juce 中的ReferenceCountedObjectPtr

提供了对引用计数对象的管理，其实也就是操作引用计数对象，当引用计数为零的时候将对象销毁，值得学习的是juce是如果将引用计数对象和它的智能指针结合在一起的，这个后面再加分析

//==============================================================================
/**
    A smart-pointer class which points to a reference-counted object.

    The template parameter specifies the class of the object you want to point to - the easiest
    way to make a class reference-countable is to simply make it inherit from ReferenceCountedObject
    or SingleThreadedReferenceCountedObject, but if you need to, you can roll your own reference-countable
    class by implementing a set of methods called incReferenceCount(), decReferenceCount(), and
    decReferenceCountWithoutDeleting(). See ReferenceCountedObject for examples of how these methods
    should behave.

    When using this class, you'll probably want to create a typedef to abbreviate the full
    templated name - e.g.
    @code
    struct MyClass  : public ReferenceCountedObject
    {
        typedef ReferenceCountedObjectPtr<MyClass> Ptr;
        ...
    @endcode

    @see ReferenceCountedObject, ReferenceCountedObjectArray
*/
template <class ReferenceCountedObjectClass>
class ReferenceCountedObjectPtr
{
public:
    /** The class being referenced by this pointer. */
    typedef ReferenceCountedObjectClass ReferencedType;

    //==============================================================================
    /** Creates a pointer to a null object. */
    ReferenceCountedObjectPtr() noexcept
        : referencedObject (nullptr)
    {
    }

    /** Creates a pointer to an object.
        This will increment the object's reference-count.
    */
    ReferenceCountedObjectPtr (ReferencedType* refCountedObject) noexcept
        : referencedObject (refCountedObject)
    {
        incIfNotNull (refCountedObject);
    }

    /** Copies another pointer.
        This will increment the object's reference-count.
    */
    ReferenceCountedObjectPtr (const ReferenceCountedObjectPtr& other) noexcept
        : referencedObject (other.referencedObject)
    {
        incIfNotNull (referencedObject);
    }

    /** Copies another pointer.
        This will increment the object's reference-count (if it is non-null).
    */
    template <class Convertible>
    ReferenceCountedObjectPtr (const ReferenceCountedObjectPtr<Convertible>& other) noexcept
        : referencedObject (static_cast<ReferencedType*> (other.get()))
    {
        incIfNotNull (referencedObject);
    }

    /** Changes this pointer to point at a different object.
        The reference count of the old object is decremented, and it might be
        deleted if it hits zero. The new object's count is incremented.
    */
    ReferenceCountedObjectPtr& operator= (const ReferenceCountedObjectPtr& other)
    {
        return operator= (other.referencedObject);
    }

    /** Changes this pointer to point at a different object.
        The reference count of the old object is decremented, and it might be
        deleted if it hits zero. The new object's count is incremented.
    */
    template <class Convertible>
    ReferenceCountedObjectPtr& operator= (const ReferenceCountedObjectPtr<Convertible>& other)
    {
        return operator= (static_cast<ReferencedType*> (other.get()));
    }

    /** Changes this pointer to point at a different object.

        The reference count of the old object is decremented, and it might be
        deleted if it hits zero. The new object's count is incremented.
    */
    ReferenceCountedObjectPtr& operator= (ReferencedType* const newObject)
    {
        if (referencedObject != newObject)
        {
            incIfNotNull (newObject);
            ReferencedType* const oldObject = referencedObject;
            referencedObject = newObject;
            decIfNotNull (oldObject);
        }

        return *this;
    }

   #if JUCE_COMPILER_SUPPORTS_MOVE_SEMANTICS
    /** Takes-over the object from another pointer. */
    ReferenceCountedObjectPtr (ReferenceCountedObjectPtr&& other) noexcept
        : referencedObject (other.referencedObject)
    {
        other.referencedObject = nullptr;
    }

    /** Takes-over the object from another pointer. */
    ReferenceCountedObjectPtr& operator= (ReferenceCountedObjectPtr&& other)
    {
        std::swap (referencedObject, other.referencedObject);
        return *this;
    }
   #endif

