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// Copyright 2018 The Dawn 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>
#include <memory>
#include <type_traits>
#include <utility>
// 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> should look like the following.
// public:
// Result(T&& success);
// Result(std::unique_ptr<E> error);
//
// Result(Result<T, E>&& other);
// Result<T, E>& operator=(Result<T, E>&& other);
//
// ~Result();
//
// bool IsError() const;
// bool IsSuccess() const;
//
// T&& AcquireSuccess();
// std::unique_ptr<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 [[nodiscard]] Result<void, E> {
public:
Result();
Result(std::unique_ptr<E> error);
Result(Result<void, E>&& other);
Result<void, E>& operator=(Result<void, E>&& other);
~Result();
bool IsError() const;
bool IsSuccess() const;
void AcquireSuccess();
std::unique_ptr<E> AcquireError();
private:
std::unique_ptr<E> mError;
};
// 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.
namespace detail {
// 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 the headers
enum PayloadType {
Success = 0,
Error = 1,
Empty = 2,
};
intptr_t MakePayload(const void* pointer, PayloadType type);
PayloadType GetPayloadType(intptr_t payload);
template <typename T>
static T* GetSuccessFromPayload(intptr_t payload);
template <typename E>
static E* GetErrorFromPayload(intptr_t payload);
constexpr static intptr_t kEmptyPayload = Empty;
} // namespace detail
template <typename T, typename E>
class [[nodiscard]] 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(std::unique_ptr<E> error);
// Support returning a Result<T*, E*> from a Result<TChild*, E*>
template <typename TChild>
Result(Result<TChild*, E>&& other);
template <typename TChild>
Result<T*, E>& operator=(Result<TChild*, E>&& other);
~Result();
bool IsError() const;
bool IsSuccess() const;
T* AcquireSuccess();
std::unique_ptr<E> AcquireError();
private:
template <typename T2, typename E2>
friend class Result;
intptr_t mPayload = detail::kEmptyPayload;
};
template <typename T, typename E>
class [[nodiscard]] Result<const 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(const T* success);
Result(std::unique_ptr<E> error);
Result(Result<const T*, E>&& other);
Result<const T*, E>& operator=(Result<const T*, E>&& other);
~Result();
bool IsError() const;
bool IsSuccess() const;
const T* AcquireSuccess();
std::unique_ptr<E> AcquireError();
private:
intptr_t mPayload = detail::kEmptyPayload;
};
template <typename T>
class Ref;
template <typename T, typename E>
class [[nodiscard]] Result<Ref<T>, E> {
public:
static_assert(alignof_if_defined_else_default<T, 4> >= 4,
"Result<Ref<T>, E> reserves two bits for tagging pointers");
static_assert(alignof_if_defined_else_default<E, 4> >= 4,
"Result<Ref<T>, E> reserves two bits for tagging pointers");
template <typename U>
Result(Ref<U>&& success);
Result(std::unique_ptr<E> error);
template <typename U>
Result(Result<Ref<U>, E>&& other);
template <typename U>
Result<Ref<U>, E>& operator=(Result<Ref<U>, E>&& other);
~Result();
bool IsError() const;
bool IsSuccess() const;
Ref<T> AcquireSuccess();
std::unique_ptr<E> AcquireError();
private:
template <typename T2, typename E2>
friend class Result;
intptr_t mPayload = detail::kEmptyPayload;
};
// Catchall definition of Result<T, E> implemented as a tagged struct. It could be improved to use
// a tagged union instead if it turns out to be a hotspot. T and E must be movable and default
// constructible.
