Add a functional-y Result<T, E> class and tests for it.

Implements two optimized versions of Result, one for Result<void, E*> as
a nullable pointer, and one for Result<T*, E*> as a tagged pointer.

src/common/CMakeLists.txt

@@ -25,6 +25,7 @@ list(APPEND COMMON_SOURCES
     ${COMMON_DIR}/Math.cpp
     ${COMMON_DIR}/Math.h
     ${COMMON_DIR}/Platform.h
+    ${COMMON_DIR}/Result.h
     ${COMMON_DIR}/Serial.h
     ${COMMON_DIR}/SerialQueue.h
     ${COMMON_DIR}/SwapChainUtils.h

src/common/Result.h

@@ -0,0 +1,250 @@
+// Copyright 2018 The NXT Authors
+//
+// 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
+//
+//     http://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 COMMON_RESULT_H_
+#define COMMON_RESULT_H_
+
+#include "common/Assert.h"
+#include "common/Compiler.h"
+
+#include <cstddef>
+#include <cstdint>
+
+// Result<T, E> is the following sum type (Haskell notation):
+//
+//      data Result T E = Success T | Error E | Empty
+//
+// It is meant to be used as the return type of functions that might fail. The reason for the Empty
+// case is that a Result should never be discarded, only destructured (its error or success moved
+// out) or moved into a different Result. The Empty case tags Results that have been moved out and
+// Result's destructor should ASSERT on it being Empty.
+//
+// Since C++ doesn't have efficient sum types for the special cases we care about, we provide
+// template specializations for them.
+
+template <typename T, typename E>
+class Result;
+
+// The interface of Result<T, E> shoud look like the following.
+//  public:
+//    Result(T success);
+//    Result(E error);
+//
+//    Result(Result<T, E>&& other);
+//    Result<T, E>&& operator=(Result<T, E>&& other);
+//
+//    ~Result();
+//
+//    bool IsError() const;
+//    bool IsSuccess() const;
+//
+//    T&& AcquireSuccess();
+//    E&& AcquireError();
+
+// Specialization of Result for returning errors only via pointers. It is basically a pointer
+// where nullptr is both Success and Empty.
+template <typename E>
+class NXT_NO_DISCARD Result<void, E*> {
+  public:
+    Result();
+    Result(E* error);
+
+    Result(Result<void, E*>&& other);
+    Result<void, E*>& operator=(Result<void, E>&& other);
+
+    ~Result();
+
+    bool IsError() const;
+    bool IsSuccess() const;
+
+    void AcquireSuccess();
+    E* AcquireError();
+
+  private:
+    E* mError = nullptr;
+};
+
+// Uses SFINAE to try to get alignof(T) but fallback to Default if T isn't defined.
+template <typename T, size_t Default, typename = size_t>
+constexpr size_t alignof_if_defined_else_default = Default;
+
+template <typename T, size_t Default>
+constexpr size_t alignof_if_defined_else_default<T, Default, decltype(alignof(T))> = alignof(T);
+
+// Specialization of Result when both the error an success are pointers. It is implemented as a
+// tagged pointer. The tag for Success is 0 so that returning the value is fastest.
+template <typename T, typename E>
+class NXT_NO_DISCARD Result<T*, E*> {
+  public:
+    static_assert(alignof_if_defined_else_default<T, 4> >= 4,
+                  "Result<T*, E*> reserves two bits for tagging pointers");
+    static_assert(alignof_if_defined_else_default<E, 4> >= 4,
+                  "Result<T*, E*> reserves two bits for tagging pointers");
+
+    Result(T* success);
+    Result(E* error);
+
+    Result(Result<T*, E*>&& other);
+    Result<T*, E*>& operator=(Result<T*, E>&& other);
+
+    ~Result();
+
+    bool IsError() const;
+    bool IsSuccess() const;
+
+    T* AcquireSuccess();
+    E* AcquireError();
+
+  private:
+    enum PayloadType {
+        Success = 0,
+        Error = 1,
+        Empty = 2,
+    };
+
