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325 lines
7.9 KiB
325 lines
7.9 KiB
/*
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* Copyright (C) 2015 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#ifndef AAPT_MAYBE_H
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#define AAPT_MAYBE_H
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#include <type_traits>
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#include <utility>
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#include "android-base/logging.h"
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#include "util/TypeTraits.h"
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namespace aapt {
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/**
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* Either holds a valid value of type T, or holds Nothing.
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* The value is stored inline in this structure, so no
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* heap memory is used when creating a Maybe<T> object.
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*/
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template <typename T>
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class Maybe {
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public:
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/**
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* Construct Nothing.
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*/
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Maybe();
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~Maybe();
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Maybe(const Maybe& rhs);
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template <typename U>
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Maybe(const Maybe<U>& rhs); // NOLINT(google-explicit-constructor)
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Maybe(Maybe&& rhs) noexcept;
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template <typename U>
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Maybe(Maybe<U>&& rhs); // NOLINT(google-explicit-constructor)
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Maybe& operator=(const Maybe& rhs);
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template <typename U>
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Maybe& operator=(const Maybe<U>& rhs);
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Maybe& operator=(Maybe&& rhs) noexcept;
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template <typename U>
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Maybe& operator=(Maybe<U>&& rhs);
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/**
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* Construct a Maybe holding a value.
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*/
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Maybe(const T& value); // NOLINT(google-explicit-constructor)
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/**
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* Construct a Maybe holding a value.
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*/
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Maybe(T&& value); // NOLINT(google-explicit-constructor)
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/**
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* True if this holds a value, false if
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* it holds Nothing.
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*/
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explicit operator bool() const;
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/**
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* Gets the value if one exists, or else
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* panics.
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*/
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T& value();
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/**
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* Gets the value if one exists, or else
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* panics.
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*/
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const T& value() const;
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T value_or_default(const T& def) const;
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private:
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template <typename U>
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friend class Maybe;
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template <typename U>
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Maybe& copy(const Maybe<U>& rhs);
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template <typename U>
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Maybe& move(Maybe<U>&& rhs);
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void destroy();
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bool nothing_;
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typename std::aligned_storage<sizeof(T), alignof(T)>::type storage_;
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};
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template <typename T>
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Maybe<T>::Maybe() : nothing_(true) {}
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template <typename T>
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Maybe<T>::~Maybe() {
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if (!nothing_) {
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destroy();
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}
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}
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template <typename T>
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Maybe<T>::Maybe(const Maybe& rhs) : nothing_(rhs.nothing_) {
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if (!rhs.nothing_) {
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new (&storage_) T(reinterpret_cast<const T&>(rhs.storage_));
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}
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}
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template <typename T>
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template <typename U>
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Maybe<T>::Maybe(const Maybe<U>& rhs) : nothing_(rhs.nothing_) {
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if (!rhs.nothing_) {
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new (&storage_) T(reinterpret_cast<const U&>(rhs.storage_));
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}
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}
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template <typename T>
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Maybe<T>::Maybe(Maybe&& rhs) noexcept : nothing_(rhs.nothing_) {
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if (!rhs.nothing_) {
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rhs.nothing_ = true;
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// Move the value from rhs.
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new (&storage_) T(std::move(reinterpret_cast<T&>(rhs.storage_)));
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rhs.destroy();
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}
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}
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template <typename T>
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template <typename U>
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Maybe<T>::Maybe(Maybe<U>&& rhs) : nothing_(rhs.nothing_) {
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if (!rhs.nothing_) {
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rhs.nothing_ = true;
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// Move the value from rhs.
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new (&storage_) T(std::move(reinterpret_cast<U&>(rhs.storage_)));
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rhs.destroy();
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}
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}
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template <typename T>
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inline Maybe<T>& Maybe<T>::operator=(const Maybe& rhs) {
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// Delegate to the actual assignment.
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return copy(rhs);
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}
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template <typename T>
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template <typename U>
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inline Maybe<T>& Maybe<T>::operator=(const Maybe<U>& rhs) {
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return copy(rhs);
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}
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template <typename T>
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template <typename U>
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Maybe<T>& Maybe<T>::copy(const Maybe<U>& rhs) {
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if (nothing_ && rhs.nothing_) {
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// Both are nothing, nothing to do.
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return *this;
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} else if (!nothing_ && !rhs.nothing_) {
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// We both are something, so assign rhs to us.
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reinterpret_cast<T&>(storage_) = reinterpret_cast<const U&>(rhs.storage_);
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} else if (nothing_) {
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// We are nothing but rhs is something.
