Source/WTF/wtf/ValueOrReference.h

/*
 * Copyright (C) 2025 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
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 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
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 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
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 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
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#pragma once

#include <wtf/StdLibExtras.h>

namespace WTF {

// ValueOrReference<T> is just like const T&, except that it can also optionally hold T.
//
// When T has an efficient default constructor, use ValueOrReference<T>; otherwise use OptionalOrReference<T>.
//
// ValueOrReference<T> is an optimization when you need to return a value that is
// usually an existing reference, but sometimes a temporary, e.g.:
//
// ValueOrReference<String> append(const String& string LIFETIME_BOUND, std::optional<String&> suffix)
// {
//     if (!suffix) [[likely]]
//         return string; // existing reference -- ValueOrReference<T> avoids a copy
//     return makeString(string, suffix.value()); // temporary -- ValueOrReference<T> holds T
// }
template<typename T> class ValueOrReference {
public:
    ValueOrReference()
        : m_reference(m_value)
    {
    }

    ValueOrReference(ValueOrReference&& other)
        : m_value(WTF::move(other.m_value))
        , m_reference(&other.m_reference == &other.m_value ? m_value : other.m_reference)
    {
    }

    ValueOrReference(const T& reference LIFETIME_BOUND)
        : m_reference(reference)
    {
    }

    ValueOrReference(T&& temporary)
        : m_value(WTF::move(temporary))
        , m_reference(m_value)
    {
    }

    operator const T&() const LIFETIME_BOUND { return m_reference; }
    const T& get() const LIFETIME_BOUND { return m_reference; }
    const T* operator->() const LIFETIME_BOUND { return &m_reference; }

private:
    T m_value;
    const T& m_reference;
};

} // namespace WTF

using WTF::ValueOrReference;