+    // Utility functions to manipulate the tagged pointer some of them don't need to be templated
+    // but we really want them inlined so we keep them in templates
+    static intptr_t MakePayload(void* pointer, PayloadType type);
+    static PayloadType GetPayloadType(intptr_t payload);
+    static T* GetSuccessFromPayload(intptr_t payload);
+    static E* GetErrorFromPayload(intptr_t payload);
+
+    constexpr static intptr_t kEmptyPayload = Empty;
+    intptr_t mPayload = kEmptyPayload;
+};
+
+// Implementation of Result<void, E*>
+template <typename E>
+Result<void, E*>::Result() {
+}
+
+template <typename E>
+Result<void, E*>::Result(E* error) : mError(error) {
+}
+
+template <typename E>
+Result<void, E*>::Result(Result<void, E*>&& other) : mError(other.mError) {
+    other.mError = nullptr;
+}
+
+template <typename E>
+Result<void, E*>& Result<void, E*>::operator=(Result<void, E>&& other) {
+    ASSERT(mError == nullptr);
+    mError = other.mError;
+    other.mError = nullptr;
+    return *this;
+}
+
+template <typename E>
+Result<void, E*>::~Result() {
+    ASSERT(mError == nullptr);
+}
+
+template <typename E>
+bool Result<void, E*>::IsError() const {
+    return mError != nullptr;
+}
+
+template <typename E>
+bool Result<void, E*>::IsSuccess() const {
+    return mError == nullptr;
+}
+
+template <typename E>
+void Result<void, E*>::AcquireSuccess() {
+}
+
+template <typename E>
+E* Result<void, E*>::AcquireError() {
+    E* error = mError;
+    mError = nullptr;
+    return error;
+}
+
+// Implementation of Result<T*, E*>
+template <typename T, typename E>
+Result<T*, E*>::Result(T* success) : mPayload(MakePayload(success, Success)) {
+}
+
+template <typename T, typename E>
+Result<T*, E*>::Result(E* error) : mPayload(MakePayload(error, Error)) {
+}
+
+template <typename T, typename E>
+Result<T*, E*>::Result(Result<T*, E*>&& other) : mPayload(other.mPayload) {
+    other.mPayload = kEmptyPayload;
+}
+
+template <typename T, typename E>
+Result<T*, E*>& Result<T*, E*>::operator=(Result<T*, E>&& other) {
+    ASSERT(mPayload == kEmptyPayload);
+    mPayload = other.mPayload;
+    other.mPayload = kEmptyPayload;
+    return *this;
+}
+
+template <typename T, typename E>
+Result<T*, E*>::~Result() {
+    ASSERT(mPayload == kEmptyPayload);
+}
+
+template <typename T, typename E>
+bool Result<T*, E*>::IsError() const {
+    return GetPayloadType(mPayload) == Error;
+}
+
+template <typename T, typename E>
+bool Result<T*, E*>::IsSuccess() const {
+    return GetPayloadType(mPayload) == Success;
+}
+
+template <typename T, typename E>
+T* Result<T*, E*>::AcquireSuccess() {
+    T* success = GetSuccessFromPayload(mPayload);
+    mPayload = kEmptyPayload;
+    return success;
+}
+
+template <typename T, typename E>
+E* Result<T*, E*>::AcquireError() {
+    E* error = GetErrorFromPayload(mPayload);
+    mPayload = kEmptyPayload;
+    return error;
+}
+
+template <typename T, typename E>
+intptr_t Result<T*, E*>::MakePayload(void* pointer, PayloadType type) {
+    intptr_t payload = reinterpret_cast<intptr_t>(pointer);
+    ASSERT((payload & 3) == 0);
+    return payload | type;
+}
+
+template <typename T, typename E>
+typename Result<T*, E*>::PayloadType Result<T*, E*>::GetPayloadType(intptr_t payload) {
+    return static_cast<PayloadType>(payload & 3);
+}
+
+template <typename T, typename E>
+T* Result<T*, E*>::GetSuccessFromPayload(intptr_t payload) {
+    ASSERT(GetPayloadType(payload) == Success);
+    return reinterpret_cast<T*>(payload);
+}
+
+template <typename T, typename E>
+E* Result<T*, E*>::GetErrorFromPayload(intptr_t payload) {
+    ASSERT(GetPayloadType(payload) == Error);
+    return reinterpret_cast<E*>(payload ^ 1);
+}
+
+#endif  // COMMON_RESULT_H_