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nothing_ = rhs.nothing_;
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// Copy the value from rhs.
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new (&storage_) T(reinterpret_cast<const U&>(rhs.storage_));
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} else {
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// We are something but rhs is nothing, so destroy our value.
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nothing_ = rhs.nothing_;
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destroy();
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}
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return *this;
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}
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template <typename T>
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inline Maybe<T>& Maybe<T>::operator=(Maybe&& rhs) noexcept {
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// Delegate to the actual assignment.
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return move(std::forward<Maybe<T>>(rhs));
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}
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template <typename T>
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template <typename U>
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inline Maybe<T>& Maybe<T>::operator=(Maybe<U>&& rhs) {
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return move(std::forward<Maybe<U>>(rhs));
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}
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template <typename T>
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template <typename U>
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Maybe<T>& Maybe<T>::move(Maybe<U>&& rhs) {
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if (nothing_ && rhs.nothing_) {
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// Both are nothing, nothing to do.
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return *this;
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} else if (!nothing_ && !rhs.nothing_) {
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// We both are something, so move assign rhs to us.
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rhs.nothing_ = true;
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reinterpret_cast<T&>(storage_) =
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std::move(reinterpret_cast<U&>(rhs.storage_));
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rhs.destroy();
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} else if (nothing_) {
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// We are nothing but rhs is something.
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nothing_ = false;
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rhs.nothing_ = true;
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// Move the value from rhs.
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new (&storage_) T(std::move(reinterpret_cast<U&>(rhs.storage_)));
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rhs.destroy();
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} else {
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// We are something but rhs is nothing, so destroy our value.
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nothing_ = true;
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destroy();
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}
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return *this;
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}
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template <typename T>
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Maybe<T>::Maybe(const T& value) : nothing_(false) {
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new (&storage_) T(value);
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}
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template <typename T>
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Maybe<T>::Maybe(T&& value) : nothing_(false) {
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new (&storage_) T(std::forward<T>(value));
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}
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template <typename T>
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Maybe<T>::operator bool() const {
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return !nothing_;
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}
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template <typename T>
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T& Maybe<T>::value() {
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CHECK(!nothing_) << "Maybe<T>::value() called on Nothing";
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return reinterpret_cast<T&>(storage_);
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}
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template <typename T>
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const T& Maybe<T>::value() const {
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CHECK(!nothing_) << "Maybe<T>::value() called on Nothing";
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return reinterpret_cast<const T&>(storage_);
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}
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template <typename T>
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T Maybe<T>::value_or_default(const T& def) const {
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if (nothing_) {
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return def;
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}
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return reinterpret_cast<const T&>(storage_);
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}
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template <typename T>
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void Maybe<T>::destroy() {
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reinterpret_cast<T&>(storage_).~T();
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}
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template <typename T>
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inline Maybe<typename std::remove_reference<T>::type> make_value(T&& value) {
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return Maybe<typename std::remove_reference<T>::type>(std::forward<T>(value));
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}
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template <typename T>
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inline Maybe<T> make_nothing() {
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return Maybe<T>();
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}
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// Define the == operator between Maybe<T> and Maybe<U> only if the operator T == U is defined.
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// That way the compiler will show an error at the callsite when comparing two Maybe<> objects
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// whose inner types can't be compared.
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template <typename T, typename U>
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typename std::enable_if<has_eq_op<T, U>::value, bool>::type operator==(const Maybe<T>& a,
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const Maybe<U>& b) {
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if (a && b) {
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return a.value() == b.value();
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} else if (!a && !b) {
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return true;
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}
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return false;
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}
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template <typename T, typename U>
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typename std::enable_if<has_eq_op<T, U>::value, bool>::type operator==(const Maybe<T>& a,
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const U& b) {
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return a ? a.value() == b : false;
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}
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// Same as operator== but negated.
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template <typename T, typename U>
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typename std::enable_if<has_eq_op<T, U>::value, bool>::type operator!=(const Maybe<T>& a,
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const Maybe<U>& b) {
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return !(a == b);
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}
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template <typename T, typename U>
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typename std::enable_if<has_lt_op<T, U>::value, bool>::type operator<(const Maybe<T>& a,
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const Maybe<U>& b) {
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if (a && b) {
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return a.value() < b.value();
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} else if (!a && !b) {
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return false;
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}
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return !a;
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}
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} // namespace aapt
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#endif // AAPT_MAYBE_H
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