template <typename T, typename E>
class [[nodiscard]] Result {
public:
Result(T&& success);
Result(std::unique_ptr<E> error);
Result(Result<T, E>&& other);
Result<T, E>& operator=(Result<T, E>&& other);
~Result();
bool IsError() const;
bool IsSuccess() const;
T&& AcquireSuccess();
std::unique_ptr<E> AcquireError();
private:
enum PayloadType {
Success = 0,
Error = 1,
Acquired = 2,
};
PayloadType mType;
std::unique_ptr<E> mError;
T mSuccess;
};
// Implementation of Result<void, E>
template <typename E>
Result<void, E>::Result() {
}
template <typename E>
Result<void, E>::Result(std::unique_ptr<E> error) : mError(std::move(error)) {
}
template <typename E>
Result<void, E>::Result(Result<void, E>&& other) : mError(std::move(other.mError)) {
}
template <typename E>
Result<void, E>& Result<void, E>::operator=(Result<void, E>&& other) {
ASSERT(mError == nullptr);
mError = std::move(other.mError);
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>
std::unique_ptr<E> Result<void, E>::AcquireError() {
return std::move(mError);
}
// Implementation details of the tagged pointer Results
namespace detail {
template <typename T>
T* GetSuccessFromPayload(intptr_t payload) {
ASSERT(GetPayloadType(payload) == Success);
return reinterpret_cast<T*>(payload);
}
template <typename E>
E* GetErrorFromPayload(intptr_t payload) {
ASSERT(GetPayloadType(payload) == Error);
return reinterpret_cast<E*>(payload ^ 1);
}
} // namespace detail
// Implementation of Result<T*, E>
template <typename T, typename E>
Result<T*, E>::Result(T* success) : mPayload(detail::MakePayload(success, detail::Success)) {
}
template <typename T, typename E>
Result<T*, E>::Result(std::unique_ptr<E> error)
: mPayload(detail::MakePayload(error.release(), detail::Error)) {
}
template <typename T, typename E>
template <typename TChild>
Result<T*, E>::Result(Result<TChild*, E>&& other) : mPayload(other.mPayload) {
other.mPayload = detail::kEmptyPayload;
static_assert(std::is_same<T, TChild>::value || std::is_base_of<T, TChild>::value, "");
}
template <typename T, typename E>
template <typename TChild>
Result<T*, E>& Result<T*, E>::operator=(Result<TChild*, E>&& other) {
ASSERT(mPayload == detail::kEmptyPayload);
static_assert(std::is_same<T, TChild>::value || std::is_base_of<T, TChild>::value, "");
mPayload = other.mPayload;
other.mPayload = detail::kEmptyPayload;
return *this;
}
template <typename T, typename E>
Result<T*, E>::~Result() {
ASSERT(mPayload == detail::kEmptyPayload);
}
template <typename T, typename E>
bool Result<T*, E>::IsError() const {
return detail::GetPayloadType(mPayload) == detail::Error;
}
template <typename T, typename E>
bool Result<T*, E>::IsSuccess() const {
return detail::GetPayloadType(mPayload) == detail::Success;
}
template <typename T, typename E>
T* Result<T*, E>::AcquireSuccess() {
T* success = detail::GetSuccessFromPayload<T>(mPayload);
mPayload = detail::kEmptyPayload;
return success;
}
template <typename T, typename E>
std::unique_ptr<E> Result<T*, E>::AcquireError() {
std::unique_ptr<E> error(detail::GetErrorFromPayload<E>(mPayload));
mPayload = detail::kEmptyPayload;
return std::move(error);
}
// Implementation of Result<const T*, E*>
template <typename T, typename E>
Result<const T*, E>::Result(const T* success)
: mPayload(detail::MakePayload(success, detail::Success)) {
}
template <typename T, typename E>
Result<const T*, E>::Result(std::unique_ptr<E> error)
: mPayload(detail::MakePayload(error.release(), detail::Error)) {
}
template <typename T, typename E>
Result<const T*, E>::Result(Result<const T*, E>&& other) : mPayload(other.mPayload) {
other.mPayload = detail::kEmptyPayload;
}
template <typename T, typename E>
Result<const T*, E>& Result<const T*, E>::operator=(Result<const T*, E>&& other) {
ASSERT(mPayload == detail::kEmptyPayload);
mPayload = other.mPayload;
other.mPayload = detail::kEmptyPayload;
return *this;
}
template <typename T, typename E>
