chromium/third_party/dawn/src/dawn/common/Result.h

// Copyright 2018 The Dawn & Tint Authors
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:
//
// 1. Redistributions of source code must retain the above copyright notice, this
//    list of conditions and the following disclaimer.
//
// 2. Redistributions in binary form must reproduce the above copyright notice,
//    this list of conditions and the following disclaimer in the documentation
//    and/or other materials provided with the distribution.
//
// 3. Neither the name of the copyright holder nor the names of its
//    contributors may be used to endorse or promote products derived from
//    this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
// DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
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#ifndef SRC_DAWN_COMMON_RESULT_H_
#define SRC_DAWN_COMMON_RESULT_H_

#include <cstddef>
#include <cstdint>
#include <memory>
#include <type_traits>
#include <utility>
#include <variant>

#include "dawn/common/Assert.h"
#include "dawn/common/Compiler.h"

namespace dawn {

// 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 DAWN_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.
Result<void, E>;

// Uses SFINAE to try to get alignof(T) but fallback to Default if T isn't defined.
alignof_if_defined_else_default;

alignof_if_defined_else_default;

// 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 {};

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 =;
}  // namespace detail

Result<T *, E>;

Result<const T *, E>;

template <typename T>
class Ref;

Result<Ref<T>, E>;

// 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 {};

// Implementation of Result<void, E>
template <typename E>
Result<void, E>::Result() {}

template <typename E>
Result<void, E>::Result(std::unique_ptr<E> error) :{}

template <typename E>
Result<void, E>::Result(Result<void, E>&& other) :{}

template <typename E>
Result<void, E>& Result<void, E>::operator=(Result<void, E>&& other) {}

template <typename E>
Result<void, E>::~Result() {}

template <typename E>
bool Result<void, E>::IsError() const {}

template <typename E>
bool Result<void, E>::IsSuccess() const {}

template <typename E>
void Result<void, E>::AcquireSuccess() {}

template <typename E>
std::unique_ptr<E> Result<void, E>::AcquireError() {}

// Implementation details of the tagged pointer Results
namespace detail {

template <typename T>
T* GetSuccessFromPayload(intptr_t payload) {}

template <typename E>
E* GetErrorFromPayload(intptr_t payload) {}

}  // namespace detail

// Implementation of Result<T*, E>
template <typename T, typename E>
Result<T*, E>::Result(T* success) :{}

template <typename T, typename E>
Result<T*, E>::Result(std::unique_ptr<E> error)
    :{}

template <typename T, typename E>
template <typename TChild>
Result<T*, E>::Result(Result<TChild*, E>&& other) : mPayload(other.mPayload) {}

template <typename T, typename E>
template <typename TChild>
Result<T*, E>& Result<T*, E>::operator=(Result<TChild*, E>&& other) {}

template <typename T, typename E>
Result<T*, E>::~Result() {}

template <typename T, typename E>
bool Result<T*, E>::IsError() const {}

template <typename T, typename E>
bool Result<T*, E>::IsSuccess() const {}

template <typename T, typename E>
T* Result<T*, E>::AcquireSuccess() {}

template <typename T, typename E>
std::unique_ptr<E> Result<T*, E>::AcquireError() {}

// Implementation of Result<const T*, E*>
template <typename T, typename E>
Result<const T*, E>::Result(const T* success)
    :{}

template <typename T, typename E>
Result<const T*, E>::Result(std::unique_ptr<E> error)
    :{}

template <typename T, typename E>
Result<const T*, E>::Result(Result<const T*, E>&& other) :{}

template <typename T, typename E>
Result<const T*, E>& Result<const T*, E>::operator=(Result<const T*, E>&& other) {}

template <typename T, typename E>
Result<const T*, E>::~Result() {}

template <typename T, typename E>
bool Result<const T*, E>::IsError() const {}

template <typename T, typename E>
bool Result<const T*, E>::IsSuccess() const {}

template <typename T, typename E>
const T* Result<const T*, E>::AcquireSuccess() {}

template <typename T, typename E>
std::unique_ptr<E> Result<const T*, E>::AcquireError() {}

// 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)) {}

template <typename T, typename E>
template <typename U>
Result<Ref<T>, E>::Result(const Ref<U>& success) : Result(Ref<U>(success)) {}

template <typename T, typename E>
Result<Ref<T>, E>::Result(std::unique_ptr<E> error)
    :{}

template <typename T, typename E>
template <typename U>
Result<Ref<T>, E>::Result(Result<Ref<U>, E>&& other) : mPayload(other.mPayload) {}

template <typename T, typename E>
template <typename U>
Result<Ref<U>, E>& Result<Ref<T>, E>::operator=(Result<Ref<U>, E>&& other) {}

template <typename T, typename E>
Result<Ref<T>, E>::~Result() {}

template <typename T, typename E>
bool Result<Ref<T>, E>::IsError() const {}

template <typename T, typename E>
bool Result<Ref<T>, E>::IsSuccess() const {}

template <typename T, typename E>
Ref<T> Result<Ref<T>, E>::AcquireSuccess() {}

template <typename T, typename E>
std::unique_ptr<E> Result<Ref<T>, E>::AcquireError() {}

// Implementation of Result<T, E>
template <typename T, typename E>
Result<T, E>::Result(T success) :{}

template <typename T, typename E>
Result<T, E>::Result(std::unique_ptr<E> error) :{}

template <typename T, typename E>
Result<T, E>::~Result() {}

template <typename T, typename E>
Result<T, E>::Result(Result<T, E>&& other) {}

template <typename T, typename E>
Result<T, E>& Result<T, E>::operator=(Result<T, E>&& other) {}

template <typename T, typename E>
bool Result<T, E>::IsError() const {}

template <typename T, typename E>
bool Result<T, E>::IsSuccess() const {}

template <typename T, typename E>
T Result<T, E>::AcquireSuccess() {}

template <typename T, typename E>
std::unique_ptr<E> Result<T, E>::AcquireError() {}

}  // namespace dawn

#endif  // SRC_DAWN_COMMON_RESULT_H_