()` value.
+   */
+    explicit ErrorInfo(std::string reason,
+                       std::string domain,
+                       std::unordered_map<std::string, std::string> metadata)
+        : reason_(std::move(reason)),
+          domain_(std::move(domain)),
+          metadata_(std::move(metadata))
+    {}
+
+    /**
+   * The reason of the error.
+   *
+   * This is a constant value that identifies the proximate cause of the error.
+   * Error reasons are unique within a particular domain of errors. This should
+   * be at most 63 characters and match a regular expression of
+   * `[A-Z][A-Z0-9_]+[A-Z0-9]`, which represents UPPER_SNAKE_CASE.
+   */
+    std::string const &reason() const { return reason_; }
+
+    /**
+   * The logical grouping to which the "reason" belongs.
+   *
+   * The error domain is typically the registered service name of the tool or
+   * product that generates the error. Example: "pubsub.googleapis.com". If the
+   * error is generated by some common infrastructure, the error domain must be
+   * a globally unique value that identifies the infrastructure. For Google API
+   * infrastructure, the error domain is "googleapis.com".
+   *
+   * For errors generated by the C++ client libraries the domain is
+   * `gcloud-cpp`.
+   */
+    std::string const &domain() const { return domain_; }
+
+    /**
+   * Additional structured details about this error.
+   *
+   * Keys should match the regular expression `[a-zA-Z0-9-_]` and be limited
+   * to 64 characters in length.
+   *
+   * When identifying the current value of an exceeded limit, the units should
+   * be contained in the key, not the value.  For example, if the client exceeds
+   * the number of instances that can be created in a single (batch) request
+   * return `{"instanceLimitPerRequest": "100"}` rather than
+   * `{"instanceLimit": "100/request"}`.
+   */
+    std::unordered_map<std::string, std::string> const &metadata() const
+    {
+        return metadata_;
+    }
+
+    friend bool operator==(ErrorInfo const &, ErrorInfo const &);
+    friend bool operator!=(ErrorInfo const &, ErrorInfo const &);
+
+private:
+    friend void internal::AddMetadata(ErrorInfo &,
+                                      std::string const &key,
+                                      std::string value);
+
+    std::string reason_;
+    std::string domain_;
+    std::unordered_map<std::string, std::string> metadata_;
+};
+
+/**
+ * Represents success or an error with info about the error.
+ *
+ * This class is typically used to indicate whether or not a function or other
+ * operation completed successfully. Success is indicated by an "OK" status. OK
+ * statuses will have `.code() == StatusCode::kOk` and `.ok() == true`, with
+ * all other properties having empty values. All OK statuses are equal. Any
+ * non-OK `Status` is considered an error. Users can inspect the error using
+ * the member functions, or they can simply stream the `Status` object, and it
+ * will print itself in some human readable way (the streamed format may change
+ * over time and you should *not* depend on the specific format of a streamed
+ * `Status` object remaining unchanged).
+ *
+ * This is a regular value type that can be copied, moved, compared for
+ * equality, and streamed.
+ */
+class Status {
+public:
+    /// Default constructor, initializes to `StatusCode::kOk`.
+    Status();
+    /// Destructor.
+    ~Status();
+    ///@{
+    /**
+   * @name Copy construction and assignment.
+   */
+    Status(Status const &);
+    Status &operator=(Status const &);
+    ///@}
+    ///@{
+    /**
+   * @name Move construction and assignment.
+   */
+    Status(Status &&) noexcept;
+    Status &operator=(Status &&) noexcept;
+    ///@}
+
+    /**
+   * Construct from a status code, message and (optional) error info.
+   *
+   * @param code the status code for the new `Status`.
+   * @param message the message for the new `Status`, ignored if @p code is
+   *     `StatusCode::kOk`.
+   * @param info the `ErrorInfo` for the new `Status`, ignored if @p code is
+   *     `SStatusCode::kOk`.
+   */
+    explicit Status(StatusCode code, std::string message, ErrorInfo info = {});
+
+    /// Returns true if the status code is `StatusCode::kOk`.
+    bool ok() const { return !impl_; }
+
+    /// Returns the status code.
+    StatusCode code() const;
+
+    /**
+   * Returns the message associated with the status.
+   *
+   * This is always empty if `code()` is `StatusCode::kOk`.
+   */
+    std::string const &message() const;
+
+    /**
+   * Returns the additional error info associated with the status.
+   *
+   * This is always a default-constructed error info if `code()` is
+   * `StatusCode::kOk`.
+   */
+    ErrorInfo const &error_info() const;
+
+    friend inline bool operator==(Status const &a, Status const &b)
+    {
+        return (a.ok() && b.ok()) || Equals(a, b);
+    }
+
+    friend inline bool operator!=(Status const &a, Status const &b)
+    {
+        return !(a == b);
+    }
+
+private:
+    static bool Equals(Status const &a, Status const &b);
+    friend void internal::SetPayload(Status &, std::string, std::string);
+    friend ulib::optional<std::string>
+    internal::GetPayload(Status const &, std::string const &);
+
+    class Impl;
+    // A null `impl_` is an OK status. Only non-OK Statuses allocate an Impl.
+    std::unique_ptr<Impl> impl_;
+};
+
+/**
+ * Stream @p s to @p os.
+ *
+ * This in intended for logging and troubleshooting. Applications should not
+ * depend on the format of this output.
+ */
+std::ostream &operator<<(std::ostream &os, Status const &s);
+
+/**
+ * A runtime error that wraps a `google::cloud::Status`.
+ */
+class RuntimeStatusError : public std::runtime_error {
+public:
+    /// Constructor from a `Status`.
+    explicit RuntimeStatusError(Status status);
+
+    /// Returns the original status.
+    Status const &status() const { return status_; }
+
+private:
+    Status status_;
+};
+
+}// namespace ulib
+
+#endif// GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_STATUS_H

