juce 中的WeakReference分析

juce中的WeakReference设计得比较巧妙，巧妙就是使用delete之后就可以通知道WeakReference，原理其实也很间单，其实就是在对象里添加了一个子对象masterReference，对象在析构的时候主动调用masterReference.clear();，这样来达到通知弱指针的这个对象已经销毁了，可以设置为空了的目的。 感觉juce最后调用个clear还是觉得有点生硬，外层最好还是再嵌套一层，析构的时候自动调用clear就可以了，对象申明也写成宏，这样的话就简洁多了。

使用方法也很简单：

/==============================================================================
/**
    This class acts as a pointer which will automatically become null if the object
    to which it points is deleted.

    To accomplish this, the source object needs to cooperate by performing a couple of simple tasks.
    It must embed a WeakReference::Master object, which stores a shared pointer object, and must clear
    this master pointer in its destructor.

    E.g.
    @code
    class MyObject
    {
    public:
        MyObject()
        {
            // If you're planning on using your WeakReferences in a multi-threaded situation, you may choose
            // to create a WeakReference to the object here in the constructor, which will pre-initialise the
            // embedded object, avoiding an (extremely unlikely) race condition that could occur if multiple
            // threads overlap while creating the first WeakReference to it.
        }

        ~MyObject()
        {
            // This will zero all the references - you need to call this in your destructor.
            masterReference.clear();
        }

    private:
        // You need to embed a variable of this type, with the name "masterReference" inside your object. If the
        // variable is not public, you should make your class a friend of WeakReference<MyObject> so that the
        // WeakReference class can access it.
        WeakReference<MyObject>::Master masterReference;
        friend class WeakReference<MyObject>;
    };

    // Here's an example of using a pointer..

    MyObject* n = new MyObject();
    WeakReference<MyObject> myObjectRef = n;

    MyObject* pointer1 = myObjectRef;  // returns a valid pointer to 'n'
    delete n;
    MyObject* pointer2 = myObjectRef;  // returns a null pointer
    @endcode

    @see WeakReference::Master
*/

源码如下：

#ifndef JUCE_WEAKREFERENCE_H_INCLUDED
#define JUCE_WEAKREFERENCE_H_INCLUDED

/
template <class ObjectType, class ReferenceCountingType = ReferenceCountedObject>
class WeakReference
{
public:
    /** Creates a null SafePointer. */
    inline WeakReference() noexcept {}

    /** Creates a WeakReference that points at the given object. */
    WeakReference (ObjectType* const object)  : holder (getRef (object)) {}

    /** Creates a copy of another WeakReference. */
    WeakReference (const WeakReference& other) noexcept         : holder (other.holder) {}

    /** Copies another pointer to this one. */
    WeakReference& operator= (const WeakReference& other)       { holder = other.holder; return *this; }

    /** Copies another pointer to this one. */
    WeakReference& operator= (ObjectType* const newObject)      { holder = getRef (newObject); return *this; }

   #if JUCE_COMPILER_SUPPORTS_MOVE_SEMANTICS
    WeakReference (WeakReference&& other) noexcept              : holder (static_cast<SharedRef&&> (other.holder)) {}
    WeakReference& operator= (WeakReference&& other) noexcept   { holder = static_cast<SharedRef&&> (other.holder); return *this; }
   #endif

    /** Returns the object that this pointer refers to, or null if the object no longer exists. */
    ObjectType* get() const noexcept                            { return holder != nullptr ? holder->get() : nullptr; }

    /** Returns the object that this pointer refers to, or null if the object no longer exists. */
    operator ObjectType*() const noexcept                       { return get(); }

    /** Returns the object that this pointer refers to, or null if the object no longer exists. */
    ObjectType* operator->() noexcept                           { return get(); }

    /** Returns the object that this pointer refers to, or null if the object no longer exists. */
    const ObjectType* operator->() const noexcept               { return get(); }

    /** This returns true if this reference has been pointing at an object, but that object has
        since been deleted.

        If this reference was only ever pointing at a null pointer, this will return false. Using
        operator=() to make this refer to a different object will reset this flag to match the status
        of the reference from which you're copying.
    */
    bool wasObjectDeleted() const noexcept                      { return holder != nullptr && holder->get() == nullptr; }

    bool operator== (ObjectType* const object) const noexcept   { return get() == object; }
    bool operator!= (ObjectType* const object) const noexcept   { return get() != object; }

    //==============================================================================
    /** This class is used internally by the WeakReference class - don't use it directly
        in your code!
        @see WeakReference,
    */
    class SharedPointer   : public ReferenceCountingType
    {
    public:
        explicit SharedPointer (ObjectType* const obj) noexcept : owner (obj) {}

        inline ObjectType* get() const noexcept     { return owner; }
        void clearPointer() noexcept                { owner = nullptr; }

    private:
        ObjectType* volatile owner;

        JUCE_DECLARE_NON_COPYABLE (SharedPointer)
    };

    typedef ReferenceCountedObjectPtr<SharedPointer> SharedRef;

    //==============================================================================
    /**
        This class is embedded inside an object to which you want to attach WeakReference pointers.
        See the WeakReference class notes for an example of how to use this class.
        @see WeakReference
    */
    class Master
    {
    public:
        Master() noexcept {}

        ~Master() noexcept
        {
            // You must remember to call clear() in your source object's destructor! See the notes
            // for the WeakReference class for an example of how to do this.
            jassert (sharedPointer == nullptr || sharedPointer->get() == nullptr);
        }

        /** The first call to this method will create an internal object that is shared by all weak
            references to the object.
        */
        SharedPointer* getSharedPointer (ObjectType* const object)
        {
            if (sharedPointer == nullptr)
            {
				//先用智能指针给包装起来
                sharedPointer = new SharedPointer (object);
            }
            else
            {
                // You're trying to create a weak reference to an object that has already been deleted!!
                jassert (sharedPointer->get() != nullptr);
            }

            return sharedPointer;
        }

        /** The object that owns this master pointer should call this before it gets destroyed,
            to zero all the references to this object that may be out there. See the WeakReference
            class notes for an example of how to do this.
        */
        void clear() noexcept
        {
            if (sharedPointer != nullptr)
                sharedPointer->clearPointer();
        }

    private:
        SharedRef sharedPointer;

        JUCE_DECLARE_NON_COPYABLE (Master)
    };

private:
    SharedRef holder;

　　//这里看出取得了object里的masterReference，所以使用的对象必需要包含这个成员，getSharedPointer返回一个SharedPointer的智能指针，而SharedPointer里包装了真正的对象的指针，由于SharedPointer被智能指针包着，所以弱引用的对象被删除了，这个

　　SharedPointer仍然存在，但它里面的指针对象由于clear被设置为null,这里再调用get自然就知道对象是否被删除

static inline SharedPointer* getRef (ObjectType* const o) { return (o != nullptr) ? o->masterReference.getSharedPointer (o) : nullptr; } }; #endif // JUCE_WEAKREFERENCE_H_INCLUDED