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/*
* Copyright (C) 2015 The Android Open Source Project
*
* 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 AAPT_MAYBE_H
#define AAPT_MAYBE_H
#include <cassert>
#include <type_traits>
#include <utility>
namespace aapt {
/**
* Either holds a valid value of type T, or holds Nothing.
* The value is stored inline in this structure, so no
* heap memory is used when creating a Maybe<T> object.
*/
template <typename T>
class Maybe {
public:
/**
* Construct Nothing.
*/
Maybe();
~Maybe();
Maybe(const Maybe& rhs);
template <typename U>
Maybe(const Maybe<U>& rhs);
Maybe(Maybe&& rhs);
template <typename U>
Maybe(Maybe<U>&& rhs);
Maybe& operator=(const Maybe& rhs);
template <typename U>
Maybe& operator=(const Maybe<U>& rhs);
Maybe& operator=(Maybe&& rhs);
template <typename U>
Maybe& operator=(Maybe<U>&& rhs);
/**
* Construct a Maybe holding a value.
*/
Maybe(const T& value);
/**
* Construct a Maybe holding a value.
*/
Maybe(T&& value);
/**
* True if this holds a value, false if
* it holds Nothing.
*/
explicit operator bool() const;
/**
* Gets the value if one exists, or else
* panics.
*/
T& value();
/**
* Gets the value if one exists, or else
* panics.
*/
const T& value() const;
private:
template <typename U>
friend class Maybe;
template <typename U>
Maybe& copy(const Maybe<U>& rhs);
template <typename U>
Maybe& move(Maybe<U>&& rhs);
void destroy();
bool mNothing;
typename std::aligned_storage<sizeof(T), alignof(T)>::type mStorage;
};
template <typename T>
Maybe<T>::Maybe()
: mNothing(true) {
}
template <typename T>
Maybe<T>::~Maybe() {
if (!mNothing) {
destroy();
}
}
template <typename T>
Maybe<T>::Maybe(const Maybe& rhs)
: mNothing(rhs.mNothing) {
if (!rhs.mNothing) {
new (&mStorage) T(reinterpret_cast<const T&>(rhs.mStorage));
}
}
template <typename T>
template <typename U>
Maybe<T>::Maybe(const Maybe<U>& rhs)
: mNothing(rhs.mNothing) {
if (!rhs.mNothing) {
new (&mStorage) T(reinterpret_cast<const U&>(rhs.mStorage));
}
}
template <typename T>
Maybe<T>::Maybe(Maybe&& rhs)
: mNothing(rhs.mNothing) {
if (!rhs.mNothing) {
rhs.mNothing = true;
// Move the value from rhs.
new (&mStorage) T(std::move(reinterpret_cast<T&>(rhs.mStorage)));
rhs.destroy();
}
}
template <typename T>
template <typename U>
Maybe<T>::Maybe(Maybe<U>&& rhs)
: mNothing(rhs.mNothing) {
if (!rhs.mNothing) {
rhs.mNothing = true;
// Move the value from rhs.
new (&mStorage) T(std::move(reinterpret_cast<U&>(rhs.mStorage)));
rhs.destroy();
}
}
template <typename T>
inline Maybe<T>& Maybe<T>::operator=(const Maybe& rhs) {
// Delegate to the actual assignment.
return copy(rhs);
}
template <typename T>
template <typename U>
inline Maybe<T>& Maybe<T>::operator=(const Maybe<U>& rhs) {
return copy(rhs);
}
template <typename T>
template <typename U>
Maybe<T>& Maybe<T>::copy(const Maybe<U>& rhs) {
if (mNothing && rhs.mNothing) {
// Both are nothing, nothing to do.
return *this;
} else if (!mNothing && !rhs.mNothing) {
// We both are something, so assign rhs to us.
reinterpret_cast<T&>(mStorage) = reinterpret_cast<const U&>(rhs.mStorage);
} else if (mNothing) {
// We are nothing but rhs is something.
mNothing = rhs.mNothing;
// Copy the value from rhs.
new (&mStorage) T(reinterpret_cast<const U&>(rhs.mStorage));
} else {
// We are something but rhs is nothing, so destroy our value.
mNothing = rhs.mNothing;
destroy();
}
return *this;
}
template <typename T>
inline Maybe<T>& Maybe<T>::operator=(Maybe&& rhs) {
// Delegate to the actual assignment.
return move(std::forward<Maybe<T>>(rhs));
}
template <typename T>
template <typename U>
inline Maybe<T>& Maybe<T>::operator=(Maybe<U>&& rhs) {
return move(std::forward<Maybe<U>>(rhs));
}
template <typename T>
template <typename U>
Maybe<T>& Maybe<T>::move(Maybe<U>&& rhs) {
if (mNothing && rhs.mNothing) {
// Both are nothing, nothing to do.
return *this;
} else if (!mNothing && !rhs.mNothing) {
// We both are something, so move assign rhs to us.
rhs.mNothing = true;
reinterpret_cast<T&>(mStorage) = std::move(reinterpret_cast<U&>(rhs.mStorage));
rhs.destroy();
} else if (mNothing) {
// We are nothing but rhs is something.
mNothing = false;
rhs.mNothing = true;
// Move the value from rhs.
new (&mStorage) T(std::move(reinterpret_cast<U&>(rhs.mStorage)));
rhs.destroy();
} else {
// We are something but rhs is nothing, so destroy our value.
mNothing = true;
destroy();
}
return *this;
}
template <typename T>
Maybe<T>::Maybe(const T& value)
: mNothing(false) {
new (&mStorage) T(value);
}
template <typename T>
Maybe<T>::Maybe(T&& value)
: mNothing(false) {
new (&mStorage) T(std::forward<T>(value));
}
template <typename T>
Maybe<T>::operator bool() const {
return !mNothing;
}
template <typename T>
T& Maybe<T>::value() {
assert(!mNothing && "Maybe<T>::value() called on Nothing");
return reinterpret_cast<T&>(mStorage);
}
template <typename T>
const T& Maybe<T>::value() const {
assert(!mNothing && "Maybe<T>::value() called on Nothing");
return reinterpret_cast<const T&>(mStorage);
}
template <typename T>
void Maybe<T>::destroy() {
reinterpret_cast<T&>(mStorage).~T();
}
template <typename T>
inline Maybe<typename std::remove_reference<T>::type> make_value(T&& value) {
return Maybe<typename std::remove_reference<T>::type>(std::forward<T>(value));
}
template <typename T>
inline Maybe<T> make_nothing() {
return Maybe<T>();
}
/**
* Define the == operator between Maybe<T> and Maybe<U> if the operator T == U is defined.
* Otherwise this won't be defined and the compiler will yell at the callsite instead of inside
* Maybe.h.
*/
template <typename T, typename U>
auto operator==(const Maybe<T>& a, const Maybe<U>& b)
-> decltype(std::declval<T> == std::declval<U>) {
if (a && b) {
return a.value() == b.value();
} else if (!a && !b) {
return true;
}
return false;
}
/**
* Same as operator== but negated.
*/
template <typename T, typename U>
auto operator!=(const Maybe<T>& a, const Maybe<U>& b)
-> decltype(std::declval<T> == std::declval<U>) {
return !(a == b);
}
} // namespace aapt
#endif // AAPT_MAYBE_H