src/ulib/status_or.h

@@ -0,0 +1,355 @@
+// Copyright 2018 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_STATUS_OR_H
+#define GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_STATUS_OR_H
+
+#include "status.h"
+#include <stdexcept>
+#include <type_traits>
+#include <utility>
+
+namespace ulib {
+
+/**
+ * Holds a value or a `Status` indicating why there is no value.
+ *
+ * `StatusOr<T>` represents either a usable `T` value or a `Status` object
+ * explaining why a `T` value is not present. Typical usage of `StatusOr<T>`
+ * looks like usage of a smart pointer, or even a `std::optional<T>`, in that
+ * you first check its validity using a conversion to bool (or by calling
+ * `StatusOr::ok()`), then you may dereference the object to access the
+ * contained value.
+ *
+ * It is undefined behavior (UB) to dereference a `StatusOr<T>` that is not
+ * "ok". For example:
+ *
+ * @code
+ * StatusOr<Foo> foo = FetchFoo();
+ * if (!foo) {  // Same as !foo.ok()
+ *   // handle error and probably look at foo.status()
+ * } else {
+ *   foo->DoSomethingFooey();  // UB if !foo
+ * }
+ * @endcode
+ *
+ * Alternatively, you may call the `StatusOr::value()` member function,
+ * which is defined to: (1) throw an exception if there is no `T` value, or (2)
+ * crash the program if exceptions are disabled. It is never UB to call
+ * `value()`.
+ *
+ * @code
+ * StatusOr<Foo> foo = FetchFoo();
+ * foo.value().DoSomethingFooey();  // May throw/crash if there is no value
+ * @endcode
+ *
+ * Functions that can fail will often return a `StatusOr<T>` instead of
+ * returning an error code and taking a `T` out-param, or rather than directly
+ * returning the `T` and throwing an exception on error. `StatusOr<T>` is used
+ * so that callers can choose whether they want to explicitly check for errors,
+ * crash the program, or throw exceptions.
+ *
+ * Since constructors do not have a return value, they should be designed in
+ * such a way that they cannot fail by moving the object's complex
+ * initialization logic into a separate factory function that itself can return
+ * a `StatusOr<T>`. For example:
+ *
+ * @code
+ * class Bar {
+ *  public:
+ *   Bar(Arg arg);
+ *   ...
+ * };
+ * StatusOr<Bar> MakeBar() {
+ *   ... complicated logic that might fail
+ *   return Bar(std::move(arg));
+ * }
+ * @endcode
+ *
+ * `StatusOr<T>` supports equality comparisons if the underlying type `T` does.
+ *
+ * @tparam T the type of the value.
+ */
+template<typename T>
+class StatusOr final {
+public:
+    static_assert(!std::is_reference<T>::value,
+                  "StatusOr<T> requires T to **not** be a reference type");
+
+    /**
+   * A `value_type` member for use in generic programming.
+   *
+   * This is analogous to that of `std::optional::value_type`.
+   */
+    using value_type = T;
+
+    /**
+   * Initializes with an error status (`StatusCode::kUnknown`).
+   */
+    StatusOr() : StatusOr(MakeDefaultStatus()) {}
+
+    StatusOr(StatusOr const &) = default;
+    StatusOr &operator=(StatusOr const &) = default;
+
+    // NOLINTNEXTLINE(performance-noexcept-move-constructor)
+    StatusOr(StatusOr &&other)
+        : status_(std::move(other.status_)),
+          value_(std::move(other.value_))
+    {
+        other.status_ = MakeDefaultStatus();
+    }
+
+    // NOLINTNEXTLINE(performance-noexcept-move-constructor)
+    StatusOr &operator=(StatusOr &&other)
+    {
+        status_ = std::move(other.status_);
+        value_ = std::move(other.value_);
+        other.status_ = MakeDefaultStatus();
+        return *this;
+    }
+
+    /**
+   * Creates a new `StatusOr<T>` holding the error condition @p rhs.
+   *
+   * @par Post-conditions
+   * `ok() == false` and `status() == rhs`.
+   *
+   * @param rhs the status to initialize the object.
+   * @throws std::invalid_argument if `rhs.ok()`. If exceptions are disabled the
+   *     program terminates via `google::cloud::Terminate()`
+   */
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    StatusOr(Status rhs) : status_(std::move(rhs))
+    {
+        if (status_.ok()) {
+            // google::cloud::internal::ThrowInvalidArgument(__func__);
+            throw std::invalid_argument(__func__);
+        }
+    }
+
+    /**
+   * Assigns the given non-OK Status to this `StatusOr<T>`.
+   *
+   * @throws std::invalid_argument if `status.ok()`. If exceptions are disabled
+   *     the program terminates via `google::cloud::Terminate()`
+   */
+    StatusOr &operator=(Status status)
+    {
+        *this = StatusOr(std::move(status));
+        return *this;
+    }
+
+    /**
+   * Assign a `T` (or anything convertible to `T`) into the `StatusOr`.
+   *
+   * This function does not participate in overload resolution if `U` is equal
+   * to `StatusOr<T>` (or to a cv-ref-qualified `StatusOr<T>`).
