Source/WTF/wtf/Optional.h

/*
 * Copyright (C) 2014 Apple Inc. All rights reserved.
 *
 * 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.
 *
 * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS 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 APPLE INC. OR ITS CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
 * THE POSSIBILITY OF SUCH DAMAGE.
 */

#ifndef Optional_h
#define Optional_h

#include <type_traits>
#include <wtf/Assertions.h>
#include <wtf/StdLibExtras.h>

// WTF::Optional is a class based on std::optional, described here:
// http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2013/n3527.html
// If this ends up in a C++ standard, we should replace our implementation with it.

namespace WTF {

struct InPlaceTag { };
constexpr InPlaceTag InPlace { };

struct NulloptTag { explicit constexpr NulloptTag(int) { } };
constexpr NulloptTag Nullopt { 0 };

template<typename T>
class Optional {
public:
    Optional()
        : m_isEngaged(false)
    {
    }

    Optional(NulloptTag)
        : m_isEngaged(false)
    {
    }

    Optional(const T& value)
        : m_isEngaged(true)
    {
        new (NotNull, &m_value) T(value);
    }

    Optional(const Optional& other)
        : m_isEngaged(other.m_isEngaged)
    {
        if (m_isEngaged)
            new (NotNull, &m_value) T(*other.asPtr());
    }

    Optional(Optional&& other)
        : m_isEngaged(other.m_isEngaged)
    {
        if (m_isEngaged)
            new (NotNull, &m_value) T(WTFMove(*other.asPtr()));
    }

    Optional(T&& value)
        : m_isEngaged(true)
    {
        new (NotNull, &m_value) T(WTFMove(value));
    }

    template<typename... Args>
    Optional(InPlaceTag, Args&&... args)
        : m_isEngaged(true)
    {
        new (NotNull, &m_value) T(std::forward<Args>(args)...);
    }

    ~Optional()
    {
        destroy();
    }

    Optional& operator=(NulloptTag)
    {
        destroy();
        return *this;
    }

    Optional& operator=(const Optional& other)
    {
        if (this == &other)
            return *this;

        destroy();
        if (other.m_isEngaged) {
            new (NotNull, &m_value) T(*other.asPtr());
            m_isEngaged = true;
        }
        return *this;
    }

    Optional& operator=(Optional&& other)
    {
        if (this == &other)
            return *this;

        destroy();
        if (other.m_isEngaged) {
            new (NotNull, &m_value) T(WTFMove(*other.asPtr()));
            m_isEngaged = true;
        }
        return *this;
    }

    template<typename U, class = typename std::enable_if<std::is_same<typename std::remove_reference<U>::type, T>::value>::type>
    Optional& operator=(U&& u)
    {
        destroy();
        new (NotNull, &m_value) T(std::forward<U>(u));
        m_isEngaged = true;
        return *this;
    }

    explicit operator bool() const { return m_isEngaged; }

    const T* operator->() const
    {
        ASSERT(m_isEngaged);
        return asPtr();
    }

    T* operator->()
    {
        ASSERT(m_isEngaged);
        return asPtr();
    }

    const T& operator*() const { return value(); }
    T& operator*() { return value(); }

    T& value()
    {
        ASSERT(m_isEngaged);
        return *asPtr();
    }

    const T& value() const
    {
        ASSERT(m_isEngaged);
        return *asPtr();
    }

    template<typename U>
    T valueOr(U&& value) const
    {
        if (m_isEngaged)
            return *asPtr();

        return std::forward<U>(value);
    }

    template<typename U>
    T valueOrCompute(U callback) const
    {
        if (m_isEngaged)
            return *asPtr();

        return callback();
    }

private:
    const T* asPtr() const { return reinterpret_cast<const T*>(&m_value); }
    T* asPtr() { return reinterpret_cast<T*>(&m_value); }
    void destroy()
    {
        if (m_isEngaged) {
            asPtr()->~T();
            m_isEngaged = false;
        }
    }

    bool m_isEngaged;
    typename std::aligned_storage<sizeof(T), std::alignment_of<T>::value>::type m_value;
};

template<typename T>
constexpr bool operator==(const Optional<T>& lhs, const Optional<T>& rhs)
{
    return static_cast<bool>(lhs) == static_cast<bool>(rhs) && (!static_cast<bool>(lhs) || lhs.value() == rhs.value());
}

template<typename T>
constexpr bool operator!=(const Optional<T>& lhs, const Optional<T>& rhs)
{
    return !(lhs == rhs);
}

template<typename T>
constexpr bool operator==(const Optional<T>& opt, NulloptTag)
{
    return !opt;
}

template<typename T>
constexpr bool operator!=(const Optional<T>& opt, NulloptTag)
{
    return static_cast<bool>(opt);
}

template<typename T>
constexpr bool operator==(NulloptTag, const Optional<T>& opt)
{
    return !opt;
}

template<typename T>
constexpr bool operator!=(NulloptTag, const Optional<T>& opt)
{
    return static_cast<bool>(opt);
}

template<typename T>
constexpr bool operator==(const Optional<T>& opt, const T& value)
{
    return opt && opt.value() == value;
}

template<typename T>
constexpr bool operator!=(const Optional<T>& opt, const T& value)
{
    return !(opt == value);
}

template<typename T>
constexpr bool operator==(const T& value, const Optional<T>& opt)
{
    return opt && opt.value() == value;
}

template<typename T>
constexpr bool operator!=(const T& value, const Optional<T>& opt)
{
    return !(value == opt);
}

template<typename T>
Optional<typename std::decay<T>::type>
makeOptional(T&& value)
{
    return Optional<typename std::decay<T>::type>(std::forward<T>(value));
}

} // namespace WTF

using WTF::InPlace;
using WTF::Nullopt;
using WTF::Optional;
using WTF::makeOptional;

#endif // Optional_h