    /** Destructor.
        This will decrement the object's reference-count, which will cause the
        object to be deleted when the ref-count hits zero.
    */
    ~ReferenceCountedObjectPtr()
    {
        decIfNotNull (referencedObject);
    }

    //==============================================================================
    /** Returns the object that this pointer references.
        The pointer returned may be null, of course.
    */
    operator ReferencedType*() const noexcept       { return referencedObject; }

    /** Returns the object that this pointer references.
        The pointer returned may be null, of course.
    */
    ReferencedType* get() const noexcept            { return referencedObject; }

    /** Returns the object that this pointer references.
        The pointer returned may be null, of course.
    */
    ReferencedType* getObject() const noexcept      { return referencedObject; }

    // the -> operator is called on the referenced object
    ReferencedType* operator->() const noexcept
    {
        jassert (referencedObject != nullptr); // null pointer method call!
        return referencedObject;
    }

private:
    //==============================================================================
    ReferencedType* referencedObject;

    static void incIfNotNull (ReferencedType* o) noexcept
    {
        if (o != nullptr)
            o->incReferenceCount();
    }

    static void decIfNotNull (ReferencedType* o) noexcept
    {
        if (o != nullptr && o->decReferenceCountWithoutDeleting())
            ContainerDeletePolicy<ReferencedType>::destroy (o);
    }
};


//==============================================================================
/** Compares two ReferenceCountedObjectPtrs. */
template <class ReferenceCountedObjectClass>
bool operator== (const ReferenceCountedObjectPtr<ReferenceCountedObjectClass>& object1, ReferenceCountedObjectClass* const object2) noexcept
{
    return object1.get() == object2;
}

/** Compares two ReferenceCountedObjectPtrs. */
template <class ReferenceCountedObjectClass>
bool operator== (const ReferenceCountedObjectPtr<ReferenceCountedObjectClass>& object1, const ReferenceCountedObjectPtr<ReferenceCountedObjectClass>& object2) noexcept
{
    return object1.get() == object2.get();
}

/** Compares two ReferenceCountedObjectPtrs. */
template <class ReferenceCountedObjectClass>
bool operator== (ReferenceCountedObjectClass* object1, const ReferenceCountedObjectPtr<ReferenceCountedObjectClass>& object2) noexcept
{
    return object1 == object2.get();
}

/** Compares two ReferenceCountedObjectPtrs. */
template <class ReferenceCountedObjectClass>
bool operator!= (const ReferenceCountedObjectPtr<ReferenceCountedObjectClass>& object1, const ReferenceCountedObjectClass* object2) noexcept
{
    return object1.get() != object2;
}

/** Compares two ReferenceCountedObjectPtrs. */
template <class ReferenceCountedObjectClass>
bool operator!= (const ReferenceCountedObjectPtr<ReferenceCountedObjectClass>& object1, const ReferenceCountedObjectPtr<ReferenceCountedObjectClass>& object2) noexcept
{
    return object1.get() != object2.get();
}

/** Compares two ReferenceCountedObjectPtrs. */
template <class ReferenceCountedObjectClass>
bool operator!= (ReferenceCountedObjectClass* object1, const ReferenceCountedObjectPtr<ReferenceCountedObjectClass>& object2) noexcept
{
    return object1 != object2.get();
}


#endif   // JUCE_REFERENCECOUNTEDOBJECT_H_INCLUDED

　　值得注意的是对象转移的时候，同样是增加新对象的引用计数，减去原有对象的引用计数

    /** Changes this pointer to point at a different object.

        The reference count of the old object is decremented, and it might be
        deleted if it hits zero. The new object's count is incremented.
    */
    ReferenceCountedObjectPtr& operator= (ReferencedType* const newObject)
    {
        if (referencedObject != newObject)
        {
            incIfNotNull (newObject);
            ReferencedType* const oldObject = referencedObject;
            referencedObject = newObject;
            decIfNotNull (oldObject);
        }

        return *this;
    }