+   *
+   * @return a reference to this object.
+   * @tparam U a type convertible to @p T.
+   */
+    template<typename U = T,
+             /// @cond implementation_details
+             std::enable_if_t<!std::is_same<StatusOr, std::decay_t<U>>::value,
+                              int> = 0
+             /// @endcond
+             >
+    StatusOr &operator=(U &&rhs)
+    {
+        status_ = Status();
+        value_ = std::forward<U>(rhs);
+        return *this;
+    }
+
+    /**
+   * Creates a new `StatusOr<T>` holding the value @p rhs.
+   *
+   * @par Post-conditions
+   * `ok() == true` and `value() == rhs`.
+   *
+   * @param rhs the value used to initialize the object.
+   *
+   * @throws ... If `T`'s move constructor throws.
+   */
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    StatusOr(T &&rhs) : value_(std::move(rhs)) {}
+
+    /**
+   * Creates a new `StatusOr<T>` holding the value @p rhs.
+   *
+   * @par Post-conditions
+   * `ok() == true` and `value() == rhs`.
+   *
+   * @param rhs the value used to initialize the object.
+   *
+   * @throws ... If `T` copy constructor throws.
+   */
+    // NOLINTNEXTLINE(google-explicit-constructor)
+    StatusOr(T const &rhs) : value_(rhs) {}
+
+    /// Returns `true` when `this` holds a value.
+    bool ok() const { return status_.ok(); }
+
+    /// Returns `true` when `this` holds a value.
+    explicit operator bool() const { return status_.ok(); }
+
+    ///@{
+    /**
+   * @name Dereference operators.
+   *
+   * @par Pre-conditions
+   * @parblock
+   * `ok() == true`
+   *
+   * @warning Using these operators when `ok() == false` results in undefined
+   *     behavior.
+   * @endparblock
+   *
+   * @return A properly ref and const-qualified reference to the underlying
+   *     value.
+   */
+    T &operator*() & { return *value_; }
+
+    T const &operator*() const & { return *value_; }
+
+    T &&operator*() && { return *std::move(value_); }
+
+    T const &&operator*() const && { return *std::move(value_); }
+
+    ///@}
+
+    ///@{
+    /**
+   * @name Member access operators.
+   *
+   * @par Pre-conditions
+   * @parblock
+   * `ok() == true`
+   *
+   * @warning Using these operators when `ok() == false` results in undefined
+   *     behavior.
+   * @endparblock
+   *
+   * @return A properly ref and const-qualified pointer to the underlying value.
+   */
+    T *operator->() & { return &*value_; }
+
+    T const *operator->() const & { return &*value_; }
+
+    ///@}
+
+    ///@{
+    /**
+   * @name Value accessors.
+   *
+   * @return All these member functions return a (properly ref and
+   *     const-qualified) reference to the underlying value.
+   *
+   * @throws RuntimeStatusError with the contents of `status()` if the object
+   *   does not contain a value, i.e., if `ok() == false`.
+   */
+    T &value() &
+    {
+        CheckHasValue();
+        return **this;
+    }
+
+    T const &value() const &
+    {
+        CheckHasValue();
+        return **this;
+    }
+
+    T &&value() &&
+    {
+        CheckHasValue();
+        return std::move(**this);
+    }
+
+    T const &&value() const &&
+    {
+        CheckHasValue();
+        return std::move(**this);
+    }
+
+    ///@}
+
+    ///@{
+    /**
+   * @name Status accessors.
+   *
+   * @return A reference to the contained `Status`.
+   */
+    Status const &status() const & { return status_; }
+
+    Status &&status() && { return std::move(status_); }
+
+    ///@}
+
+private:
+    static Status MakeDefaultStatus()
+    {
+        return Status{StatusCode::kUnknown, "default"};
+    }
+
+    void CheckHasValue() const &
+    {
+        if (!ok()) {
+            // internal::ThrowStatus(status_);
+            throw RuntimeStatusError(std::move(status_));
+        }
+    }
+
+    // When possible, do not copy the status.
+    void CheckHasValue() &&
+    {
+        if (!ok()) {
+            // internal::ThrowStatus(std::move(status_));
+            throw RuntimeStatusError(std::move(status_));
+        }
+    }
+
+    Status status_;
+    ulib::optional<T> value_;
+};
+
+// Returns true IFF both `StatusOr<T>` objects hold an equal `Status` or an
+// equal instance  of `T`. This function requires that `T` supports equality.
+template<typename T>
+bool
+operator==(StatusOr<T> const &a, StatusOr<T> const &b)
+{
+    if (!a || !b) return a.status() == b.status();
+    return *a == *b;
+}
+
+// Returns true of `a` and `b` are not equal. See `operator==` docs above for
+// the definition of equal.
+template<typename T>
+bool
+operator!=(StatusOr<T> const &a, StatusOr<T> const &b)
+{
+    return !(a == b);
+}
+
+template<typename T>
+StatusOr<T>
+make_status_or(T rhs)
+{
+    return StatusOr<T>(std::move(rhs));
+}
+
+}// namespace ulib
+
+#endif// GOOGLE_CLOUD_CPP_GOOGLE_CLOUD_STATUS_OR_H

