Strong.h

/*
 * Copyright (C) 2011-2019 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:
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 *    notice, this list of conditions and the following disclaimer.
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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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 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
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#pragma once

#include "Handle.h"
#include "HandleSet.h"
#include "JSLock.h"
#include "StrongForward.h"
#include <wtf/Assertions.h>

namespace JSC {

class VM;

// A strongly referenced handle that prevents the object it points to from being garbage collected.
template <typename T, ShouldStrongDestructorGrabLock shouldStrongDestructorGrabLock> class Strong : public Handle<T> {
    using Handle<T>::slot;
    using Handle<T>::setSlot;
    template <typename U, ShouldStrongDestructorGrabLock> friend class Strong;

public:
    typedef typename Handle<T>::ExternalType ExternalType;
    
    Strong()
        : Handle<T>()
    {
    }
    
    inline Strong(VM&, ExternalType = ExternalType());

    inline Strong(VM&, Handle<T>);

    Strong(const Strong& other)
        : Handle<T>()
    {
        if (!other.slot())
            return;
        setSlot(HandleSet::heapFor(other.slot())->allocate());
        set(other.get());
    }

    template <typename U> Strong(const Strong<U>& other)
        : Handle<T>()
    {
        if (!other.slot())
            return;
        setSlot(HandleSet::heapFor(other.slot())->allocate());
        set(other.get());
    }
    
    enum HashTableDeletedValueTag { HashTableDeletedValue };
    bool isHashTableDeletedValue() const { return slot() == hashTableDeletedValue(); }
    Strong(HashTableDeletedValueTag)
        : Handle<T>(hashTableDeletedValue())
    {
    }

    ~Strong()
    {
        clear();
    }

    bool operator!() const { return !slot() || !*slot(); }

    explicit operator bool() const { return !!*this; }

    void swap(Strong& other)
    {
        Handle<T>::swap(other);
    }

    ExternalType get() const { return HandleTypes<T>::getFromSlot(this->slot()); }

    inline void set(VM&, ExternalType);

    template <typename U> Strong& operator=(const Strong<U>& other)
    {
        if (!other.slot()) {
            clear();
            return *this;
        }

        set(*HandleSet::heapFor(other.slot())->vm(), other.get());
        return *this;
    }
    
    Strong& operator=(const Strong& other)
    {
        if (!other.slot()) {
            clear();
            return *this;
        }

        set(HandleSet::heapFor(other.slot())->vm(), other.get());
        return *this;
    }

    void clear()
    {
        if (!slot())
            return;

        auto* heap = HandleSet::heapFor(slot());
        if (shouldStrongDestructorGrabLock == ShouldStrongDestructorGrabLock::Yes) {
            JSLockHolder holder(heap->vm());
            heap->deallocate(slot());
            setSlot(nullptr);
        } else {
            heap->deallocate(slot());
            setSlot(nullptr);
        }
    }

private:
    static HandleSlot hashTableDeletedValue() { return reinterpret_cast<HandleSlot>(-1); }

    void set(ExternalType externalType)
    {
        ASSERT(slot());
        JSValue value = HandleTypes<T>::toJSValue(externalType);
        HandleSet::heapFor(slot())->template writeBarrier<std::is_base_of_v<JSCell, T>>(slot(), value);
        *slot() = value;
    }
};

template<class T> inline void swap(Strong<T>& a, Strong<T>& b)
{
    a.swap(b);
}

} // namespace JSC

namespace WTF {

template<typename T> struct VectorTraits<JSC::Strong<T>> : SimpleClassVectorTraits {
    static constexpr bool canCompareWithMemcmp = false;
};

template<typename P> struct HashTraits<JSC::Strong<P>> : SimpleClassHashTraits<JSC::Strong<P>> { };

} // namespace WTF