Result<const T*, E>::~Result() {
ASSERT(mPayload == detail::kEmptyPayload);
}
template <typename T, typename E>
bool Result<const T*, E>::IsError() const {
return detail::GetPayloadType(mPayload) == detail::Error;
}
template <typename T, typename E>
bool Result<const T*, E>::IsSuccess() const {
return detail::GetPayloadType(mPayload) == detail::Success;
}
template <typename T, typename E>
const T* Result<const T*, E>::AcquireSuccess() {
T* success = detail::GetSuccessFromPayload<T>(mPayload);
mPayload = detail::kEmptyPayload;
return success;
}
template <typename T, typename E>
std::unique_ptr<E> Result<const T*, E>::AcquireError() {
std::unique_ptr<E> error(detail::GetErrorFromPayload<E>(mPayload));
mPayload = detail::kEmptyPayload;
return std::move(error);
}
// Implementation of Result<Ref<T>, E>
template <typename T, typename E>
template <typename U>
Result<Ref<T>, E>::Result(Ref<U>&& success)
: mPayload(detail::MakePayload(success.Detach(), detail::Success)) {
static_assert(std::is_convertible<U*, T*>::value, "");
}
template <typename T, typename E>
Result<Ref<T>, E>::Result(std::unique_ptr<E> error)
: mPayload(detail::MakePayload(error.release(), detail::Error)) {
}
template <typename T, typename E>
template <typename U>
Result<Ref<T>, E>::Result(Result<Ref<U>, E>&& other) : mPayload(other.mPayload) {
static_assert(std::is_convertible<U*, T*>::value, "");
other.mPayload = detail::kEmptyPayload;
}
template <typename T, typename E>
template <typename U>
Result<Ref<U>, E>& Result<Ref<T>, E>::operator=(Result<Ref<U>, E>&& other) {
static_assert(std::is_convertible<U*, T*>::value, "");
ASSERT(mPayload == detail::kEmptyPayload);
mPayload = other.mPayload;
other.mPayload = detail::kEmptyPayload;
return *this;
}
template <typename T, typename E>
Result<Ref<T>, E>::~Result() {
ASSERT(mPayload == detail::kEmptyPayload);
}
template <typename T, typename E>
bool Result<Ref<T>, E>::IsError() const {
return detail::GetPayloadType(mPayload) == detail::Error;
}
template <typename T, typename E>
bool Result<Ref<T>, E>::IsSuccess() const {
return detail::GetPayloadType(mPayload) == detail::Success;
}
template <typename T, typename E>
Ref<T> Result<Ref<T>, E>::AcquireSuccess() {
ASSERT(IsSuccess());
Ref<T> success = AcquireRef(detail::GetSuccessFromPayload<T>(mPayload));
mPayload = detail::kEmptyPayload;
return success;
}
template <typename T, typename E>
std::unique_ptr<E> Result<Ref<T>, E>::AcquireError() {
ASSERT(IsError());
std::unique_ptr<E> error(detail::GetErrorFromPayload<E>(mPayload));
mPayload = detail::kEmptyPayload;
return std::move(error);
}
// Implementation of Result<T, E>
template <typename T, typename E>
Result<T, E>::Result(T&& success) : mType(Success), mSuccess(std::move(success)) {
}
template <typename T, typename E>
Result<T, E>::Result(std::unique_ptr<E> error) : mType(Error), mError(std::move(error)) {
}
template <typename T, typename E>
Result<T, E>::~Result() {
ASSERT(mType == Acquired);
}
template <typename T, typename E>
Result<T, E>::Result(Result<T, E>&& other)
: mType(other.mType), mError(std::move(other.mError)), mSuccess(std::move(other.mSuccess)) {
other.mType = Acquired;
}
template <typename T, typename E>
Result<T, E>& Result<T, E>::operator=(Result<T, E>&& other) {
mType = other.mType;
mError = std::move(other.mError);
mSuccess = std::move(other.mSuccess);
other.mType = Acquired;
return *this;
}
template <typename T, typename E>
bool Result<T, E>::IsError() const {
return mType == Error;
}
template <typename T, typename E>
bool Result<T, E>::IsSuccess() const {
return mType == Success;
}
template <typename T, typename E>
T&& Result<T, E>::AcquireSuccess() {
ASSERT(mType == Success);
mType = Acquired;
return std::move(mSuccess);
}
template <typename T, typename E>
std::unique_ptr<E> Result<T, E>::AcquireError() {
ASSERT(mType == Error);
mType = Acquired;
return std::move(mError);
}
#endif // COMMON_RESULT_H_