tests/CMakeLists.txt

@@ -3,7 +3,8 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_STANDARD_REQUIRED ON)
 
-add_executable(ulib_test
+add_executable(
+  ulib_test
   ulib/base/types_unittest.cpp
   ulib/log/log_unittest.cpp
   ulib/concorrency/mutex_unittest.cpp
@@ -16,10 +17,7 @@ add_executable(ulib_test
   3party/optional/optional_unittest.cpp
   3party/sqlpp11/sqlpp11_unittest.cpp
   ulib/utils/defer_unittest.cpp
-)
-target_link_libraries(ulib_test PRIVATE
-        ulib
-        gtest
-        gtest_main)
+  ulib/status_or_unittest.cpp)
+target_link_libraries(ulib_test PRIVATE ulib gtest gtest_main)
 
 add_test(NAME ulib_test COMMAND ulib_test)

tests/ulib/status_or_unittest.cpp

@@ -0,0 +1,11 @@
+#include <gtest/gtest.h>
+#include <ulib/status_or.h>
+
+TEST(StatusOr, EmptyValue)
+{
+    ulib::StatusOr<int> s;
+    EXPECT_FALSE(s.ok());
+    s = 1;
+    EXPECT_TRUE(s.ok());
+    EXPECT_EQ(s.value(), 1);
+}